Antithrombin III (ATIII) is currently referred to as antithrombin (AT).
Antithrombin (AT) is a 58-kDa molecule belonging to the serine protease inhibitor (serpin) superfamily that plays a central role as an anticoagulant in mammalian circulation systems; its sites of action are shown in the image below. In fact it is present in a wide variety of organisms ranging from thermophilic bacteria to mammals. In addition to its effect as an antagonist of thrombin, it also inhibits other proteases of the coagulation cascade.[3, 4, 5, 6]
Antithrombin (AT) sites of action.
These actions are catalyzed by the interaction between antithrombin and vessel wall-associated glycosaminoglycans. Recent studies have also shown that antithrombin has anti-inflammatory actions that are independent of its effect on coagulation.[7, 8, 9, 10]
The existence of antithrombin was conceptualized as long ago as 1905 by Morawitz. Olav Egeberg described the first family with thrombotic disease due to inherited antithrombin deficiency in 1965. Work done in the early years of antithrombin research has been elegantly reviewed by Ulrich Abildgaard in 2007. Over the last few years, there has been a growing body of data describing novel mutations in the antithrombin gene and literature helping to elucidate the molecular pathology of antithrombin deficiency.[13, 14, 15, 16, 17]
For excellent patient education resources, see eMedicineHealth's patient education article Deep Vein Thrombosis.
Antithrombin belongs to the serpin family of inhibitors, which include heparin cofactor II (HCII), alpha2-antiplasmin, plasminogen activator inhibitor-1 (PAI-1), C1-inhibitor, and alpha1-antitrypsin. Antithrombin forms a 1:1 irreversible complex with its target active enzyme, and the complex is cleared by the liver with loss of enzyme activity.
Serpins have a highly conserved structure with 3 beta-sheets and 9 alpha-helices. A region known as the reactive center loop (RCL) protrudes above the core of the serpin molecule and has a sequence of amino acids that is complementary to binding sites in the active sites of the target proteases. Cleavage at the reactive center by target proteases results in the activation of a unique mechanism of inhibition. Antithrombin exists in 2 forms: 90% as the alpha-form that is glycosylated at all positions and 10% as the beta form that is not glycosylated at position Asn135.
Plasma antithrombin contains 432 amino acids, 6 of which are cysteine residues that form 3 intramolecular disulfide bonds. Also present are 4 glycosylation sites at Asn96, Asn135, Asn155 and Asn192, to which are attached oligosaccharide side chains. The major physiologic role of the molecule, as the name implies, is the inhibition of thrombin (factor IIa). In addition, it also inhibits other serine proteinases, including activated factors X, IX, XI, and XII. Antithrombin also antagonizes factor VII by accelerating the dissociation of the factor VIIa-tissue factor complex and preventing its reassociation.
The mechanism of inactivation of serine proteinases occurs in 2 steps, with an initial weak interaction followed by a conformational change that traps the proteinase. This mechanism is depicted in the image below.
Antithrombin (AT) neutralizes the enzyme (IIa) by forming a 1:1 stoichiometric complex (AT:IIa) between the arginine-serine sites of the 2 proteins. B....
Transformation to the final complex involves formation of a highly stable bond between the Arg393 residue on antithrombin and the Ser residue on thrombin. The formation of the antithrombin-proteinase complex is catalyzed by heparin and related glycosaminoglycans. Under optimal conditions, the interaction between thrombin and antithrombin could be accelerated by as much as 2000 times. It should be noted that the catalytic effect of heparin is achieved when its concentration is far below that of antithrombin and its target proteinases.
Many in vitro studies have established the relative rates of thrombin generation and neutralization, but a study by Undas et al quantified the changes in the rate of activation and inactivation of several hemostatic factors in blood serially sampled from a bleeding time cut. In this in vivo test system with an active, ongoing interaction between blood components and the injured vessel wall in flowing blood, it was noted that thrombin-antithrombin (TAT) complexes started increasing within 30 seconds of the bleeding time cut and reached a maximum by 180 seconds.
The pattern of increase was typical of the 2 phases of activation, which have been described in other models of thrombosis, with an initial 60- to 90-second initiation phase followed by a subsequent propagation phase, during which activation reaches its maximum level. In the healthy volunteers, under basal conditions, the amount of thrombin formed exceeded TAT formation at all time points tested until bleeding stopped.
TAT complexes formed following the neutralization of thrombin by antithrombin have been used as a surrogate marker for thrombin generation; serial changes in TAT levels have been used to determine alterations of the extent of hemostatic activation in the course of a disease or to assess the impact of specific therapy (eg, the effect of heparin in ameliorating disseminated intravascular coagulation [DIC]).
HCII is another physiologic inhibitor of hemostasis that appears to contribute about 20-30% of plasma (AT) heparin-cofactor activity in the presence of large amounts of heparin; HCII does not contribute to anti–factor Xa activity. Therefore, it has been suggested that, in the assessment of the true heparin cofactor activity of antithrombin, the anti–factor Xa activity of antithrombin be measured within 30 seconds of incubation with factor Xa in the presence of small amounts of heparin in order to exclude the contribution of HCII to this assay.
The use of low doses of heparin in the test system and the use of factor Xa rather than thrombin allows for an accurate assessment of antithrombin's heparin cofactor activity with avoidance of the contribution of HCII to this assessment. Thrombomodulin, an endothelial cell receptor for thrombin, also binds antithrombin and accelerates its anticoagulant effect. In a purified system, tissue factor pathway inhibitor (TFPI) also appeared to potentiate the ability of antithrombin to neutralize activated coagulation factors.
Antithrombin is synthesized primarily in the liver. It is secreted into the plasma in the form of a molecule containing 432 amino acids with a molecular weight of 58,200. The normal plasma level is 150 mcg/mL and the plasma half-life is approximately 3 days.
Independent of its anticoagulant properties, antithrombin also exerts anti-inflammatory and anti-proliferative effects. A number of studies have documented the ability of antithrombin to inhibit leukocyte rolling and adhesion. The ability of this molecule to inhibit leukocyte-endothelial cell interaction is at least partly due to the release of prostacyclins from endothelial cells. Oelschlager et al have shown that antithrombin produces a dose-dependent reduction in both lipopolysaccharide and tumor necrosis factor (TNF)–alpha activation of nuclear factor kB (NF-kB) in cultured monocytes and endothelial cells. As a result, the synthesis of proinflammatory mediators such as interleukin (IL)-6, IL-8, and TNF is decreased, leading to an anti-inflammatory effect.
A number of studies have also shown that cleaved antithrombin has potent antiangiogenic and antitumor properties. Larsson and colleagues have shown that fibroblast growth factor (FGF)-induced angiogenesis in the chick embryo and angiogenesis in mouse fibrosarcoma tumors is inhibited by treatment with latent antithrombin. There is literature to suggest that latent antithrombin may also induce apoptosis of endothelial cells by disrupting cell-matrix interactions.
Patients with antithrombin deficiency (AT deficiency) have prolonged circulation of activated coagulation factors, which increases the risk of thrombus formation at sites that fulfill Virchow's postulates (stasis, alteration of coagulability of the blood, and vessel wall damage). A 50% reduction in the level of antithrombin activity is sufficient to tilt the balance in favor of thrombosis; patients who are heterozygous for antithrombin deficiency (AT deficiency) have a variable incidence of thrombotic disease, whereas most homozygous individuals have a 100% frequency of thrombotic disease, which may be fatal at an early age.
Inherited or acquired antithrombin deficiencies (AT deficiencies) predispose affected individuals to serious venous and arterial thrombotic disease. Although it is well recognized that inherited antithrombin deficiency (AT deficiency) is a more serious disorder than inherited deficiencies of proteins C or S, there is much variability in thrombotic manifestations in patients with inherited antithrombin deficiency. A population-based case control study found a 5-fold increased risk of thrombosis when antithrombin deficiency (AT deficiency) was associated with another genetic defect that predisposes to thrombosis.[24, 25] This risk increased to 20-fold when antithrombin deficiency was coupled with an acquired risk factor for thrombosis.
Variable co-inheritance of other thrombophilic mutations (eg, activated protein C [APC] resistance, factor V Leiden, protein C or S deficiency, thrombomodulin gene mutations, methylene tetrahydrofolate reductase (MTHFR) deficiency, high lipoprotein(a) levels) is the reason for discordance in thrombotic manifestations among individuals within a family with antithrombin deficiency (AT deficiency).
The type of mutation also influences the phenotype. For example, the heterozygous form of a commonly inherited variant of antithrombin affecting the heparin-binding site (HBS) is not a risk factor for thrombosis. The location and type of mutation also affects the phenotype; for instance, the replacement of the normal threonine-85 (85 Thr) by a nonpolar methionine (known as antithrombin wibble) results in a mild adult-onset thrombotic disease, whereas replacement of the same85 Thr by a polar lysine (known as antithrombin wobble) results in severe and early onset of thrombosis in childhood.
Interestingly, a rise in body temperature in the presence of an antithrombin wobble, as with fevers, can add additional conformational stress on the antithrombin wobble protein, tilting the balance in favor of thrombosis. A cooperative interplay of risk factors occurs in individuals, depending on their genetic and acquired thrombophilic risk factors. Thus, the presence of an additional inherited or acquired risk factor(s) in a patient with antithrombin deficiency adds to the thrombophilic burden and necessitates aggressive prophylaxis in high-risk situations.
Ample evidence documents the high risk of venous thromboembolism (VTE) events in patients with antithrombin deficiency (AT deficiency), whether it is inherited or acquired. Inherited antithrombin deficiency contributes to about 1% of VTE in the affected population.
Studies of families with inherited antithrombin deficiency (AT deficiency) show that an increasing proportion of affected individuals develop thrombotic complications starting in their teen years, with spontaneous thrombosis in approximately 40% of patients. In the remaining 60%, additional precipitating factors, such as oral contraceptive use, pregnancy, labor and delivery, surgery, or trauma, may precipitate a thrombotic event. By age 50 years, more than 50% of individuals with inherited antithrombin deficiency have had VTE, in contrast with only 5% of nondeficient individuals. There has been a suggestion that antipsychotic drugs may potentiate thrombosis; this requires further validation.
A homozygous type of antithrombin deficiency (antithrombin III Kumamoto) has been reported to be present in a family with consanguinity. It was shown to be associated with arterial thrombotic disease. The patient developed cerebral arterial thrombosis at age 17 years and subsequently developed venous thrombosis. Evidence for a role of antithrombin deficiency (AT deficiency) in arterial thrombotic disease is now emerging.
The most common thrombotic manifestations in patients with antithrombin deficiency (AT deficiency) include lower extremity VTE, with recurrent VTE being common. Other sites of thrombosis include the inferior vena cava, hepatic and portal veins, and renal, axillary, brachial, mesenteric, pelvic, cerebral, and retinal veins. Arterial thrombosis is strikingly less common.
The gene for antithrombin is located on chromosome 1 band q23.1-23.9, has 7 exons and 6 introns, and is 13.5 kilobases (kb) long. The promoter region does not have a TATA or CAAT box. A control element at the 5' flanking region is apparently critical for efficient synthesis of antithrombin, with homology to an enhancer of murine and human genes. The mRNA is 1567 nucleotides long, encodes approximately 432 amino acids, codes for a signal peptide for antithrombin, and has an approximately 175 base pair (bp) 3' untranslated region. Two modes of splicing of the primary transcript are feasible at 2 sites in the first intron; the result is either a full native antithrombin molecule or a truncated product with a portion left within the cell.
Mutations that lead to a loss of function result in antithrombin deficiency (AT deficiency); those that affect the Arg393 site (P1 site) near the carboxy terminal end have a major impact on antithrombin activity. However, mutations at Ser394 (P1' site) have variable effects on different enzymes, depending on the mutation. An up-to-date listing of mutations affecting the antithrombin gene is available at the Antithrombin Mutation Database. A review of published mutations shows that they are distributed throughout the molecule, with reactive center defects having the biggest impact and heparin-binding defects carrying the least thrombotic risk.
Antithrombin deficiency (AT deficiency) states can be broadly classified into 2 types.
Type I antithrombin deficiency states in which heterozygous mutations lead to a complete loss of the mutant antithrombin protein result in immunologic and functional levels that are 50% or less than normal. The genetic basis of type I mutations includes major gene deletions or point mutations, with point mutations accounting for most of these cases. The mutations appear to cause a quantitative reduction in antithrombin synthesis by various processes, including premature termination of translation, aberrant RNA processing, and production of unstable antithrombin molecules that have short plasma half lives.
A report described 22 novel mutations in the antithrombin gene, of which 9 missense mutations resulted in type I deficiency and led to low antithrombin activity and antigen levels. Clinically these mutations were associated with venous thrombosis occurring before the age of 32 years. Homozygous type I antithrombin deficiency (AT deficiency) is almost always fatal in utero.
Type II antithrombin deficiency states are usually the result of single amino acid changes that result in functional deficits in a molecule that is otherwise synthesized and secreted into the plasma in a normal fashion. The variant antithrombin molecules may have abnormalities at the reactive site or the heparin binding site. Most cases of type II antithrombin deficiency are also heterozygous, although rare cases of homozygous type II deficiency have been described.
There also exists a third category of type II antithrombin deficiency in which multiple or "pleiotropic" abnormalities affect the reactive site, the heparin binding site, or the plasma concentration. Type II heparin binding site variants are not associated with a high risk of thrombosis unless the affected individual is a homozygote.
An autosomal dominant trait, inherited antithrombin deficiency (AT deficiency) has a prevalence between 0.2/1000 and 0.5/1000. In the general population, the incidence is thought to be in the range of 0.2-0.4%, with approximately 65% of biochemically affected individuals experiencing a thrombotic event.
In patients who develop venous thrombosis, the prevalence of hereditary antithrombin deficiency (AT deficiency) is between 1:20 and 1:200. Among the subtypes of antithrombin deficiency, type II antithrombin deficiency is at least twice as common as type I antithrombin deficiency in the general population. However, in symptomatic patients, cases of type I antithrombin deficiency represent about 80% of the total cases.
The frequency of acquired antithrombin deficiency (AT deficiency) depends on the frequency of the associated disease process.
In a study of 4000 Scottish blood donors, the prevalence of type I antithrombin deficiency was found to be 0.2/1000 and that of type II heparin binding site antithrombin deficiency was found to be 2-3/1000. Antithrombin deficiency (AT deficiency) is not restricted to any particular ethnic group and has been found in many countries.
Patients who are heterozygous for type I or II antithrombin deficiency develop significant thromboembolic complications, generally involving the deep veins. The lifetime risk of developing VTE depends on the subtype of antithrombin deficiency (AT deficiency). In hereditary type I antithrombin deficiency, the lifetime risk is between 50% and 85%. In patients with type II antithrombin deficiency, the risk of developing VTE is higher in those patients who have reactive site defects as compared to heparin-binding site defects.
In some subgroups of type II antithrombin deficiency patients, the lifetime risk of developing VTE is about 20%. The incidence of pregnancy-related VTE in women with antithrombin deficiency (AT deficiency) could be as high as 50%. Patients may develop recurrent VTE disease at an early age and, if the condition is unrecognized or inadequately treated, they may die from such events. Long-term consequences, such as chronic leg ulcerations, severe venous varicosities, and postphlebitic syndrome, are common from repeated episodes of VTE, which cause significant morbidity. The prognosis of patients with reductions in antithrombin as part of other systemic disorders depends on the underlying disorder.
Although no overt racial predilection for antithrombin deficiency (AT deficiency) is known, the literature, especially from the Far East, has described the presence of novel mutations in the antithrombin gene that have observed in thrombophilic patients in specific population groups.[37, 38]
Antithrombin deficiency (AT deficiency) is inherited as an autosomal dominant trait. Some mutations require homozygosity (2 doses of the gene [ie, autosomal recessive]) to be clinically significant. Both men and women can present with the inherited disorder.
Clinical manifestations of antithrombin deficiency (AT deficiency) are evident at an early or later age, depending on the severity of the inherited genetic defect and also on the co-inheritance or presence of other thrombophilic mutations, drugs, or diseases.
Neonates normally have approximately 60% of adult antithrombin levels despite the absence of a prothrombotic state. Premature infants have even lower values. Thus, a reduction in antithrombin level in these instances does not automatically imply an inherited deficiency. Serial follow-up may be necessary in families with inherited antithrombin deficiency (AT deficiency) to prove an inherited deficiency of antithrombin. If the genetic mutation in the family is known, the diagnosis is much simplified by the presence or absence of the specific mutation.
The clinical presentation of antithrombin deficiency (AT deficiency) depends on whether patients develop venous or arterial thrombosis and on the extent of damage to the particular organ.
Physical findings depend upon the site of thrombosis. As indicated previously, VTE is much more common than arterial thrombotic disease.
Important considerations during the laboratory workup of antithrombin deficiency (AT deficiency) include the following:
Initial workup: Routine coagulation tests should include prothrombin time (PT), activated partial thromboplastin time (aPTT), and fibrinogen level.
Special laboratory tests
Additional hypercoagulability workup is complex. Some of the currently known thrombophilic factors are as follows:
Objective documentation of all thromboembolic disease is essential. The various imaging techniques available include compression and color ultrasonography, venography, angiography, computed tomography (CT) scanning, and magnetic resonance imaging (MRI). The specific imaging modality depends on the location of the suspected thrombus.
Decisions about proceeding with additional tests, including genetic tests, are based on the patient's history and their current medications.
Gene-based tests require that the potential implications, such as the inherited nature of the defect and insurance issues, be discussed with the patient before blood is drawn. The need for genetic counseling should be discussed after test results become available.
In a patient with a known inherited antithrombin deficiency (AT deficiency), management of the acute thrombotic event depends on the type of antithrombin deficiency, because a variable response to large doses of heparin occurs in some of these patients. When a therapeutic response to intravenous heparin is not achievable, additional support with an antithrombin concentrate may be necessary.
Patients who have had an episode of DVT and whose antithrombin deficiency has been recognized should receive lifelong oral anticoagulation to protect them from recurrent VTE, the development of thrombosis at other sites, or both. Patients who present with atypical site thrombosis, such as mesenteric or hepatic vein thrombosis, should be placed on lifelong anticoagulation immediately. A variety of precipitating factors, such as taking oral contraceptives or hormone replacements (which should be discontinued), may precede the development of VTE. Patients with known antithrombin deficiency need aggressive antithrombotic prophylaxis during high-risk situations such as surgery and pregnancy.
In the future, in patients with antithrombin deficiency (AT deficiency), synthetic direct thrombin inhibitors that do not require antithrombin for their anticoagulant effect (eg, argatroban) could be tried. Such inhibitors are also more desirable, because they may obviate the need for exposure to biologic products such as plasma or antithrombin concentrate, currently required for the adequate anticoagulant action of presently available agents.
Few adequate clinical trials have been carried out to answer the question of the possible utility of antithrombin concentrates in pregnancy-related disorders.
In animal models of hepatic failure as well as in human studies, replacement with antithrombin concentrates at variable time points has generated differing results, with some showing control of DIC without any obvious impact on bleeding, whereas others have not had any significant impact on outcome.
In patients undergoing orthotopic liver transplantation, it has been suggested that prophylactic administration of antithrombin concentrates may be beneficial in minimizing the DIC. Once again, large, prospective, multicenter randomized clinical trials are needed, because published clinical data have shown both efficacy and a lack of it. In this context, note that, in a very small study involving children undergoing orthotopic liver transplantation, administration of antithrombin concentrate (along with fresh frozen plasma [FFP], prostaglandin E1, and LMWH) was beneficial in reducing the frequency of hepatic arterial thrombosis.
Antithrombin supplementation has been suggested to be useful in patients with the following conditions or those undergoing the following procedures (The value of replacement in all of these procedures and conditions has not been clearly proven in unbiased trials):
A phase III, double-blind, placebo-controlled, randomized multicenter trial found that administration of high-dose antithrombin within 6 hours of the onset of sepsis and septic shock in adults had no effect on 28-day all-cause mortality rates. Increased bleeding occurred in patients who received the antithrombin concentrate and heparin (low or therapeutic doses of heparin).
Each unit of FFP obtained from a blood bank contains whatever "normal" level of antithrombin the individual donor had. If a patient requires 3000 U of antithrombin, that patient would require 3000 mL of FFP given rapidly to raise the level of antithrombin in the recipient. Clearly, volume overload becomes a problem in such patients, particularly more so in patients with an inability to tolerate large volumes. Thus, FFP replacement is not a reasonable source of repeated antithrombin replacement; rather, FFP is a choice only when no concentrate is available.
Pooled plasma treated with solvent-detergent (PLAS+SD) is available to treat any condition in which FFP is typically used and for which no factor concentrate is available. Viral inactivation using the solvent-detergent (SD) process has been used in preparation of coagulation factor concentrates in the past. In vitro treatment of donor plasma with 1% of the solvent tri(n-butyl) phosphate (TNBP) and 1% of the detergent Triton X-100 leads to significant inactivation of a broad spectrum of lipid-enveloped viruses.
Replacement with antithrombin concentrate is necessary in patients with known antithrombin deficiency (AT deficiency). In patients with acute severe trauma, some studies suggest a beneficial effect with prophylactic replacement. The frequency of antithrombin replacement depends on the half-life of the product, but in the presence of active bleeding, more frequent replacement should be based on antithrombin levels.
In acquired disorders, correction of antithrombin levels allows heparin to exert its full antithrombotic effect. Such replacement is necessary to maintain a minimum of 80% antithrombin activity until the full therapeutic effect of oral anticoagulants is obtained. Serial assessment of antithrombin levels is necessary to assure the adequacy of the dosing.
A healthy, normal diet is appropriate.
A patient's activity level depends on the clinical circumstance. Activity in patients with acute venous thromboembolic disease depends on their overall clinical status. Patients with antithrombin deficiency (AT deficiency) and thrombosis should receive in-hospital treatment for their acute illness.
A nonrandomized open-label phase III trial of recombinant human antithrombin (rhAT) in patients with hereditary antithrombin deficiency (AT deficiency) in high-risk situations for thrombosis (nonpregnant surgical patients or pregnant patients scheduled for cesarean section or delivery induction) was completed in February 2008 and approved by the US Food and Drug Administration (FDA) in early 2009 and marketed under the trade name ATryn.
Recombinant human antithrombin is also approved for use in Europe for the prophylaxis of venous thromboembolism in the surgery of patients with congenital antithrombin deficiency (AT deficiency).
Plasma-derived antithrombin has been approved by the FDA for use in patients with hereditary antithrombin III deficiency (ATIII deficiency). In patients with a congenital deficiency of antithrombin III, replacement/prophylaxis is recommended (1) before or following major surgery, (2) during bed rest for longer than 24 hours (because of the increased risk of thrombosis), (3) for thrombosis during pregnancy to allow heparin to be effective, and (4) for acute DVT/PE.
Many acquired causes have been associated with antithrombin deficiency (AT deficiency); however, none of them has garnered FDA approval based on published clinical trials. It must be stated that, in patients with shock and DIC due to trauma, sepsis, or hepatic coma, the duration of DIC symptoms was significantly shorter in patients treated with antithrombin III alone than in patients who received heparin alone. The duration of symptoms with the combined use of heparin and antithrombin III was between the other two. As expected, patients with DIC due to polytrauma who were also receiving heparin had a greater tendency to bleed.
The reader is encouraged to review the FDA package insert with each product that is used for therapy.
FFP has traditionally been the source of factors to treat coagulation factor deficiencies for which no concentrates are available. Alpha2-plasmin inhibitor falls into that category.
Careful screening of blood donors and viral testing of donated blood (HBV surface antigen [HBsAg] and antibody to HBV core antigen [HBcAg], HCV, antibody to HIV-1 and HIV-2, HIV p24 antigen, antibodies to human T-cell lymphotropic virus [HTLV]-I and HTLV-II, screening for an elevated alanine aminotransferase [ALT] level) have improved the safety of blood products, but risks remain for a variety of reasons, including failure to detect infections during the "window," or incubation period, before the results of currently available tests become positive.
Other types of infections continue to cause concerns, including those for which we currently do not screen, do not have tests, or do not know of their presence. Some of the previously mentioned emerging pathogens include HIV-2, HIV type O, hepatitis G virus (HGV), TT virus (TTV), human herpesvirus (HHV)-8, the SEN family of viruses, and prions causing Creutzfeldt-Jakob disease (CJD) and new variant CJD (nvCJD).[42, 43, 44]
Higher risks of virally transmitted illnesses remain among patients who are recipients of multiple units of FFP. The use of solvent (TNBP) and detergent (Triton X-100) to treat pooled human plasmas results in significant inactivation of lipid-enveloped viruses (eg, HIV, HCV, HBV). The greater degree of viral safety assured by this treatment has led to the exclusive use of PLAS+SD instead of FFP in some countries (eg, Norway, Belgium).
SD-treated plasma delivers consistent and reproducible levels of coagulation factors. In contrast to the extreme variability in FFP, no leukocytes are present, and physiologic inhibitor levels are mostly in the normal range, with the exception of a moderate reduction in the levels of alpha2-plasmin inhibitor (~0.48 IU/mL) and protein S (~0.52 IU/mL). In addition, coagulation zymogens are not activated, levels of other plasma proteins and immunoglobulins are normal, and all lots have anti–hepatitis A virus (HAV) antibody levels of more than 0.8 IU/mL, providing passive administration of antibody, which may neutralize HAV. SD-treated plasma also lacks the largest von Willebrand multimers and has a proven efficacy in the treatment of a variety of bleeding disorders.
PLAS+SD's disadvantages include minor allergic reactions as observed with other blood products but which respond to antihistamines. This product should not be given to patients with known immunoglobulin A (IgA) deficiency.
Alpha2-plasmin inhibitor recovery after use of PLAS+SD: Mean recovery of alpha2-plasmin inhibitor was 237% ± 146% in 7 patients who had received SD plasma and albumin during plasma exchange after they had undergone plasmapheresis to hypofibrinogenemic levels (< 125%). All coagulation factor levels are stable for approximately 12 months when stored at -18°C, but PLAS+SD should be used within 24 hours of being thawed.
All PLAS+SD units should be ABO compatible with each patient's red blood cells. Adverse effects include minor allergic reactions and volume overload. Rarely, citrate toxicity, hypothermia, other metabolic problems (if large volumes are used rapidly), and noncardiogenic pulmonary edema arise. Antibody-induced positive direct antiglobulin test results and hemolysis may also occur rarely.
See below for further details of the use of PLAS+SD instead of FFP.
Newer emerging technologies, such as those using nucleic acid chemistry, are being used to inactivate viruses, bacteria, and parasites with an attempt to also remove prions, thus making blood and blood components safer than they are today. These newer technologies attempt to preserve the clinically useful components of blood while improving its safety. These methodologies could be used to improve the safety of a wide variety of products.
Recognition of the importance of the lysine-binding sites in various interactions in the fibrinolytic pathway led to the synthesis of lysine analogues such as EACA (epsilon amino-caproic acid) (6-aminohexanoic acid [Amicar]) and trans- p-aminomethyl-cyclohexane carboxylic acid (AMCA, tranexamic acid [Cyklokapron]). These synthetic lysine analogues induce a conformational change in plasminogen when they bind to its lysine-binding site. The plasminogen has the shape of a prolate ellipsoid after EACA binds to it. It elongates into a long structure in which the interaction between the parts of plasminogen as they existed is lost. In vivo, they probably prevent plasminogen activation and, in large doses, also bind plasmin, thereby preventing it from binding to its substrate, fibrin.
When looking at binding sites on plasminogen for EACA, the tightest binding is to kringle 1 followed by kringles 4 and 5. The interaction with kringle 2 is weak, and kringle 3 does not interact at all. A model of the structure of kringle 4 shows that the shallow trough formed by the hydrophobic amino acids is surrounded by positively and negatively charged amino acids at an ideal distance to interact with EACA.
EACA is the most widely used antifibrinolytic drug in the United States. The minimal dose needed to inhibit either normal or excessive fibrinolysis is unknown. EACA is absorbed well orally, and 50% is excreted in the urine in 24 hours. Generally, an initial loading dose is followed by a maintenance dose to adequately inhibit fibrinolysis until excess bleeding is controlled. The maintenance dose is then gradually tapered until it can be stopped. Rarely, myopathy and muscle necrosis develop. Lower doses are adequate when bleeding involves the urinary tract, since as concentrations are 75-100 times higher in urine than in plasma.
AMCA is also rapidly excreted in the urine, with more than 90% excreted in 24 hours. However, its antifibrinolytic effect lasts longer than EACA. AMCA inhibits fibrinolysis at lower plasma concentrations, although its serum half-life is similar to that of EACA. Therefore, AMCA can be given less frequently and at lower doses.
The dose of EACA and AMCA must be reduced when renal failure is present.
Aprotinin (Trasylol), a third antifibrinolytic drug obtained from bovine lung, is a nonhuman protein inhibitor of several serine proteases, including plasmin. It is approved by the FDA for use in patients undergoing open heart surgery to reduce operative blood loss. Aprotinin administration has also reduced blood loss and transfusion requirements in patients undergoing orthotopic liver transplantation and in patients undergoing elective resection of a solitary liver metastasis originating from colon cancer. Aprotinin is the most expensive of the 3 drugs discussed here, and it is now only available via a limited-access protocol. Fergusson et al reported an increased risk for death compared with tranexamic acid or aminocaproic acid in high-risk cardiac surgery.
Clinical Context: A serine protease inhibitor (an alpha2-globulin) that inactivates thrombin, plasmin, and other serine proteases of coagulation, including factors IXa, Xa, XIa, XIIa, and VIIa. Made from pooled human plasma and is heat treated. Do not refrigerate after reconstitution, and administer within 3 h of reconstitution.
Clinical Context: Antithrombin (AT) regulates hemostasis by inhibiting thrombin and factor Xa, key proteases for blood coagulation. Indicated for prevention of perioperative and peripartum thromboembolic events in patients with hereditary AT deficiency. Not indicated for treatment of thromboembolic events.
Antithrombin concentrates are used to raise the plasma antithrombin level from a reduced value to approximately 120%. The goal is to maintain the level of antithrombin activity at a minimum of about 80% at all times. Serial monitoring of levels is necessary to ensure an adequate level. The anticoagulant effect of heparin is enhanced by antithrombin; thus, monitoring of the aPTT is necessary to determine the need to reduce the heparin dosage when heparin is being concomitantly administered with antithrombin. Both HBV and HIV are inactivated in this product, but viral transmission has not been completely eliminated. A recombinant product would solve that problem.
The required dose = (%desired –%baseline) × body weight (kg) divided by 1.4.
This calculation is based on an expected rise of 1.4% with 1 IU/kg given intravenously. Recoveries vary from patient to patient and are also affected by the underlying disease. Therefore, baseline and 20-minute postinfusion samples should be tested for antithrombin activity to determine the initial response to a dose. Subsequently, predose trough level and immediate postdose values provide trough and peak values to help in further dosing. Minimum levels of approximately 80% are suggested. Surgery, bleeding, and active thrombosis all affect the level and half-life. The disappearance time in normal volunteers was 22 hours, but this is physiologic information. Following the initial loading dose to raise the value to 120%, approximately 60% of that dose is administered every 24 hours as a maintenance dose.
Clinical Context: See details of discussion under Medical Care. SD treatment of pooled human plasma removes lipid-enveloped viruses, making this product safer than untreated FFP. SD treatment, however, does not remove all viruses from plasma. Efficacy and safety has been proven in the treatment of several coagulopathies. Per the package insert from the American Red Cross, the half-life of the coagulation factors in recipients of this product were similar to normal values at the time they were measured.
If available, SD-treated plasma can be used in patients with alpha2-antiplasmin deficiency, because no concentrate is available to treat this coagulation factor deficiency. As with any bleeding disorder, serial measurement of the specific coagulation factor in question is essential to assure hemostatic adequacy of levels. On average, 1 U of SD plasma raises factor levels by ~2-3%, whereas 4-6 U raises factor levels by ~8-18% in a 70-kg person. These numbers do not specifically apply to alpha2-antiplasmin and are being provided only as a general guide.
Serial monitoring of required alpha2-antiplasmin levels is necessary to follow these patients. This product should be stored at -18°C or colder, and thawed at 30-37°C in a water bath with very gentle shaking; once thawed, keep at room temperature and use as soon as possible, preferably within 24 h. Do not store thawed material in the cold.
Use inhibitors of fibrinolysis together with FFP replacement for minor surgical procedures (eg, dental extractions, sinus surgery) so that they can be accomplished on an outpatient basis with the use of a single dose of product.
Concern about the possible relationship to acute thrombotic events remains, although a causal relationship is being questioned because the underlying disease state determines the site and extent of thrombosis.
Treatment of the acute thrombotic event in patients with antithrombin deficiency (AT deficiency) has traditionally been accomplished with intravenous heparin supplemented by antithrombin. With the availability of direct thrombin inhibitors, which do not require antithrombin for their action, a whole new therapeutic arena has opened for these patients.
Therapy of the acute thromboembolic event must be followed by the lifelong administration of an oral anticoagulant (vitamin K antagonists) to maintain anticoagulation in the therapeutic range at all times for patients with inherited antithrombin deficiency (AT deficiency) and thrombosis.
Discontinuation of oral anticoagulants should be undertaken with great caution and only for essential procedures because of the risk of recurrent thromboembolic events. Replacement with antithrombin concentrate may be needed during such times.
Long-term administration of therapeutic oral outpatient anticoagulants is effective in preventing recurrent thromboembolic episodes in patients with proven antithrombin deficiency (AT deficiency) and an index thromboembolic event. As long as the patient's international normalized ratio (INR) is therapeutic at all times (the authors prefer INRs in the 2.6-3.2 range, using a sensitive thromboplastin), patients do very well.
Management of bridge therapy when necessary to discontinue warfarin sodium poses a real problem at this time, as heparin and LMWHs are the only currently available choices. The possible use of subcutaneous hirudin is complicated by antibody formation to the compound. No data are available on the utility of subcutaneous argatroban.
When oral anticoagulants are temporarily discontinued for a surgical procedure, an alternative method of prophylaxis should be considered. Currently, a non – antithrombin-dependent agent is unavailable. Most physicians use heparin or LMWH despite their expected limitation.
Drug and diet interactions are a major problem with vitamin K antagonists.
Lack of availability of adequate support from a knowledgeable hematologist may require that the patient with antithrombin deficiency (AT deficiency) be transferred to an appropriate facility.
Identification of the specific mutation in a family with antithrombin deficiency (AT deficiency) may allow a mother to undergo prenatal testing if she or her spouse is affected, using techniques well established for people with hemophilia, but only after the patient fully comprehends the implications and complications of such testing.
Patients with an inherited deficiency require lifelong oral anticoagulants to prevent recurrent thrombotic complications.
Serious long-term morbidity can result from the following issues:
Hepatitis viruses, HIV, acquired immunodeficiency syndrome (AIDS), parvovirus, and prion-induced diseases that are transmitted from blood products can lead to morbidity and mortality.
A review by Senior focused attention on concerns about transmission of CJD or nvCJD from blood products. The FDA's Transmissible Spongiform Encephalopathies Advisory Committee (TSEAC) proposed limiting donors and excluding those who had resided or traveled in Europe for 5 years starting in 1980 or those who had lived in the United Kingdom for a total of more than 3 months. The availability of new tests to detect nvCJD are also anticipated.
The prognosis depends on the sites and types of complications that patients with inherited antithrombin deficiency (AT deficiency) have. Women have the added risks of pregnancy or estrogen use as an early precipitating factor for thrombotic events.
The prognosis with acquired causes of antithrombin deficiency depends on the underlying disease.
Antithrombin (AT) neutralizes the enzyme (IIa) by forming a 1:1 stoichiometric complex (AT:IIa) between the arginine-serine sites of the 2 proteins. Binding of heparin to lysyl residues on AT results in a conformational change in AT, which makes it more available to bind thrombin (IIa), IXa, and Xa, thus markedly accelerating the rate of enzyme-inhibitor complex formation. AT also neutralizes XIa and XIIa.
Antithrombin (AT) neutralizes the enzyme (IIa) by forming a 1:1 stoichiometric complex (AT:IIa) between the arginine-serine sites of the 2 proteins. Binding of heparin to lysyl residues on AT results in a conformational change in AT, which makes it more available to bind thrombin (IIa), IXa, and Xa, thus markedly accelerating the rate of enzyme-inhibitor complex formation. AT also neutralizes XIa and XIIa.
Cell surface–directed hemostasis (image adapted from Hoffman M, Monroe DM 3rd. A cell-based model of hemostasis. Thromb Haemost. 2001). Initially, a small amount of thrombin is generated on the surface of the tissue factor–bearing (TF-bearing) cell. Following amplification, the second burst generates a larger amount of thrombin, leading to fibrin (clot) formation.