Immune heparin-induced thrombocytopenia can occur in patients receiving clopidogrel and aspirin

Platelet adenosine diphosphate (ADP) receptors may play a role in potentiating platelet activation induced by IgG from patients with immune heparin-induced thrombocytopenia (HIT), as shown by previous studies using the ADP receptor antagonists AR-C66096 and ticlopidine. Consistent with these observations, we found that platelet activation by HIT sera is also significantly reduced in patients receiving clopidogrel, an ADP receptor antagonist prodrug now in wide clinical use. Despite these in vitro and ex vivo findings, we observed two patients develop acute HIT while receiving both clopidogrel and aspirin: both patients' sera tested strongly positive in a heparin-dependent washed platelet activation assay (100% serotonin release) and PF4/heparin-enzyme-immunoassay (2.594 and 2.190 absorbance units). Both patients also developed HIT-associated clinical sequelae (acute systemic reaction postintravenous heparin bolus; thrombotic stroke) in association with their episode of HIT. We conclude that combined therapy with aspirin and clopidogrel does not necessarily protect against clinical HIT, at least in patients with HIT antibodies that have strong platelet-activating characteristics.     

Heparin-induced thrombocytopenia: a frequent complication after cardiac surgery

Thrombocytopenia is a common problem in cardiovascular patients, and heparin-induced thrombocytopenia (HIT) is therefore frequently suspected. Unfractionated heparin during cardiopulmonary bypass is particularly immunogenic as 25% to 50% post-cardiac surgery patients develop heparin-dependent antibodies but only 1 to 3% will develop HIT. These antibodies recognize a 'self protein', platelet factor 4 (PF4), bound to heparin. Antibodies associated with a high risk of HIT are mainly IgG1 which strongly activate platelets and coagulation, thereby causing thrombocytopenia and thrombosis. A biphasic evolution of platelet count with a secondary decrease after a previous increase following CPB or non-recovery of thrombocytopenia within 6 days post-operatively always requires screening for HIT antibodies. Both functional (platelet activation tests) and immunologic assays (antigen assays) are necessary in every patient to establish the diagnosis of HIT. When the clinical probability of HIT is high, the first requirement is to discontinue heparin, without waiting for results of laboratory investigations. An alternative anticoagulant such as danaparoid sodium (Orgaran) or lepirudin (Refludan) must then be administered since heparin withdrawal alone is insufficient to control the prothrombotic state associated with HIT. The risk of HIT will probably soon decrease due to the wider use of fondaparinux, which does not interact in vitro with PF4, but it could remain significant in patients undergoing cardiac surgery with CPB.     

Spontaneous heparin-induced thrombocytopenia syndrome without any proximate heparin exposure,infection, or inflammatory condition: Atypical clinical features with heparin-dependent platelet activating antibodies

Recent studies suggest that a thromboembolic disorder resembling heparin-induced thrombocytopenia (HIT), so-called spontaneous HIT syndrome, can occur in patients without any history of heparin exposure. It is likely due to anti-platelet factor 4 (PF4)/polyanion antibodies induced by other polyanions, such as bacterial surfaces and nucleic acids. We describe an atypical case of spontaneous HIT syndrome. A 70-year-old man suddenly presented with acute cerebral sinus thrombosis (CST). Soon after the initiation of unfractionated heparin (UFH) for the treatment of CST, his platelet count fell precipitously and he developed deep vein thrombosis, a clinical picture consistent with rapid-onset HIT but without any proximate episodes of heparin exposure, infection, trauma, surgery, or other acute illness. Antigen assays and a washed platelet activation assay indicated that the patient already possessed anti-PF4/heparin IgG antibodies with heparin-dependent platelet activation properties on admission. Cessation of UFH and initiation of argatroban resulted in prompt recovery of his platelet count without further thromboembolic events. We identified two similar cases in the literature. However, these patients do not meet the recently proposed criteria for spontaneous HIT syndrome. Even in atypical cases, however, inappropriate or delayed diagnosis of HIT appears to be associated with worse outcomes. We propose that these atypical cases should be included in the category of spontaneous HIT syndrome.     

Platelet-activating anti-PF4 disorders: An overview

Platelet factor 4 (PF4) is a highly cationic tetrameric protein that can be targeted by platelet-activating anti-PF4 antibodies of immunoglobulin G (IgG) class. Certain features of PF4, including its multivalent nature (duplicate antigen sites per tetramer), the ability of many PF4 tetramers to undergo close approximation through charge neutralization, and the dimeric binding of IgG molecules, results in formation of IgG-containing immune complexes in situ on platelets, neutrophils, and monocytes, resulting in Fcγ receptor-mediated pancellular activation that also activates hemostasis (potential for disseminated intravascular coagulation). This review discusses 4 anti-PF4 disorders: classic heparin-induced thrombocytopenia ([HIT]; triggered by heparin and certain other polyanionic pharmaceuticals, featuring predominantly heparin-dependent antibodies), autoimmune HIT (aHIT; severe subtype of HIT that features both heparin-dependent and heparin-independent platelet-activating antibodies), and spontaneous HIT (non-heparin triggers such as knee replacement surgery and infection; predominantly heparin-independent platelet-activating antibodies). Most recently, a novel fourth anti-PF4 disorder, vaccine-induced immune thrombotic thrombocytopenia (VITT), was identified as an ultrarare complication of adenovirus vector vaccines. VITT is characterized by thrombocytopenia, disseminated intravascular coagulation, a high frequency of thrombosis—including in unusual sites (cerebral veins, splanchnic veins)—and highly pathogenic anti-PF4 antibodies with heparin-independent platelet-activating properties.     

Heparin-induced thrombocytopenia: new evidence for the dynamic binding of purified anti-PF4-heparin antibodies to platelets and the resultant platelet activation

Immune heparin-induced thrombocytopenia (HIT) is associated with antibodies directed against a complex of platelet factor 4 (PF4) and heparin. We were able to affinity purify anti-PF4-heparin IgG (HIT IgG) from the plasma of 2 patients with HIT. Under conditions that were more physiological and sensitive than those in previous studies, we observed that this HIT IgG caused platelet aggregation on the addition of heparin. Platelets activated with HIT IgG increased their release and surface expression of PF4. We quantitated, for the first time, the binding of affinity-purified HIT iodine 125-IgG to platelets as they activated in a plasma milieu. Binding of the HIT IgG was dependent on heparin and required some degree of platelet activation. Blocking the platelet FcgammaRII with the monoclonal antibody IV.3 did not prevent HIT IgG binding to activated platelets. We concluded that anti-PF4-heparin IgG is the component in these HIT plasmas that induces platelet aggregation. The Fab region of HIT IgG binds to PF4-heparin on the surface of activated platelets. We propose that only then does the Fc portion of the bound IgG further activate the same or adjacent platelets through the Fc receptor. Our data support a dynamic model of platelet activation in which released PF4 enhances further antibody binding and more release.     

Heparin-induced thrombocytopenia

BACKGROUND AND OBJECTIVE: There are two types of heparin-induced thrombocytopenia (HIT). HIT I is characterized by a transitory, slight and asymptomatic reduction in platelet count, occurring in the first 1-2 days of therapy, that resolves spontaneously; in contrast, HIT II, which has an immunologic origin, is characterized by a significant thrombocytopenia generally after the fifth day of therapy that usually resolves in 5-15 days only after therapy withdrawal. HIT II is the most frequent and dangerous side-effect of heparin therapy; in fact, in spite of thrombocytopenia, it can be complicated by venous and arterial thrombosis. Therefore, the recognition of HIT II may be difficult due to the underlying thrombotic symptoms for which heparin is administered. The aim of this article is to review the most recent advances in the field and to give critical guidelines for the clinical diagnosis and treatment of HIT II. STATE OF THE ART: The prevalence of HIT II, as confirmed by laboratory tests, seems to be about 2% in patients receiving unfractionated heparin (UH), while it is much lower in those receiving low molecular weight heparin (LMWH). The immunologic etiology of HIT II is largely accepted. Platelet factor 4 (PF4) displaced from endothelial heparan sulphate or directly from the platelets, binds to the heparin molecule to form an immunogenic complex. The anti-heparin/ PF4 IgG immunocomplexes activate platelets and provoke an immunologic endothelial lesion with thrombocytopenia and/or thrombosis. The IgG anti-heparin/PF4 immunocomplex activates platelets mainly through binding with the FcgRIIa (CD32) receptor. The onset of thrombocytopenia is independent of the dosage, schedule and route of administration of heparin. Orthopedic and cardiovascular surgery patients receiving post-surgical prophylaxis or treatment for deep venous thrombosis are at higher risk of HIT II. Besides thrombocytopenia, cutaneous allergic manifestations and skin necrosis may be present. Hemorrhagic events are not frequent, while the major clinical complications in 30% of patients are both arterial and venous thromboses which carry a 20% mortality. The diagnosis of HIT II should be formulated on the basis of clinical criteria and in vitro demonstration of heparin-dependent antibodies. Functional tests, such as platelet aggregation and (14)C-serotonin release assay and immunologic tests, such as the search for anti-PF4/heparin complex antibodies by an ELISA method are available. If HITT II is probable, heparin must be immediately suspended and an alternative anticoagulant therapy should be initiated before resolution of thrombocytopenia and the following treatment with a vitamin K antagonist. The general opinion is to administer low molecular weight heparin (in the absence of in vitro cross-reactivity with the antibodies), heparinoids such as Orgaran or direct thrombin inhibitors such as hirudin. PERSPECTIVES: Further studies are required to elucidate the pathogenesis of HIT II and especially to discover the clinical and immunologic factors that induce the occurrence of thrombotic complications. The best therapeutic strategy remains to be confirmed in larger clinical trials.     

Pathogenicity of IgA and/or IgM antibodies to heparin–PF4 complexes in patients with heparin-induced thrombocytopenia

Antibodies to heparin–PF4 (H-PF4) complexes have been tested and isotyped in 38 patients who developed severe heparin-induced thrombocytopenia (type II HIT). All the patients had a platelet count < 120 × 10⁹/l or a reduction of >30% of the initial value, occurring at least 5 d after the onset of heparin. Thrombocytopenia, which rapidly reversed following the withdrawal of heparin, was associated with thrombosis in nine patients. Although IgG isotypes were found in most cases ( n  = 26), the presence of only IgM and/or IgA was observed in 12 patients, including three cases showing a thrombotic complication. Our results indicate that type II HIT may be induced by IgA and/or IgM anti-H-PF4 antibodies even in the absence of IgG isotypes. This finding demonstrates that platelet Fc receptors (FcγRII) are not necessarily involved in the pathogenicity of heparin-dependent antibodies and emphasizes the major role of platelet PF4 receptors. The increased expression of the latter following a slight activation by thrombin, and the subsequent binding of IgM and IgA antibodies to H-PF4 on the platelet surface, may directly trigger platelet activation, aggregation and thrombosis. Alternatively, thrombocytopenia could be indirectly induced through the mediation of neutrophils, monocytes and lymphocytes which expose receptors for IgA (FcαR) or IgM (FcμR). IgM–platelet complexes may also bind and activate complement, leading to platelet activation or destruction. Moreover, the reactivity of the antibodies with glycosaminoglycans–PF4 complexes present on the endothelial surface could also induce endothelial lesions and promote procoagulant activity and predisposition to thrombosis.     

Heparin-induced thrombocytopenia

Hemorrhage is the most common and best-recognized complication of heparin treatment. However, a potentially more dangerous complication is the development of heparin-induced thrombocytopenia (HIT). All patients exposed to heparin, irrespective of the dose and route of administration, are at risk of developing HIT. It is due to the formation of antibodies against the heparin-platelet factor 4 complex, which cause secondary activation of platelets, coagulation and, finally, increased thrombin production. The main symptom is the sudden onset of thrombocytopenia involving a drop in the platelet count to less than 50% of the basal level, with or without the appearance of thrombotic complications some 5 to 14 days after the start of heparin therapy. Heparin-induced thrombocytopenia can be detected early in patients receiving heparin by monitoring the platelet count. Demonstration of heparin-dependent platelet activation using an antigen or functional assay confirms the clinical diagnosis. Once the diagnosis of HIT has been confirmed serologically or there is a high level of suspicion of HIT, heparin must be suspended and treatment with an alternative anticoagulant should be considered. This review contains a discussion of the diagnosis and treatment of this syndrome.     

Pathogenicity of IgA and/or IgM antibodies to heparin–PF4 complexes in patients with heparin-induced thrombocytopenia

Antibodies to heparin–PF4 (H-PF4) complexes have been tested and isotyped in 38 patients who developed severe heparin-induced thrombocytopenia (type II HIT). All the patients had a platelet count < 120 × 10⁹/l or a reduction of >30% of the initial value, occurring at least 5 d after the onset of heparin. Thrombocytopenia, which rapidly reversed following the withdrawal of heparin, was associated with thrombosis in nine patients. Although IgG isotypes were found in most cases (n = 26), the presence of only IgM and/or IgA was observed in 12 patients, including three cases showing a thrombotic complication. Our results indicate that type II HIT may be induced by IgA and/or IgM anti-H-PF4 antibodies even in the absence of IgG isotypes. This finding demonstrates that platelet Fc receptors (FcγRII) are not necessarily involved in the pathogenicity of heparin-dependent antibodies and emphasizes the major role of platelet PF4 receptors. The increased expression of the latter following a slight activation by thrombin, and the subsequent binding of IgM and IgA antibodies to H-PF4 on the platelet surface, may directly trigger platelet activation, aggregation and thrombosis. Alternatively, thrombocytopenia could be indirectly induced through the mediation of neutrophils, monocytes and lymphocytes which expose receptors for IgA (FcαR) or IgM (FcμR). IgM–platelet complexes may also bind and activate complement, leading to platelet activation or destruction. Moreover, the reactivity of the antibodies with glycosaminoglycans–PF4 complexes present on the endothelial surface could also induce endothelial lesions and promote procoagulant activity and predisposition to thrombosis.     

Detection of Platelet-Bound and Serum Antibodies in Autoimmune Thrombocytopenia by Enzyme-Linked Assay

Platelet antibodies were looked for in 47 patients with autoimmune thrombocytopenia using a modification of the enzyme-linked assay previously described. Surface-bound antibodies measured as increased platelet-associated IgG were found in 32 (68%) of the patients. After incubation in test sera, the platelet-associated IgG of normal donor platelets was significantly increased in 27 of the 47 patients (57%), thus demonstrating the presence of platelet antibodies free in their sera. 6 patients had antibodies only in the serum without any elevation of their platelet-associated IgG. When both tests are evaluated together no antibody was detected by either the direct or the indirect test in 9 of the 47 patients (19%) studied. The technique used is described and the interpretation of our results discussed.     

Generation of platelet-derived microparticles and procoagulant activity by heparin-induced thrombocytopenia IgG/serum and other IgG platelet agonists: a comparison with standard platelet agonists

Heparin-induced thrombocytopenia (HIT) is a relatively common, immunoglobulin-mediated adverse drug reaction associated with in vivo thrombin generation and both venous and arterial thrombosis. Serum and purified IgG from patients with HIT induce normal platelets to generate procoagulant platelet-derived microparticles, but the magnitude of this response in comparison with other IgG and standard platelet agonists is unknown. We describe a comparison of IgG platelet agonists, including HIT-IgG/serum, heat-aggregated IgG, and platelet-activating murine monoclonal antibodies, with standard 'strong' and 'weak' platelet agonists, and have determined their relative ability to generate platelet procoagulant activity. Using washed normal platelets as targets, we observed that HIT sera as well as other IgG agonists produced similar or even greater numbers of microparticles and procoagulant activity than the standard strong platelet agonists (thrombin, collagen, and thrombin receptor agonist peptide). The only exception was the non-physiological platelet agonist, calcium ionophore, which consistently produced a platelet procoagulant response even greater than the IgG agonists. We conclude that the IgG class of platelet agonists (including pathogenic HIT antibodies) is an effective trigger of the platelet procoagulant response comparable at least to strong physiological platelet agonists. These results help to explain the association between HIT, in vivo thrombin generation, and thrombosis.     

Affinity purification of heparin-dependent antibodies to platelet factor 4 developed in heparin-induced thrombocytopenia: biological characteristics and effects on platelet activation

Antibodies to heparin platelet factor 4 (H-PF4) complexes were purified from the plasma of three patients with heparin-induced thrombocytopenia (HIT) using affinity chromatography. From each plasma, the largest amount of antibodies was eluted with 2 M NaCl at pH 7.5 (peak 1) and the remainder was obtained using 0.1 M glycine/0. 5 M NaCl at pH 2.5 (peak 2). In an enzyme-linked immunosorbent assay (ELISA), we then showed that each patient had developed antibodies to PF4 displaying different characteristics. In patient 1, peak 1 IgG reacted almost exclusively with H-PF4 complexes, whereas peak 2 IgG had similar reactivity with PF4 whether or not heparin was present. Patient 2 expressed a mixture of IgA, IgM and IgG and both fractions bound to PF4 alone or to H-PF4 complexes. Finally, IgG in patient 3 only bound to H-PF4 and was unreactive with PF4 alone. Using [14C]-serotonin release assays, the antibodies developed in the three patients and exhibiting the strongest ability to activate platelets with heparin were those having the highest affinity to H-PF4. These results strongly support the hypothesis that HIT antibodies to PF4 are heterogeneous regarding their affinity and specificity for target antigens and this may greatly influence their ability to activate platelets and their pathogenicity.     

Delayed-onset heparin-induced thrombocytopenia and thrombosis

BACKGROUND: Heparin-induced thrombocytopenia is a prothrombotic drug reaction caused by platelet-activating antibodies that recognize complexes of platelet factor 4 and heparin. OBJECTIVE: To describe a syndrome termed delayed-onset heparin-induced thrombocytopenia, in which thrombocytopenia and thrombotic events begin 5 or more days after withdrawal of heparin. DESIGN: Case series. SETTING: Secondary and tertiary care hospitals. PATIENTS: 12 patients who presented with serologically confirmed, delayed-onset heparin-induced thrombocytopenia, including 6 outpatients presenting after hospital discharge. MEASUREMENTS: The platelet serotonin-release assay was used to measure IgG-induced heparin-dependent and heparin-independent platelet activation; an enzyme immunoassay that detects IgG against platelet factor 4-heparin complexes was also used. RESULTS: Patients with delayed-onset heparin-induced thrombocytopenia presented with thrombocytopenia and associated thrombosis a mean of 9.2 days (range, 5 to 19 days) after stopping heparin therapy. Nine patients received additional heparin, with further decrease in platelet counts. Compared with controls, patients with delayed-onset heparin-induced thrombocytopenia had higher titers of IgG antibodies to platelet factor 4-heparin and greater IgG-induced heparin-dependent and heparin-independent platelet activation. CONCLUSIONS: Delayed-onset heparin-induced thrombocytopenia should be suspected when patients present with thrombocytopenia and thrombosis up to 3 weeks after exposure to heparin. This syndrome could be caused by high titers of platelet-activating IgG induced by heparin.     

Generation of platelet-derived microparticles and procoagulant activity by heparin-induced thrombocytopenia IgG/serum and other IgG platelet agonists: a comparison with standard platelet agonists

Heparin-induced thrombocytopenia (HIT) is a relatively common, immunoglobulin-mediated adverse drug reaction associated with in vivo thrombin generation and both venous and arterial thrombosis. Serum and purified IgG from patients with HIT induce normal platelets to generate procoagulant platelet-derived microparticles, but the magnitude of this response in comparison with other IgG and standard platelet agonists is unknown. We describe a comparison of IgG platelet agonists, including HIT-IgG/serum, heat-aggregated IgG, and platelet-activating murine monoclonal antibodies, with standard 'strong' and 'weak' platelet agonists, and have determined their relative ability to generate platelet procoagulant activity. Using washed normal platelets as targets, we observed that HIT sera as well as other IgG agonists produced similar or even greater numbers of microparticles and procoagulant activity than the standard strong platelet agonists (thrombin, collagen, and thrombin receptor agonist peptide). The only exception was the non-physiological platelet agonist, calcium ionophore, which consistently produced a platelet procoagulant response even greater than the IgG agonists. We conclude that the IgG class of platelet agonists (including pathogenic HIT antibodies) is an effective trigger of the platelet procoagulant response comparable at least to strong physiological platelet agonists. These results help to explain the association between HIT, in vivo thrombin generation, and thrombosis.     

Immunoglobulin G from patients with heparin-induced thrombocytopenia binds to a complex of heparin and platelet factor 4

Heparin-induced thrombocytopenia (HIT) is an important complication of heparin therapy. Although there is general agreement that platelet activation in vitro by the HIT IgG is mediated by the platelet Fc receptor, the interaction among the antibody, heparin, and platelet membrane components is uncertain and debated. In this report, we describe studies designed to address these interactions. We found, as others have noted, that a variety of other sulfated polysaccharides could substitute for heparin in the reaction. Using polysaccharides selected for both size and charge, we found that reactivity depended on two independent factors: a certain minimum degree of sulfation per saccharide unit and a certain minimum size. Hence, highly sulfated but small (< 1,000 daltons) polysaccharides were not reactive nor were large but poorly sulfated polysaccharides. The ability of HIT IgG to recognize heparin by itself was tested by Ouchterlony gel diffusion, ammonium sulfate and polyethylene glycol precipitation, and equilibrium dialysis. No technique demonstrated reactivity. However, when platelet releasate was added to heparin and HIT IgG, a 50-fold increase in binding of radio-labeled heparin to HIT IgG was observed. The releasate was then depleted of proteins capable of binding to heparin by immunoaffinity chromatography. Only platelet factor 4-immunodepleted releasate lost its reactivity with HIT IgG and heparin. Finally, to determine whether the reaction occurred on the surface of platelets or in the fluid phase, washed platelets were incubated with HIT IgG or heparin and after a wash step, heparin or HIT IgG was added, respectively. Reactivity was only noted when platelets were preincubated with heparin. Consistent with these observations was the demonstration of the presence of PF4 on platelets using flow cytometry. These studies indicate that heparin and other large, highly sulfated polysaccharides bind to PF4 to form a reactive antigen on the platelet surface. HIT IgG then binds to this complex with activation of platelets through the platelet Fc receptors.     

Heparin-induced thrombocytopenia: a review

Immune heparin-induced thrombocytopenia (HIT) is a relevant adverse drug reaction consisting in a hypercoagulable state caused by an anticoagulant agent. The incidence is approximately 6.5% in patients receiving unfractionated heparin after orthopedic surgery, and is equal to or lower than 1% in other settings. HIT occurrence is a function of heparin type, duration of heparin administration, patient population, and gender. The pathogenesis is due to an antibody response to the complex heparin/platelet factor 4 in most cases, with secondary activation of platelets and coagulation, and finally increased thrombin generation. Thrombocytopenia, venous or arterial thrombosis, heparin-induced skin necrosis, adrenal hemorrhage, and transient amnesia can characterize the clinical course of HIT. Platelet monitoring in patients receiving heparin is indicated to early detect HIT. A thrombotic event can be the first manifestation of HIT. Laboratory demonstration of heparin-dependent platelet activation confirms the clinical diagnosis; antigenic or functional assays are available. Once HIT is highly likely or confirmed serologically, immediate heparin cessation is mandatory and an alternative therapeutic anticoagulant is needed due to the high risk (or the presence) of thrombotic events. The available nonheparin anticoagulants aim to reduce thrombin generation. Lepirudin, argatroban, and bivalirudin (direct thrombin inhibitors) and danaparoid and fondaparinux (factor Xa inhibitors) represent the current treatment options for HIT. Vitamin K antagonists can be used safely only after a stable platelet count has been obtained.     

Antibodies to platelet factor 4-heparin after cardiopulmonary bypass in patients anticoagulated with unfractionated heparin or a low-molecular-weight heparin : clinical implications for heparin-induced thrombocytopenia.

BACKGROUND: Cardiopulmonary bypass (CPB) induces platelet activation with release of platelet factor 4 (PF4), and patients are exposed to high doses of heparin (H). We investigated whether this contributes to the development of antibodies to H-PF4 and heparin-induced thrombocytopenia (HIT). METHODS AND RESULTS: CPB was performed with unfractionated heparin (UFH) in 328 patients. After surgery, patients received UFH (calcium heparin, 200 IU. kg-1. d-1) (group 1, n=157) or low-molecular-weight heparin (LMWH, Dalteparin, 5000 IU once daily) (group 2, n=171). Eight days after surgery, antibodies to H-PF4 were present in 83 patients (25.3%), 46 in group 1 and 37 in group 2 (P=0.12). Most patients (61%) had IgG1 to H-PF4, but only 8 samples with antibodies induced platelet activation with positive results on serotonin release assay. HIT occurred in 6 patients in group 1, but no thrombocytopenia was observed in subjects receiving LMWH, although 2 had high levels of antibodies with positive serotonin release assay results. When antibodies to H-PF4 were present, mean platelet counts were lower only in patients with FcgammaRIIA R/R131 platelets. CONCLUSIONS: These results provide evidence that the development of antibodies to H-PF4 after CPB performed with UFH is not influenced by the postoperative heparin treatment. The antibodies associated with high risk of HIT are mainly IgG1, which is present at high titers in the plasma of patients continuously treated with UFH.     

Heparininduzierte Thrombozytopenie

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction that carries an increased risk of thromboembolic complications. HIT is caused by platelet-activating antibodies directed against a complex of platelet factor 4 (PF4) and heparin. HIT typically manifests in the second week after initiation of heparin therapy with a platelet count reduction of more than 50% of the highest level after the start of heparin administration as well as thromboembolic events. The clinical probability can be calculated by the 4 T’s score. The laboratory diagnosis of HIT is based on confirmation of PF4/heparin antibodies or on functional tests that provide evidence of heparin-dependent platelet-activating antibodies. A low 4 T’s score and negative HIT test virtually rule out the presence of HIT. Patients with acute HIT require anticoagulation with a compatible anticoagulant in a therapeutic dose. The drugs currently available for this include the direct thrombin inhibitors argatroban, lepirudin, bivalirudin, and desirudin and the indirect factor Xa inhibitors danaparoid and fondaparinux.     

The complicated relationships of heparin-induced thrombocytopenia and platelet factor 4 antibodies with COVID-19

COVID-19 (coronavirus disease 2019) represents a prothrombotic disorder, and there have been several reports of platelet factor 4/heparin antibodies being present in COVID-19-infected patients. This has thus been identified in some publications as representing a high incidence of heparin-induced thrombocytopenia (HIT), whereas in others, findings have been tempered by general lack of functional reactivity using confirmation assays of serotonin release assay (SRA) or heparin-induced platelet aggregation (HIPA). Moreover, in at least two publications, data are provided suggesting that antibodies can arise in heparin naïve patients or that platelet activation may not be heparin-dependent. From this literature, we would conclude that platelet factor 4/heparin antibodies can be observed in COVID-19-infected patients, and they may occur at higher incidence than in historical non-COVID-19-infected cohorts. However, the situation is complex, since not all platelet factor 4/heparin antibodies may lead to platelet activation, and not all identified antibodies are heparin-dependent, such that they do not necessarily reflect “true” HIT. Most recently, a “HIT-like” syndrome has reported in patients who have been vaccinated against COVID-19. Accordingly, much more is yet to be learnt about the insidious disease that COVID-19 represents, including autoimmune outcomes in affected patients.     

The incidence of heparin-induced thrombocytopenia in medical patients treated with low-molecular-weight heparin: a prospective cohort study

In contrast with extensive documentation in patients treated with unfractionated heparin (UFH), the incidence of heparin-induced thrombocytopenia (HIT) in medical patients receiving low-molecular-weight heparin (LMWH) is less well defined. In a prospective cohort study, the platelet count was monitored in 1754 consecutive patients referred to 17 medical centers and treated with LMWH for prophylaxis or treatment of thromboembolic disorders. The diagnosis of HIT was accepted in case of a platelet drop of at least 50%, the absence of obvious explanations for thrombocytopenia, and the demonstration of heparin-dependent IgG antibodies. HIT developed in 14 patients (0.80%; 95% CI, 0.43%-1.34%), in all of them within the first 2 weeks, and was more frequent in patients who had (1.7%) than in those who had not (0.3%) been exposed to UFH or LMWH (OR = 4.9; 95% CI, 1.5-15.7). The prevalence of thromboembolic complications in HIT patients (4 of 14; 28.6%) was remarkably higher than that (41 of 1740; 2.4%) observed in the remaining patients (OR = 16.6; 95% CI, 5.0-55.0). Immune thrombocytopenia and related thromboembolism may complicate the clinical course of medical patients treated with LMWH with a frequency that is not different from that observed with the use of UFH. The previous administration of heparin increases the rate of HIT.