Azotemia

Background

Azotemia is an elevation of blood urea nitrogen (BUN) and serum creatinine levels. The reference range for BUN is 8-20 mg/dL, and the normal range for serum creatinine is 0.7-1.4 mg/dL.

Each human kidney contains approximately 1 million functional units, nephrons, which are primarily involved in urine formation. Urine formation ensures that the body eliminates the final products of metabolic activities and excess water in an attempt to maintain a constant internal environment (homeostasis). Urine formation by each nephron involves 3 main processes, as follows:

• Filtration at the glomerular level
• Selective reabsorption from the filtrate passing along the renal tubules
• Secretion by the cells of the tubules into this filtrate

Perturbation of any of these processes impairs the kidney’s excretory function, resulting in azotemia.

The quantity of glomerular filtrate produced each minute by all nephrons in both kidneys is referred to as the glomerular filtration rate (GFR). On average, the GFR is about 125 mL/min (10% less for women), or 180 L/day. About 99% of the filtrate (178 L/day) is reabsorbed, and the rest (2 L/day) is excreted.

Measurement of renal function

Radionuclide assessment of the GFR is the best available test for measuring kidney function. However, this test is expensive and not widely available, and as a result, serum creatinine concentration and creatinine clearance (CrCl) more commonly are used to estimate GFR.

An inverse relation between serum creatinine and the GFR exists; however, serum creatinine and CrCl are not sensitive measures of kidney damage, for 2 reasons. First, substantial renal damage can take place before any decrease in the GFR occurs. Second, a substantial decline in the GFR may lead to only a slight elevation in serum creatinine (see the image below). Because of compensatory hypertrophy and hyperfiltration of the remaining healthy nephrons, an elevation in serum creatinine is apparent only when the GFR falls to about 60-70 mL/min.

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Graph shows relation of glomerular filtration rate (GFR) to steady-state serum creatinine and blood urea nitrogen (BUN) levels. In early renal disease....

Because creatinine normally is filtered as well as secreted into the renal tubules, CrCl may cause the GFR to be substantially overestimated, especially as kidney failure progresses because of maximal tubular excretion. More accurate determinations of GFR require the use of inulin clearance or a radiolabeled compound (eg, iothalamate). In practice, precise knowledge of the GFR is not required, and the disease process usually can be adequately monitored by using the estimated GFR (eGFR), which may be obtained with a number of different methods.

CrCl is best calculated by obtaining a 24-hour urine collection for creatinine and volume and then using the following formula:

CrCl (mL/min) = U/P × V

where U is the urine creatinine in mg/dL, P is the serum creatinine in mg/dL, and V is the 24-hour volume divided by 1440 (the number of minutes in 24 hours). An adequate 24-hour collection usually reflects a creatinine generation of 15-20 mg/kg in women and 20-25 mg/kg in men. When 24-hour creatinine is measured, the adequacy of the collection must be established prior to calculation of the creatinine clearance.

Alternatively, the GFR can be estimated by means of the Cockcroft and Gault formula, a bedside formula that uses the patient’s serum creatinine (mg/dL), age (y), and lean weight (kg), as follows:

CrCl (mL/min) = [(140 – age) × weight]/(72 × serum creatinine)

For women, the result of the equation is multiplied by 0.85.

Another formula was derived from data collected in the Modification of Diet in Renal Disease (MDRD) study. The MDRD formula, also called the Levey formula, is now widely accepted as more accurate than the Cockcroft and Gault formula and is considered an alternative to radioisotope clearance.

Because serum creatinine levels alone cannot detect earlier stages of chronic kidney disease (CKD), the MDRD formula also takes into account the patient’s age and race. Although this formula is more accurate, it is much more difficult to calculate manually. However, software for estimating the GFR by means of the MDRD formula is available for most personal digital assistants and can be found on the Internet.

Delanaye et al have argued that the MDRD formula is inapplicable to some individuals, such as healthy individuals and patients who are anorectic or obese.^[1] It has therefore been proposed that the formula should be applied with caution.

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Pathophysiology

There are three pathophysiologic states in azotemia, as follows:

• Prerenal azotemia
• Intrarenal azotemia
• Postrenal azotemia

Prerenal azotemia

Prerenal azotemia refers to elevations in BUN and creatinine levels resulting from problems in the systemic circulation that decrease flow to the kidneys. In prerenal azotemia, decreased renal flow stimulates salt and water retention to restore volume and pressure. When volume or pressure is decreased, the baroreceptor reflexes located in the aortic arch and carotid sinuses are activated. This leads to sympathetic nerve activation, resulting in renal afferent arteriolar vasoconstriction and renin secretion through β₁ receptors.

Constriction of the afferent arterioles causes a decrease in intraglomerular pressure, which reduces the GFR proportionally. Reduction in renal blood flow results in the generation of renin, which converts angiotensinogen to angiotensin I. Angiotensin-converting enzyme then converts angiotensin I to angiotensin II, which, in turn, stimulates aldosterone release. The increase in aldosterone levels results in salt and water absorption in the distal collecting tubule.

A decrease in volume or pressure is a nonosmotic stimulus for hypothalamic production of antidiuretic hormone, which exerts its effect in the medullary collecting duct for water reabsorption. Through unknown mechanisms, activation of the sympathetic nervous system leads to enhanced proximal tubular reabsorption of salt and water, as well as BUN, creatinine, calcium, uric acid, and bicarbonate. The net result of these 4 mechanisms of salt and water retention is decreased output and decreased urinary excretion of sodium (< 20 mEq/L).

Intrarenal azotemia

Intrarenal azotemia, also known as acute renal failure (ARF), renal-renal azotemia, and acute kidney injury (AKI), refers to elevations in BUN and creatinine resulting from problems in the kidney itself. There are several definitions, including a rise in serum creatinine levels of about 30% from baseline or a sudden decline in output below 500 mL/day. If output is preserved, AKI is nonoliguric; if output falls below 500 mL/day, ARF is oliguric. Any form of AKI may be so severe that it virtually stops formation; this condition is called anuria (< 100 mL/day).

The most common causes of nonoliguric AKI are acute tubular necrosis (ATN), aminoglycoside nephrotoxicity, lithium toxicity, and cisplatin nephrotoxicity. Tubular damage is less severe than it is in oliguric AKI. Normal output in nonoliguric AKI does not reflect a normal GFR. Patients may still make 1440 mL/day of urine even when the GFR falls to about 1 mL/min because of decreased tubular reabsorption.

Some studies indicate that nonoliguric forms of AKI are associated with less morbidity and mortality than is oliguric AKI. Uncontrolled studies also suggest that volume expansion, potent diuretic agents, and renal vasodilators can convert oliguric AKI to nonoliguric AKI if administered early.

The pathophysiology of acute oliguric or nonoliguric AKI depends on the anatomic location of the injury. In ATN, epithelial damage leads to functional decline in the ability of the tubules to reabsorb salt, water, and other electrolytes. Excretion of acid and potassium is also impaired. In more severe ATN, the tubular lumen is filled with epithelial casts, causing intraluminal obstruction and resulting in a declining GFR.

Acute interstitial nephritis is characterized by inflammation and edema, which result in azotemia, hematuria, sterile pyuria, white blood cell (WBC) casts with variable eosinophiluria, proteinuria, and hyaline casts. The net effect is a loss of urinary concentrating ability, with low osmolality (< 500 mOsm/L), low specific gravity (< 1.015), high urinary sodium (>40 mEq/L), and, occasionally, hyperkalemia and renal tubular acidosis. However, if there is superimposed prerenal azotemia, the specific gravity, osmolality, and sodium may be misleading.

Glomerulonephritis or vasculitis is suggested by the presence of hematuria, red blood cells (RBCs), WBCs, granular and cellular casts, and a variable degree of proteinuria. Nephrotic syndrome usually is not associated with active inflammation and often presents as proteinuria greater than 3.5 g/24 h.

Glomerular diseases may reduce GFR by changing basement membrane permeability and stimulating the renin-aldosterone axis. Such diseases are often manifested as nephrotic or nephric syndrome. In nephrotic syndrome, the urinary sediment is inactive, and there is gross proteinuria (>3.5 g/day), hypoalbuminemia, hyperlipidemia, and edema. Azotemia and hypertension are uncommon initially, but their presence may indicate advanced disease.

Some patients with nephrotic syndrome may present with ARF. Impairment of capillary circulation in the kidney due to edema (nephrosarca) and tubular obstruction from protein casts, as well as decreased effective circulating volume, have been proposed as potential mechanisms for the development of ARF in patients with nephrotic syndrome.

In nephritic syndrome, the urinary sediment is active with WBC or RBC casts, granular casts, and azotemia. Proteinuria is less obvious, but increased salt and water retention in glomerulonephritis can lead to hypertension, edema formation, decreased output, low urinary excretion of sodium, and increased specific gravity.

Acute vascular diseases include vasculitis syndromes, malignant hypertension, scleroderma renal crisis, and thromboembolic disease, all of which cause renal hypoperfusion and ischemia leading to azotemia. Chronic vascular diseases are due to hypertensive benign nephrosclerosis, which has not been conclusively associated with end-stage renal disease (ESRD) and ischemic renal disease from bilateral renal artery stenosis.^[2]

In bilateral renal artery stenosis, maintenance of adequate intraglomerular pressure for filtration greatly depends on efferent arteriolar vasoconstriction. Azotemia sets in when angiotensin-converting enzyme (ACE) inhibitors or angiotensin II receptor blockers (ARBs) cause efferent arteriolar dilatation, thereby decreasing intraglomerular pressure and filtration. Therefore, ACE inhibitors and ARBs are contraindicated in bilateral renal artery stenosis.

In addition to accumulation of urea creatinine and other waste products, a substantial reduction in GFR in CKD results in the following:

• Decreased production of erythropoietin (causing anemia) and vitamin D-3 (causing hypocalcemia, secondary hyperparathyroidism, hyperphosphatemia, and renal osteodystrophy)
• Reduction in acid, potassium, salt, and water excretion (causing acidosis, hyperkalemia, hypertension, and edema)
• Platelet dysfunction (leading to increased bleeding tendencies)

The syndrome associated with the signs and symptoms of accumulation of toxic waste products (uremic toxins) is termed uremia and often occurs at a GFR of about 10 mL/min. Some of the uremic toxins (eg, urea, creatinine, phenols, and guanidines) have been identified, but none have been found responsible for all the manifestations of uremia.

Postrenal azotemia

Postrenal azotemia refers to elevations in BUN and creatinine levels resulting from obstruction in the collecting system. Obstruction to flow leads to reversal of the Starling forces responsible for glomerular filtration. Progressive bilateral obstruction causes hydronephrosis with an increase in the Bowman capsular hydrostatic pressure and tubular blockage that leads to progressive decline in and ultimate cessation of glomerular filtration, azotemia, acidosis, fluid overload, and hyperkalemia.

Unilateral obstruction rarely causes azotemia. There is evidence that if complete ureteral obstruction is relieved within 48 hours of onset, relatively complete recovery of GFR can be achieved within a week; little or no further recovery occurs after 12 weeks. Complete or prolonged partial obstruction can lead to tubular atrophy and irreversible renal fibrosis. Hydronephrosis may be absent if obstruction is mild or acute or if the collecting system is encased by retroperitoneal tumor or fibrosis.

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Etiology

Prerenal azotemia occurs as a consequence of impaired renal blood flow or decreased perfusion resulting from decreased blood volume, decreased cardiac output (congestive heart failure), decreased systemic vascular resistance, decreased effective arterial volume from sepsis or hepatorenal syndrome,^[3] or renal artery abnormalities. It may be superimposed on a background of chronic renal failure. Iatrogenic factors, such as excessive diuresis and treatment with ACE inhibitors, should be ruled out.

Intrarenal azotemia occurs as a result of injury to the glomeruli, tubules, interstitium, or small vessels. It may be acute oliguric, acute nonoliguric, or chronic. Systemic disease, nocturia, proteinuria, loss of urinary concentrating ability (low urine specific gravity), anemia, and hypocalcemia are suggestive of chronic intrarenal azotemia.

Postrenal azotemia occurs when an obstruction to urine flow is present. It is observed in bilateral ureteral obstruction from tumors or stones, retroperitoneal fibrosis, neurogenic bladder, and bladder neck obstruction from prostatic hypertrophy or carcinoma and posterior urethral valves. It may be superimposed on a background of chronic renal failure.

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Epidemiology

United States statistics

The reported incidence of hospital or community-acquired ARF varies considerably. In one report, community-acquired ARF occurred in about 1% of all hospital admissions. Overall, ARF occurs in about 5% of all hospital admissions. However, differences exist between the incidence of ARF occurring in the intensive care unit (ICU; about 15%) and that in the coronary care unit (CCU; about 4%).^[4]

In CKD, progressive azotemia leading to ESRD necessitating dialysis or kidney transplantation occurs in a number of chronic diseases, including diabetes (36%), hypertension (24%), glomerulonephritis (15%), cystic kidney disease (4%), and all other known miscellaneous renal disorders (15%).

International statistics

In a report from Madrid that evaluated 748 cases of ARF at 13 tertiary hospital centers, the most frequent causes were ATN (45%); prerenal (21%); acute or chronic renal failure, mostly due to ATN and prerenal disease (13%); urinary tract obstruction (10%); glomerulonephritis or vasculitis (4%); acute interstitial nephritis (2%); and atheroemboli (1%).^[5] Etiologies of CKD differ around the world. Diabetic nephropathy as a cause of CKD is on the rise in developed and developing countries.

A study of the epidemiology of community-acquired AKI in eastern India in 1983–95 versus 1996–2008 found that the incidence rate rose from 1.95 to 4.14 per 1000 hospital admissions. In addition, the etiology of AKI shifted: the incidences of obstetrical, surgical, and diarrheal AKI decreased significantly, whereas those of AKI associated with malaria, sepsis, nephrotoxic drugs, and liver disease increased.^[6]

Age-, sex-, and race-related demographics

Among the patients reported in the 2008 annual report of the United States Renal Data System (USRDS), the frequency of azotemia was 1.5% for patients aged 0-19 years, 19.1% for those aged 20-44 years, 44.0% for those aged 45-64 years, 19.6% for those aged 65-74 years, and 15.7% for those older than 75 years. The male frequency was 56.0%, and the female frequency was 44.0%.

[#Clinical]In the 2010 annual report of the USRDS, more than 500,000 patients with ESRD were receiving dialysis or a kidney transplant in the United States. Racial distribution was reported as Asian/Pacific Islander (4.7%), black (32.0%), white (61.0%), American Indian (1.3%), and other/unknown (0.6%).

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Prognosis

The prognosis of ARF/AKI generally is poor and depends on the severity of the underlying disease and the number of failed organs. Whereas mortality in patients with simple ARF/AKI without other underlying disease is 7-23%, that in ICU patients on mechanical ventilation is as high as 80%.

The prognosis of CKD depends on the etiology. Patients with diabetic kidney disease, hypertensive nephrosclerosis, and ischemic nephropathy (ie, large-vessel arterial occlusive disease) tend to have progressive azotemia resulting in ESRD. Different types of glomerulonephritis have major differences in prognosis: some are quite benign and rarely progress to ESRD, whereas others progress to ESRD within months. About 50% of patients with polycystic kidney disease progress to ESRD by the fifth or sixth decade of life.

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History

It is necessary to quickly establish if azotemia is acute or chronic and whether it is due to prerenal, intrarenal, or postrenal causes. This is vital in initiating treatment and in preventing progression. Clinical evaluation requires a thorough history, physical examination, and specific laboratory tests (including serologies, urinalysis, and, if indicated, radiologic studies and kidney biopsy; see Workup).

Patients with prerenal azotemia commonly have a history of diarrhea, vomiting, profound heat exhaustion, excessive sweat loss, concurrent illness that impairs their ability to eat and drink adequately, hemorrhage, liver disease, congestive heart failure, and polyuria (eg, caused by lithium intoxication, diuretics, diabetes, or diabetes insipidus).

Patients with intrarenal azotemia may have a history of nocturia, polyuria, proteinuria, shock, and edema. There may be a personal or family history of congenital or systemic diseases, especially diabetes, hypertension, systemic lupus erythematosus (SLE), other collagen vascular diseases, hepatitis B (HBV), hepatitis C (HCV), syphilis, multiple myeloma, and AIDS.

Obtain a detailed medication history, looking for nephrotoxic medications (especially antibiotics, nonsteroidal anti-inflammatory drugs [NSAIDs], angiotensin-converting enzyme [ACE] inhibitors, diuretics, and herbal remedies), chemical exposure, and intravenous (IV) drug abuse (associated with exposure to HIV, HBV, and HCV infections).

Patients with postrenal azotemia frequently have a history of renal colic, dysuria, frequency, hesitancy, urgency incontinence, pelvic malignancy or irradiation, or benign prostatic hypertrophy.

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Physical Examination

Physical examination should be detailed but should focus on signs that have a high diagnostic yield.

In suspected prerenal azotemia, look for tachycardia; orthostatic hypotension (systolic blood pressure drop greater than 20 mm Hg or diastolic drop greater than 10 mm Hg from supine to standing); hypotension; signs of dehydration, including dry mucous membranes, loss of skin turgor, and loss of axillary sweat; and signs of congestive heart failure or hepatic insufficiency.

In suspected intrarenal azotemia, look for hypertension and its end-organ effects, such as hypertensive retinopathy and left ventricular hypertrophy (apical impulse displaced lateral to midclavicular line), rash, joint swelling or tenderness, needle tracks, hearing abnormality, palpable kidneys, abdominal bruits, pericardial rub, and asterixis. The last 2 signs are suggestive of uremia. The presence of uremic pericarditis requires immediate dialysis.

Postrenal azotemia (obstruction) is suggested by a palpable bladder that is dull to percussion and the presence of a rectal or pelvic mass on digital examination.

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Laboratory Studies

For the initial evaluation, obtain a complete blood count (CBC), a biochemical profile, urinalysis, and urine electrolyte concentrations. In addition to establishing the presence of systemic disease, these tests may reveal clues to the origin of the azotemia. Diagnostic indices are commonly used to differentiate prerenal azotemia from intrarenal or postrenal azotemia (see in the image below).

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Diagnostic indices in azotemia. Although such indices are helpful, it is not necessary to perform all these tests on every patient. Comparison should ....

Prerenal azotemia

In prerenal azotemia, hemoconcentration results in elevation of the hematocrit and total protein/albumin, calcium, bicarbonate, and uric acid levels from baseline values.

Oliguria (urine volume < 500 mL/day) or anuria (< 100 mL/day), high urine specific gravity (>1.015), normal urinary sediment, and low urinary sodium (< 20 mEq/L; fractional excretion of sodium [FENa] < 1%) are seen.

When volume depletion is predominant, exaggerated proximal tubular reabsorption results in azotemia, hypernatremia, and elevated levels of calcium, uric acid, and bicarbonate, whereas hemoconcentration results in elevation of total protein, albumin, and hematocrit levels from baselines. When hypoperfusion due to decreased cardiac output or effective arterial volume is present, patients exhibit edema, hyponatremia, and hypoalbuminemia. Hematocrit and calcium, uric acid, and bicarbonate levels vary widely. These patients often are critically ill.

The FENa has traditionally been used to differentiate prerenal azotemia from ATN. An FENa below 1% suggests a prerenal cause (eg, volume depletion), whereas an FENa above 2% suggests acute tubular necrosis (ATN). Because the FENa is based on the fact that sodium reabsorption is enhanced in the setting of volume depletion, active use of diuretics may elevate the FENa even when volume depletion is present, yielding misleading values.

Alternatives to the FENa in this setting include the fractional excretion of urea or urea nitrogen (FEUrea) and the fractional excretion of uric acid (FEUA); excretion of urea and uric acid excretion is not influenced by diuretics. An FEUrea below 35% or an FEUA below 9-10 % suggests a prerenal etiology of ARF, whereas an FEUrea above 50% or an FEUA above 10-12 % suggests ATN.^[7]

Intrarenal azotemia

Anemia, thrombocytopenia, hypocalcemia, and high–anion gap metabolic acidosis may suggest intrarenal azotemia. Low urine specific gravity (< 1.015), active urinary sediment (see Pathophysiology), high urinary sodium (>40 mEq/L; FENa >5%), a plasma BUN–creatinine ratio of less than 20, and low urine osmolality may also suggest intrarenal azotemia.

In patients with long-standing CKD, renal ultrasonography usually shows small, contracted kidneys. Some causes of CKD can be associated with normal-sized or large kidneys, such as HIV nephropathy, diabetes, and renal amyloidosis. The renal sonogram usually is diagnostic for patients with polycystic kidney disease. In patients with active urinary sediment, progressive azotemia, proteinuria, or normal-sized kidneys on ultrasonography, a renal biopsy should be considered. Consultation with a nephrologist is imperative in all such patients.

Postrenal azotemia

Urinary indices in postrenal azotemia due to complete bilateral obstruction are usually nondiagnostic. The prima facie finding here is anuria, occasionally accompanied by hypertension. Urine output still may be present if overflow (in bladder outlet obstruction) or partial ureteral obstruction is present.

A Foley catheter should be inserted as part of the initial evaluation to rule out obstruction below the bladder outlet. Unilateral ureteral obstruction rarely leads to azotemia; it occurs acutely (as a result of obstruction from calculi, papillary necrosis, or hematoma), producing renal colic, or may be chronic and asymptomatic, producing hydronephrosis.

Bilateral partial obstruction may be associated with azotemia in the presence of normal urine output. When patients are subjected to maneuvers that increase urinary flow (eg, diuretic renography or perfusion pressure flow studies), they may exhibit an increase in size or pressure of the collecting system or experience pain.

In addition to azotemia, polyuria due to loss of concentrating ability and type 1 renal tubular acidosis, with hyperkalemia, hypercalcemia from a metastatic pelvic tumor, and elevated prostate-specific antigen (PSA) levels, may be clues to postrenal azotemia. Hydronephrosis in the absence of hydroureter may be seen in early (< 3 days) obstruction, retroperitoneal process, or partial obstruction.

Renal ultrasonography (see below) is the test of choice for ruling out obstructive uropathy. If the renal sonogram is equivocal, a furosemide (Lasix) washout scan (see Radionuclide Studies) should be performed.

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Ultrasonography

Renal ultrasonography is the most commonly used renal imaging study because of its ease of use and broad applicability for the following purposes^[8] :

• Determination of renal size and echogenicity, which is important when considering renal biopsy; small echogenic kidneys (< 9 cm) may suggest scarring from advanced renal disease, whereas normal or large kidneys with smooth contours may indicate a potentially reversible process
• Differentiation of cystic lesions from solid lesions
• Diagnosis of urinary tract obstruction (for which it is the test of choice)
• Detection of kidney stones

Doppler renal ultrasonography can be used to evaluate renal vascular flow (eg, for identification of renal vein thrombosis, renal infarction, or renal artery stenosis).

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Computed Tomography and Magnetic Resonance Imaging

Computed tomography (CT)^[9] is complementary to ultrasonography, especially when the diagnosis is uncertain. Contrast nephrotoxicity should be weighed against the benefits. CT is used for the following purposes:

• Differentiation of neoplastic lesions from simple cysts (in most cases)
• Radiologic diagnosis of renal stone disease, including radiolucent stones
• Evaluation and staging of renal cell carcinoma
• Diagnosis of renal vein thrombosis
• Diagnosis of polycystic kidney disease; it is more sensitive than ultrasonography for this task, particularly in younger patients

Knipp et al describe successful use of a technique for computed tomographic angiography (CTA) of the abdomen and pelvis in azotemic patients that uses a reduced iodinated contrast volume and low kilovolt (peak) [80-kV(p)] with iterative reconstruction. Their retrospective study in 103 patients with end-stage renal disease found that this technique allows for satisfactory abdominal/pelvic CTA with a 50% reduction in contrast volume and a 43% mean radiation dose reduction, , compared with a standard 120-kV(p) CTA protocol.^[10]

Magnetic resonance imaging (MRI) or magnetic resonance angiography (MRA) is used only when CT and ultrasonography are nondiagnostic. These modalities are standard for diagnosis of renal vein thrombosis and are also used in the evaluation of renal cell carcinoma and renal artery stenosis or vasculitis.

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Abdominal Radiography, Pyelography, and Angiography

If symptoms suggest nephrolithiasis, a plain film of the abdomen is performed to screen for presence of a radiopaque stone. Calcium-containing, struvite, and cystine stones can be identified, but radiolucent ones, such as uric acid stones, will be missed.

Intravenous pyelography (IVP) can provide detailed information concerning calyceal anatomy and the size and shape of the kidney. It is extremely useful for detecting renal stones. IVP is the preferred technique for evaluation and diagnosis of certain structural disorders (eg, chronic pyelonephritis, medullary sponge kidney, and papillary necrosis). It can provide data on the degree of obstruction. The risk of contrast nephrotoxicity should be weighed against the benefits of making a diagnosis that will not change management.

Retrograde or anterograde pyelography is of limited usefulness now that renal ultrasonography is more widely available. It may be used in patients with a high index of suspicion for hydronephrosis in whom sonograms appear normal, such as those with retroperitoneal fibrosis.

Renal arteriography is used in polyarteritis nodosa and renal artery stenosis to demonstrate multiple aneurysms or stenoses. Because of the availability of procedures that do not require contrast material (eg, ultrasonography, MRI, and MRA) do not carry a risk of contrast nephrotoxicity, this test is less commonly used than it once was.

Renal venography is the standard for diagnosis of renal vein thrombosis. However, there is a risk of contrast nephrotoxicity.

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Radionuclide Studies

Technetium-99m dimercaptosuccinic acid (^99m Tc DMSA) is heavily distributed within the renal parenchyma at first pass and so is best for detecting renal parenchymal scarring.

Technetium diethylenetriamine pentaacetic acid (^99m Tc DTPA) is heavily filtered at first pass and therefore is best for qualitative assessment of renal function (filtration and excretion). Because it is heavily filtered, it is most sensitive in detecting urine leaks after renal transplant. For the same reason, it is also used concomitantly with a furosemide washout scan (see below) for assessing functional obstruction of the collecting system.

Mercaptoacetyltriglycine (MAG3) is evenly distributed at first pass in the kidney and so is best for qualitative assessment of perfusion, filtration, and excretion. It is the preferred test for assessing the 3 aspects of function after renal transplantation. It can be used to detect urine leaks or functional obstruction with furosemide, though^99m Tc DTPA scanning remains the test of choice for these conditions. Voiding cystourethrography can be performed with a radionuclide study to detect vesicoureteral reflux.

In a furosemide washout scan, the renal scan usually is performed first. Then, if needed, the furosemide washout is done after the radionuclide has accumulated in the collecting system. Furosemide is used as a part of the renogram to separate nonobstructive hydronephrosis from obstructive hydronephrosis. If there is no obstruction, furosemide-induced flow containing little or no radionuclide will fill the collecting system, washing out radionuclide-containing urine. If obstruction is present, the radionuclide is not washed out as quickly.

The half-life or clearance of the radioisotope is plotted on a curve. A half-life shorter than 10 minutes is considered normal, one longer than 20 minutes is considered obstruction, and one of between 10-20 minutes is subject to further interpretation.

Conditions that can make it difficult to interpret the furosemide washout curve include a megaureter or pelvis that accepts a large bolus of urine and poor renal function. In patients with a megaureter, it can be difficult to determine when the renal pelvis is full, and in patients with renal disease, the onset of furosemide action may be delayed. To overcome the problem of poor renal function or relative hypovolemia if a patient has been fasting, the patient should be well hydrated with intravenous (IV) fluids before the study.

The test also is operator dependent, in that the furosemide should be administered at a time when the renal pelvis is believed to be full. A full bladder also delays washout of isotope. Therefore, the patient’s bladder must be catheterized before the study can be performed.

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Renal Biopsy

When glomerulonephritis, vasculitis, and (occasionally) interstitial nephritis are suspected, renal biopsy is indicated to establish the correct diagnosis and guide therapy. The following are common indications for renal biopsy:

• Isolated glomerular proteinuria or hematuria
• Nephrotic syndrome
• Acute nephritic syndrome
• Unexplained acute or subacute renal failure

Percutaneous renal biopsy is associated with potential complications. Severe bleeding causing hypotension occurs in 1-2% of patients. Bleeding necessitating transfusion occurs in about 0.1-0.3% of patients. Bleeding complications can be minimized by using data obtained from tests for bleeding time, prothrombin time (PT), partial thromboplastin time (PTT), and platelet count.

Nonsteroidal anti-inflammatory drugs (NSAIDs) should be stopped at least 1 week before a scheduled elective biopsy. Patients on warfarin should be started on heparin at least 3 days before renal biopsy. Patients who are taking heparin for other reasons should stop the drug for at least 1 day.

Contraindications for percutaneous renal biopsy include the following:

• Uncorrectable bleeding diathesis
• Small kidneys
• Severe hypertension
• Multiple bilateral cysts or renal tumor
• Hydronephrosis
• Active renal or perirenal infection
• Uncooperative patient

Percutaneous biopsy may be performed in selected patients with a solitary kidney because of the generally low risk of bleeding. Open renal biopsy may be performed if a percutaneous attempt is either unsuccessful or contraindicated and if the benefits of diagnosis outweigh the risks. When percutaneous biopsy is contraindicated but a diagnosis is necessary, a transvenous transjugular renal core biopsy can be performed.^[11] With this approach, bleeding occurs intravascularly, thereby reducing the risk of hematoma.

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Pharmacologic and Supportive Therapy

Prerenal azotemia

If volume depletion is due to free water loss, the serum sodium is often elevated by 10 mEq/L from baseline. The amount of fluid replacement in liters—that is, the free water deficit—can be estimated from serum sodium (mg/dL) and patient weight (kg) as follows:

[(Na/140) – 1] × 0.5 × weight

The volume of fluid to be administered is equal to the sum of the free-water deficit and daily maintenance fluids. Fifty percent of this total volume should be administered in the first 24 hours, and a new calculation should be performed at 24 hours based on new laboratory results.

Maintenance fluid can be roughly estimated at 1.5-2 L/day; however, it can also be estimated from caloric intake since 1 kCal requires 1 mL of water in the metabolic process. Normal caloric intake is about 30 Kcal/kg (low catabolic state requires < 30 kCal/kg and high catabolic state requires >40 kCal/kg). A 70-Kg person at normal caloric intake requires 2100 Kcal/day or 2.1 L of fluid intake. This volume should be added to the free-water deficit and administered as noted above.

Alternatively, the total free water deficit is usually quite close to the sum of 50% free-water deficit and daily maintenance fluids. Therefore, for all practical purposes, the total free-water deficit can be administered intravenously in 24 hours.

The fluid to be administered should consist of hypotonic solutions such as 0.5% saline or 5% dextrose in water (D5W). Alert patients should be encouraged to drink as much free water as they can tolerate; otherwise, free water can be administered via a nasogastric tube.

Serum sodium should be measured every 6-8 hours, and fluid replacement should be adjusted to avoid a precipitous decline in the serum sodium. To prevent brain edema, the rate of decrease in serum sodium should be no more than 0.7 mEq/h (17 mEq/24 h). Volume depletion due to blood loss requires IV saline and transfusion to maintain pressure (as well as interventions to halt further loss).

Diarrhea often causes isotonic volume loss that necessitates replacement with normal saline. Normal–anion gap metabolic acidosis occurring with diarrhea warrants infusion of bicarbonate in 0.5% normal saline.

Diuretic-induced volume depletion, especially in the elderly, manifests as dehydration, hyponatremia,^[12] and, occasionally, hypokalemia. The treatment of choice consists of normal saline infusion and correction of hypokalemia.

Decreased cardiac output requires optimization of cardiac performance through careful use of diuretics, an angiotensin-converting enzyme (ACE) inhibitor, beta blockers, nitrates, positive inotropic agents (including dobutamine), and, when indicated, specific therapy for the cause of impaired cardiac function.

When ACE inhibitors are contraindicated because of hyperkalemia, the combination of nitrates and hydralazine offers an alternative. Because these patients tend to have risk factors for macrovascular disease, the diagnosis of ischemic nephropathy or atheroembolic disease should be entertained when renal function continues to worsen despite optimization of cardiac function.

Reduced effective arterial volume due to systemic shunting can result from sepsis or liver failure. Severe edema, hyponatremia, and hypoalbuminemia often pose management problems. Decreased oncotic pressure, increased vascular permeability, and exaggerated salt and water retention shift the Starling forces toward formation of interstitial fluid. Effective treatment of sepsis with antibiotics and of hypotension with dopamine and norepinephrine is mandated. Crystalloid replacement can be tried, but it often leads to more edema.

In severely hypoalbuminemic patients, salt-poor albumin infusion may be undertaken, but there is no conclusive evidence of benefit.

Adequate nutrition and effective treatment of sepsis may improve oncotic pressure and normalize vascular permeability, thereby decreasing the systemic shunting. The net result is improved renal perfusion, decreased salt and water retention, improved output, and edema. In hepatorenal syndrome (HRS), the average survival is 1-2 weeks; however, there is evidence that the kidneys will recover with early liver transplantation. Occasionally, renal function is advanced, necessitating replacement therapy.

Intrarenal azotemia

Acute kidney injury (acute renal failure)

For ischemic or nephrotoxic acute tubular necrosis (ATN) due to shock (hypovolemic, cardiogenic, septic), the initial approach is to restore volume and pressure (with fluid replacement and vasopressors, respectively) and to withdraw any nephrotoxic drugs.^[13] . If the patient becomes oliguric or anuric from shock, volume in the form of crystalloids should be aggressively administered as boluses (eg, 300 mL every 2 hours, rather than 150 mL every hour). Bolus infusion leads to acute intravascular volume expansion, release of atrial natriuretic peptide from the heart, increased renal blood flow, and natriuresis, all of which are favorable in recovery from ATN compared with slow intravenous hydration.

If at least 2 L of fluids has been administered in a relatively short period (approximately 12 hours) with no improvement in urine output, a trial of high-dose intravenous furosemide at 100-160 mg can be tried, prior to preparation for renal replacement. This approach, called “tank and blast” in shock, is clinically useful but almost no evidence supports it. In one small study, hemodynamic and renal support with a continuous infusion of noradrenaline (0.06-0.12 microg/kg/min) and furosemide (10-30 mg/hr) induced polyuria and reversed acute tubular necrosis to nonoliguric acute renal failure in 11 of 14 cancer patients who had severe sepsis and multiorgan dysfunction syndrome.^[14] If the patient does not respond within 6 hours of this approach, dialysis or continuous renal replacement therapy should be considered as soon as possible.

If the patient responds by restoration of urine output to greater than 30 mL/h, continue on the appropriate amounts of intravenous fluids, vasopressors, and as-needed diuretics to keep the patient at the desired fluid balance (negative, positive, or match intake to output).

This approach is not indicated in nonshock patients with AKI. Nonshock patients with AKI require maintenance fluids, if needed, and avoidance of nephrotoxicity.

In both scenarios, early initiation of renal replacement therapy if azotemia sets in has a better prognosis than late initiation.

Albumin can be administered in combination with high-dose furosemide to enhance the diuretic effect of furosemide. The use of albumin in this context is not for volume expansion; rather, it allows more furosemide to be bound to albumin for delivery to the organic anion transporter in the kidney, thereby enabling more furosemide to enter the tubule than would otherwise do so.

Although this approach is widely used, research on the combination of albumin and a loop diuretic has principally studied its use for improving diuretic-resistant edema in patients with nephrotic syndrome.^[15] Other therapies that have not been conclusively shown to be beneficial are renal-dose dopamine and synthetic atrial natriuretic peptide.

The renal failure phase usually lasts 7-21 days if the primary insult can be corrected. Postischemic polyuria can be seen in the recovery phase and represents an attempt to excrete excess water and solute. Saline may be replaced (75% of output) as a maintenance fluid, owing to salt wasting during this phase, and to allow the patient to lose excess water retained while the patient was oliguric. Hypokalemia may result from the saline diuresis, and potassium should be replaced. Recovery is marked by the return of blood urea nitrogen (BUN) and creatinine levels to near-baseline values.

Acute interstitial nephritis is managed by withdrawing the offending nephrotoxin, avoiding further nephrotoxic exposure, and dehydration. The creatinine level begins to improve within 3-5 days. Renal biopsy may be indicated if renal failure is severe or azotemia is not improving.

Once the diagnosis is confirmed, a trial of oral prednisone (starting at 1 mg/kg/day and tapering over 6 weeks) or IV pulse methylprednisolone (1 g for 3 days) in severe cases may be considered. If the patient is a poor candidate for biopsy but the diagnosis is strongly suspected, therapy should be started.

Contrast-induced azotemia, which typically becomes evident 3-5 days after exposure, is best prevented by adequate hydration with half-normal saline at 1 mL/kg/h 12 hours before contrast administration and the use of smaller amounts of contrast. Clearly explain the risks of such procedures to the patient.

The benefits of N-acetylcysteine and sodium bicarbonate for prevention of contrast-induced azotemia are still being debated.^{[16, 17, 18]} A systematic review and meta-analysis of prevention strategies found that the greatest clinically and statistically significant reduction in contrast-induced nephropathy occurred with N-acetylcysteine in patients receiving low-osmolar contrast media (compared with IV saline) and with statins plus N-acetylcysteine (compared with N-acetlycysteine alone).^[19]

The Prevention of Serious Adverse Events Following Angiography (PRESERVE) trial, which included 5177 patients at high risk for renal complications who were undergoing angiography, found no benefit of IV sodium bicarbonate over IV saline or of oral acetylcysteine over placebo. Outcomes measured included the prevention of death, need for dialysis, or persistent decline in kidney function at 90 days, as well as the prevention of contrast-associated acute kidney injury.^[20]

Chronic kidney disease

It is important that patients with chronic kidney disease (CKD) be referred early to a nephrologist for the management of complications and for the transition to renal replacement therapy (ie, hemodialysis, peritoneal dialysis, and renal transplantation). There is some evidence that early referral of patients with CKD improves short-term outcome.

Disease progression can be slowed by means of various maneuvers, such as aggressive control of diabetes, hypertension, and proteinuria; dietary protein and phosphate restriction; and specific therapies for some of the glomerular diseases, such as lupus. Anemia, hyperphosphatemia, acidosis, and hypocalcemia should be aggressively managed before renal replacement therapy.

Postrenal azotemia

Relief of the obstruction is the mainstay of therapy for postrenal azotemia. In anuria, bladder catheterization is mandatory to rule out bladder neck obstruction, whereas in progressive azotemia, catheterization should be done after the patient has voided to determine the postvoid residual volume. A postvoid residual volume of 100 mL or more suggests obstructive uropathy, and the cause should be further investigated.

• References

Surgical Relief of Obstruction

If hydronephrosis is due to ureteral obstruction, unilateral or bilateral stenting or percutaneous nephrostomy is performed. Recovery of renal function takes 7-10 days, but renal function may be severely impaired, necessitating dialysis until such time as partial recovery is adequate for withdrawal of dialysis.

Up to 500-1000 mL/min of postobstructive polyuria can be seen with relief of obstruction. This is an appropriate response and represents an attempt to excrete the excess fluid accumulated during the period of obstruction.

Because of salt wasting during this phase, dehydration and hypokalemia are likely. Thus, two thirds of the urine output should be replaced with half-normal saline and potassium chloride if the patient is hypokalemic. Close monitoring is indicated to prevent hypotension and prerenal azotemia.

Matching the hourly urine output with IV replacement fluid is not recommended, because the excess water retained during the period of obstruction cannot be offloaded if hourly urine output is matched.

• References

Medication Summary

The goals of therapy are to increase renal perfusion and to maintain urine output. Drugs used in the management of patients with azotemia include diuretics, adrenergic agents, plasma volume expanders, and corticosteroids. Specific therapies for various systemic conditions affecting the kidney are discussed in other articles.

• References

Furosemide (Lasix)

• Dosing, Interactions, etc.

Clinical Context:  Furosemide is the drug of choice as a diuretic. It inhibits sodium chloride reabsorption in the thick ascending limb of the loop of Henle.

• References

Hydrochlorothiazide (Microzide)

• Dosing, Interactions, etc.

Clinical Context:  Hydrochlorothiazide (HCTZ) acts on the distal nephron to impair sodium reabsorption, enhancing sodium excretion. It has been in use for more than 40 years and is generally an important agent for the treatment of essential hypertension.

• References

Chlorothiazide (Diuril)

• Dosing, Interactions, etc.

Clinical Context:  Chlorothiazide inhibits the reabsorption of sodium in distal tubules, causing increased excretion of sodium and water, as well as of potassium and hydrogen ions.

• References

Metolazone (Zaroxolyn)

• Dosing, Interactions, etc.

Clinical Context:  Metolazone is given as an adjunct to furosemide in severe edematous states or when furosemide alone does not achieve adequate diuresis. It increases excretion of sodium, water, potassium, and hydrogen ions by inhibiting reabsorption of sodium in distal tubules. Metolazone may be more effective in the setting of impaired renal function.

• References

Class Summary

Diuretics are used to induce urine output in acute tubular necrosis (ATN) and to treat edema and hypertension. They increase urine excretion by inhibiting sodium and chloride reabsorption at different sites in the nephron.

• References

Albumin (Albuminar, Plasbumin, Albutein)

• Dosing, Interactions, etc.

Clinical Context:  Albumin is supplied as a 5% solution in 250 mL or a 25% solution in 50 mL. The choice between the 2 formulations is based on whether patient requires additional fluid replacement. Albumin is not used for nutritional supplementation; thus, attempts should be made to improve patient's nutrition.

• References

Class Summary

Plasma volume expanders increase plasma oncotic pressure and mobilize fluid from the interstitial space into the intravascular space in hypoalbuminemic patients. They enhance delivery of furosemide to distal tubule.

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Prednisone

• Dosing, Interactions, etc.

Clinical Context:  Prednisone is commonly used for many forms of glomerulonephritis and interstitial nephritis. Once the diagnosis is confirmed, a trial of oral prednisone (starting at 1 mg/kg/day and tapering over 6 weeks) may be considered.

• References

Prednisolone (Orapred, Pediapred, Millipred)

• Dosing, Interactions, etc.

Clinical Context:  Corticosteroids act as potent inhibitors of inflammation. They may cause profound and varied metabolic effects, particularly in relation to salt, water, and glucose tolerance, in addition to their modification of the immune response of the body. Once the diagnosis is confirmed, a trial of oral prednisolone (starting at 1 mg/kg/day and tapering over 6 weeks) may be considered.

• References

Methylprednisolone (Medrol, Solu-Medrol, Depo-Medrol)

• Dosing, Interactions, etc.

Clinical Context:  Methylprednisolone decreases inflammation by suppressing the migration of PMNs and reversing increased capillary permeability. In severe cases, a trial of intravenous pulse methylprednisolone (1 g for 3 days) may be considered once the diagnosis is confirmed.

• References

Class Summary

Corticosteroids are potent anti-inflammatory agents and immunosuppressants. They suppress humoral and cellular response to tissue injury, thereby reducing inflammation.

• References

Dopamine

• Dosing, Interactions, etc.

Clinical Context:  Above a critical dose (renal dose), dopamine becomes a potent vasoconstrictor. Renal-dose dopamine is used widely, but a clear benefit has not been established.

• References

Class Summary

Adrenergic agents stimulate vasodilation of the renal vasculature and enhance perfusion.

• References