Significance of quantitative measurement of heparin‐induced platelet antibodies

Background: Heparin‐induced thrombocytopenia (HIT) is a prothrombotic immune‐mediated adverse drug reaction. Antigen and platelet activation assays are used for detection of antibodies. Quantitative results from platelet factor 4 (PF4)‐dependent immunoassays may lead to inter‐laboratory standardization of measurements. Objectives: The aim was to modify a PF4‐dependent immunoassay to measure PF4/heparin antibodies quantitatively. Methods: Over five consecutive years, 1070 samples from thrombocytopenic, heparin‐treated patients were analyzed by a PF4/heparin ELISA and the heparin‐induced platelet activation assay (HIPA). Results of ELISA assay were expressed as arbitrary units per liter (AU/L). Results: Precision of ELISA at the concentration of 50 AU/L was 3.6%. Of 1070 samples, 117 were positive for antibodies by ELISA and/or HIPA assay. The higher the antibody concentration was, the higher was the proportion of HIPA positive cases (>140 AU/L, 100%, n = 26; 100–140 AU/L, 55%, n = 20; 50–99 AU/L, 38%, n = 29; 30–49 AU/L, 17%, n = 36). Conclusions: The measurement of anti‐PF4/heparin antibody concentration is a new parameter that may improve the diagnosis of HIT. All samples with extremely strong antibody concentration were positive also by HIPA. For accuracy, antibody concentrations must be in the linear range of the assay and an international standard is needed.     

Laboratory diagnosis of heparin‐induced thrombocytopenia

Heparin‐induced thrombocytopenia (HIT) is a clinical‐pathological disorder; thus, laboratory testing for the pathogenic platelet‐activating antiplatelet factor 4 (PF4)/heparin antibodies is central for diagnosis. The “iceberg” model summarizes the inter‐relationship between platelet activation assays and PF4‐dependent immunoassays, with platelet‐activating antibodies comprising a subset of anti‐PF4/heparin antibodies. The platelet serotonin‐release assay (SRA), performed by reference laboratories, has high sensitivity and specificity for HIT (~95% each), and is especially suited for detecting highly pathogenic HIT sera containing both heparin‐dependent and heparin‐independent platelet‐activating antibodies; this latter subgroup of antibodies explains “autoimmune HIT” disorders (delayed‐onset, persisting, spontaneous, heparin “flush,” fondaparinux‐associated). Recently, SRA‐negative HIT has become recognized, in which serum from some HIT patients contains subthreshold levels of platelet‐activating antibodies (by SRA) that become detectable using a PF4‐enhanced platelet activation assay. Unusual immunologic features of HIT include early antibody detectability (at onset of platelet count fall) and antibody transience (seroreversion). Widely available PF4‐dependent enzyme immunoassays (EIAs) have high sensitivity but poor specificity for HIT, although specificity is enhanced with IgG‐specific EIAs and strong positive results; unfortunately, EIA results are usually not available in real time. Automated rapid immunoassays, such as the chemiluminescence immunoassay (CLIA) and latex immunoturbidimetric assay (LIA), facilitate real‐time laboratory diagnosis. Recently available likelihood ratio (LR) data for positive (LR+) and negative (LR?) test results allow clinicians to adjust their pretest probabilities for HIT, using Bayesian analysis, into real‐time posttest probabilities that are dramatically increased (test positive) or decreased (test negative). Moreover, (semi‐)quantitative CLIA‐ and LIA‐positive results (weak, moderate, strong positive) can further refine the posttest probability of HIT.     

The unique immunological features of heparin‐induced thrombocytopenia

Heparin‐induced thrombocytopenia (HIT) is a serious drug reaction that leads to a decrease in platelet count and a high risk of thrombosis. HIT patients produce pathogenic immunoglobulin G (IgG) antibodies that bind to complexes of platelet factor‐4 (PF4) and heparin. HIT immune complexes crosslink Fc‐receptors resulting in platelet and monocyte activation. These events lead to the release of procoagulant chemokines and tissue factor, which together create an intensely prothrombotic state. HIT represents an atypical immune response because it has features of both T cell‐dependent and T cell‐independent mechanisms. The disorder is characterized by newly formed anti‐PF4/heparin IgG antibodies, which are characteristic of a T cell‐dependent mechanism; however, re‐exposure to heparin, months after HIT, does not lead to a memory response, which is consistent with a T cell‐independent mechanism. In this review, we discuss the immunobiological events that can explain these features, including the role for T cell‐dependent and T cell‐independent mechanisms in HIT antibody generation, the immunogenic characteristics of the PF4/heparin antigen, and the concept of a temporary loss in immune regulation contributing to the onset of HIT. We also present a novel immunobiological model to explain the atypical immune response that is characteristic of HIT.     

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.     

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.     

Evolving concepts of pathogenesis of heparin‐induced thrombocytopenia: Diagnostic and therapeutic implications

Heparin‐induced thrombocytopenia (HIT) is an immune reaction to heparin. It often causes severe thrombosis which may lead to limb gangrene and thrombosis‐associated death. The concept of its pathogenesis has been evolving during the past five decades. Initially, HIT was thought to be caused by disseminated intravascular coagulation. Later it became clear that HIT was mediated by an immune mechanism whereby an IgG antibody induced platelet aggregation, release of procoagulant materials and consequently thrombus formation. The antigen comprises Platelet Factor 4 (PF4) and heparin which have a tendency to form ultralarge complexes. The HIT immune response has atypical features. IgG antibody appears early without IgM precedence and lasts transiently. One explanation is that there is prior priming by bacterial infection. Another unique characteristic is that it is processed as if it is a particulate antigen involving complement activation and B cells. Antigen‐presenting cells/monocytes are also involved but the role of T cells is controversial. Recent advances have provided new insights into the underlying mechanisms of HIT‐related thrombosis. Previously, platelets were believed to play a central role; their activation and consequently the induction of blood coagulation was the basis of the hypercoagulability in HIT. More recently, several studies have provided clear evidence that neutrophil and NETosis, monocytes and endothelial cells contribute significantly to the thrombosis in HIT. These new insights may result in development of better diagnostic laboratory assays and more effective treatments for HIT.     

Age‐related plasma reference ranges for two heparin‐binding proteins – vitronectin and platelet factor 4

This study was conducted to establish age‐related reference ranges for two heparin‐binding proteins – vitronectin and platelet factor 4 (PF4) – and to determine if the quantitative values of these proteins may contribute to the reported age‐dependent effect of unfractionated heparin (UFH). Plasma samples were obtained from healthy children aged between 1 month and 16 years and from healthy adult volunteers. Two commercial kits were used to measure plasma vitronectin and PF4 levels. Results were reported as mean and boundaries including 95% of the population. Plasma vitronectin levels for children aged 1–5 years were significantly higher compared with adults. Plasma PF4 levels for infants <1 year of age were significantly lower compared with adults. The differences between reference values for both proteins in all other age‐groups were not statistically significant. This study for the first time has established age‐related reference ranges for vitronectin and PF4. In establishing these ranges, the quantitative values of these proteins do not appear to be the major contributory cause for the age‐dependent variation in UFH effect. Future studies are required to evaluate the possible impact of age‐dependent differences in binding between heparin‐binding proteins and UFH.     

Simultaneous binding of heparin and platelet factor-4 to platelets: Further insights into the mechanism of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is mediated by antibody against complexes of platelet factor-4 (PF4) and heparin. Although it has been assumed that these complexes bind to platelets and provide a target for the antibody, this has never been demonstrated. Furthermore, there is evidence suggesting that heparin-PF4 complexes do not bind to platelets. We have analyzed the effect of each ligand on the platelet binding of the other. We particularly focused on the result when heparin and PF4 are in equimolar concentration because we had previously shown that this was the condition under which HIT-IgG increased on the platelet surface. We found that when the molar concentration of PF4 approximates or exceeds that of heparin, the ligands bind simultaneously to the cells and HIT-IgG binds also. However, when heparin is in molar excess, both PF4 binding and HIT-IgG binding are diminished. Our data are consistent with the hypothesis that heparin-PF4 complexes bind via their heparin component to heparin binding sites on the platelet membrane rather than by their PF4 component to PF4 sites. The conditions promoting the binding of the complexes also lead to binding of HIT-IgG. Am. J. Hematol. 58:24–30, 1998. © 1998 Wiley-Liss, Inc.     

Biological and clinical features of low-molecular-weight heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a common adverse effect of unfractionated heparin (UFH) therapy. In contrast, only a few patients have been reported with HIT following low-molecular-weight heparin (LMWH) therapy (LMW-HIT). To define the clinical and biological characteristics of LMW-HIT, 180 patients treated for suspected HIT at 15 French centres were investigated. Clinical history was recorded and HIT was confirmed in 59 patients with positive serotonin release assay results: 57 of them had high levels of antibodies (Abs) to heparin-platelet factor 4 complexes (H/PF4) and two had Abs to interleukin 8. Eleven patients were treated exclusively with LMWH (LMW-HIT) and 48 with UFH either alone (UF-HIT, n = 34) or combined with LMWH (UF/LMW-HIT, n = 14). The LMW-HIT and UF-HIT groups were similar with respect to sex, age, platelet count before heparin therapy, frequency of bleeding and occurrence of disseminated intravascular coagulation. The interval to onset of HIT was longer in LMW-HIT patients compared with UF-HIT patients (P = 0.03). Severe thrombocytopenia (platelets < 15 x 10(9)/l) was more frequent in the LMW-HIT group (P = 0.04). Thrombosis occurred in three of 11 LMW-HIT patients, i.e. as frequently as in UF-HIT patients. LMW-HIT is potentially severe and may be observed after longer heparin treatment compared with UF-HIT. It is highly recommended, therefore, that platelet counts be monitored carefully whenever LMWH is administered.     

Determination of heparin–platelet factor 4–IgG antibodies improves diagnosis of heparin-induced thrombocytopenia

Only a few patients with heparin-induced antibodies develop heparin-induced thrombocytopenia (HIT). In this study, we investigated whether different immunglobulin classes can be used to differentiate between antibody-positive patients with and without HIT. Four different patient populations were investigated: 32 patients with the immune type of HIT with thromboembolic complications, 13 patients with HIT without thromboembolism, 24 patients with heparin-platelet factor 4 (PF4) antibodies without clinical symptoms of HIT, and 20 heparin-treated patients with thrombocytopenia caused by other reasons. In all patients the immunglobulin mixture of IgG, IgM and IgA, and the single immunglobulin classes of heparin-PF4 antibodies, were investigated. No significant differences between HIT patients with thromboembolic complications and patients with isolated HIT were found concerning the different immunglobulin classes. Antibody-positive patients with HIT had significantly higher levels of IgG antibodies than those without HIT (P < 0.05), while they did not differ concerning IgM and IgA antibodies. By determining IgG antibodies, the specificity of the enzyme-linked immunosorbent assay (ELISA) system was increased without loss of sensitivity. Heparin-PF4-IgG antibodies can identify patients at risk of developing life-threatening HIT. Future ELISAs should only include this immunglobulin class, as the determination of the antibody mixture may lead to overestimation of HIT.