The presence of heparin-platelet factor 4 antibodies as a marker of hypercoagulability during hemodialysis

Danaparoid use for haemodialysis in a morbidly obese patient with heparin-induced thrombocytopenia - Need for a higher than recommended weight-based dosing

WHAT IS KNOWN AND OBJECTIVE

Heparin is widely used to prevent clotting of the extracorporeal circuit during haemodialysis (HD). Heparin-induced thrombocytopenia (HIT) is a potentially devastating immune mediated adverse drug reaction caused by the emergence of antibodies that activate platelets in the presence of heparin, leading to a pro-thrombotic state. Danaparoid is an alternative anticoagulant used in patients on HD with HIT but its dosing recommendations in obese patients on HD are relatively scarce.

CASE SUMMARY

We report a case of a 48-year-old morbidly obese patient who received weight-based dosing of danaparoid for HD with monitoring of anti-Xa activity. However, despite the patient's anti-Xa level being within the therapeutic range at various time points, the circuit lines kept clotting during HD.

WHAT IS NEW AND CONCLUSION

The report provides evidence that the manufacturer's recommendations on dosing danaparoid based on body weight may lead to sub-optimal therapeutic benefit and highlight the need for higher than recommended weight-based dosing in obese individuals on dialysis.

Temporality of heparin-induced antibodies: a retrospective study in outpatients undergoing hemodialysis on unfractionated heparin

Background

Heparin-induced antibodies (HIA) are responsible for causing heparin-induced thrombocytopenia and thrombosis. Research has shown that the temporality of heparin-induced antibodies does not follow the classic immunologic response. The immunobiology of HIA generation remains unclear with varying in vitro and in vivo data. Outpatients undergoing hemodialysis (HD) are exposed to heparin chronically. The HIA immune response can therefore be investigated in vivo in this population.

Methods

We examined the time between the start of HD using unfractionated heparin and HIA levels in 212 outpatients during a 6-year period. Antibodies were detected on enzyme-linked immunosorbent assay. HIA levels were analyzed to determine significance of the trend over time. HIA subgroups were also analyzed for correlation with subsequent thrombotic events and platelet count during follow up.

Results

Overall, the HIA response in HD was found to peak early with waning antibody response despite continued exposure to heparin. The peak prevalence of a strong immune response (optical density > 1.000) was early and short lived, while weaker immune response (optical density 0.400–1.000) persisted for the first 6 months then declined. The mean follow-up time per patient was 2.3 ± 1.4 years. Despite circulating HIA, including high titers, no patients developed HIT in this sample. There was no association between HIA and thrombocytopenia. There was increased incidence of thrombosis in patients with strong HIA compared to other groups, but this did not achieve statistical significance.

Conclusions

The data suggest a significant temporal pattern of HIA in outpatients undergoing HD using unfractionated heparin. Positive HIA was not found to be significantly associated with thrombocytopenia or thrombosis risk in these patients. However, while not achieving statistical significance, subsequent thrombotic events occurred most frequently in the strong positive HIA group (optical density > 1.000). Further research into HIA and risk of thrombosis in this population is needed.

The clinical significance and risk factors of anti-platelet factor 4/heparin antibody on maintenance hemodialysis patients: a two-year prospective follow-up

BACKGROUND

Heparin-induced thrombocytopenia is an immune response mediated by anti-PF4/heparin antibody, which is clinically characterized by thrombocytopenia and thromboembolic events. In this study, a prospective and multi-center clinical investigation 1) determined the positive rate of anti-PF4/heparin antibody in maintenance hemodialysis patients in China, 2) identified the related risk factors, and 3) further explored the effect of the anti-PF4/heparin antibody on bleeding, thromboembolic events, and risk of death in the patients.

METHODS

The serum anti-PF4/heparin antibody was measured in 661 patients from nine hemodialysis centers, detected by IgG-specific ELISA and followed by confirmation with excess heparin. Risk factors of these patients were analyzed. Based on a two-year follow-up, the association between the anti-PF4/heparin antibody and bleeding, thromboembolic events, and risk of death in the patients was investigated.

RESULTS

1) The positivity rate of the anti-PF4/heparin antibody in maintenance hemodialysis patients was 5.6%. With diabetes as an independent risk factor, the positivity rate of the anti-PF4/heparin antibody decreased in the patients undergoing weekly dialyses ≥3 times. 2) The positivity rate of the anti-PF4/heparin antibody was not related to the occurrence of clinical thromboembolic events and was not a risk factor for death within two years in maintenance hemodialysis patients. 3) Negativity for the anti-PF4/heparin antibody combined with a reduction of the platelet count or combined with the administration of antiplatelet drugs yielded a significant increase in bleeding events. However, the composite determination of the anti-PF4/heparin antibody and thrombocytopenia, as well as the administration of antiplatelet drugs, was not predictive for the risk of thromboembolic events in the maintenance hemodialysis patients.

CONCLUSIONS

A single detection of the anti-PF4/heparin antibody did not predict the occurrence of clinical bleeding, thromboembolic events, or risk of death in the maintenance hemodialysis patients.

Evaluation of circuit and AV fistula clotting and detection of anti-PF4/heparin complex antibodies in hemodialysis patients suspected of having heparin-induced thrombocytopenia

A retrospective study was performed to elucidate the characteristics of heparin-induced thrombocytopenia (HIT) in newly treated hemodialysis (HD) patients who essentially required anticoagulation with unfractionated heparin (UFH). Seventy-eight patients suspected of having HIT within 3 months of starting HD with UFH were selected for this study. Their platelet counts were routinely followed, and anti-PF4/heparin complex antibodies (HIT antibodies) were measured with enzyme-linked immunosorbent assay (ELISA) until the titer became negative. The characteristics of thrombocytopenia were a platelet count of ≤150 × 10(9)/L and/or decrease of ≥30% and as caused by the intermittent use (3 times/a week) of UFH during HD. Fifty-five patients showed unexpected clotting in the extracorporeal circuit and/or arteriovenous fistula (AVF) thrombosis, while 23 patients had neither of these complications. The patients were classified into HD-related and non-HD-related thrombus groups. The impact of various combinations of the 3 clinical factors (thrombocytopenia, timing, and HD-related thrombus) and the results of ELISA as a laboratory factor were examined. A combination of 2 platelet factors (thrombocytopenia and timing) and ELISA positivity did not reveal the presence of HIT, while a combination of the 3 clinical factors and a positive ELISA improved the accuracy of HIT diagnosis. The findings on the 4-factor combination were supported by high rates of seroconversion in a serotonin release assay. Combining appropriate clinical factors and a positive ELISA may lead to the proper management of HD patients suspected of having HIT. In conclusion, while HD patients showed a drop of ≤150 × 10(9)/L or ≥30% on days 7 to 30, unexpected clotting in the circuit and/or AVF thrombosis was considered as a sign of HIT development.

An Overview of Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune-mediated adverse drug reaction to heparin products leading to a prothrombotic state. Devastating clinical sequelae may result, including venous or arterial thromboembolism, limb amputation, and death. Heparin cessation alone is insufficient to manage HIT. Pharmacotherapy with argatroban or lepirudin is essential. This article reviews the pathogenesis, diagnosis, and pharmacotherapy of HIT.

Heparin-induced thrombocytopenia: a renal perspective

Heparin-induced thrombocytopenia (HIT) is a clinicopathologic syndrome in which one or more clinical events, usually thrombocytopenia or thrombosis, are temporally related to heparin administration and caused by HIT antibodies. Rapid and accurate diagnosis is essential given the high incidence of thrombosis at around the time of initial disease recognition. Discontinuation of heparin and initiation of alternative anticoagulants reduces HIT-associated morbidity and mortality. The clinical consequences of HIT in hemodialysis patients remain unclear, with several studies reporting no clinical sequelae and others describing complications such as thrombocytopenia or clotting of the extracorporeal circuit. Frequent clotting of the extracorporeal circuit has also been reported in HIT-antibody-positive patients on continuous veno-venous hemofiltration. Several recent findings are of particular interest to nephrologists. An acute systemic reaction has been described as a presentation of HIT in hemodialysis patients shortly after administration of an unfractionated heparin bolus. This syndrome is important to recognize as it might mimic a dialyzer reaction. More recently, the presence of a positive HIT-antibody test or increasing titers of HIT antibody were associated with increased mortality in hemodialysis patients, raising the question of whether these antibodies have a role in the increased cardiovascular mortality seen in these patients. HIT-antibody production is often transient and small numbers of hemodialysis patients with undetectable antibody levels have been rechallenged with heparin without adverse clinical consequences.

Beyond the platelet count: heparin antibodies as independent risk predictors

A major potential side effect of heparin is immunogenicity, eliciting antibody development to a protein complex comprised of platelet factor 4 and heparin. Nevertheless, the clinical implications of heparin antibody positive patients remain broad, ranging from no apparent clinical consequences to life-threatening arterial and venous thromboemboli. The "Iceberg Model" has been proposed to depict this spectrum, with a relatively large population of antibody-positive patients forming the base of the iceberg, a smaller population of thrombocytopenic patients in the middle and a limited number of patients with thrombocytopenia and thrombosis comprising the apex. An underlying assumption of this model is that thrombosis occurs only in settings of relative or absolute thrombocytopenia. However, several recent studies suggest that antibody formation to platelet factor 4/heparin complexes, even in the absence of thrombocytopenia, may be associated with thrombotic events. In this review, we summarize these data, consider potential mechanisms for thrombosis, and suggest recommendations for testing and management of antibody-positive patients.

Heparin–platelet factor 4 antibodies in patients presenting to the ED with thrombosis

BACKGROUND

Patients with heparin-platelet factor 4 (PF4) antibodies, particularly platelet-activating ones, are at risk for heparin-induced thrombocytopenia if administered heparin. We determined the heparin-PF4 antibody prevalence in emergency department (ED) patients presenting with chest pain or symptoms of thrombosis.

METHODS

Admission samples from 324 ED patients with chest pain or symptoms of thrombosis were tested for heparin-PF4 antibodies and, if positive, platelet-activating antibodies.

RESULTS

Twenty-four (7.4%; 95% confidence interval, 4.8%-10.8%) patients had heparin-PF4 antibodies. Seropositivity occurred in 18 (9.2%) of 196 patients recently (< or =6 months) hospitalized vs 6 (4.7%) of 128 not recently hospitalized (P = .19), and in 16/231 (6.9%) patients with chest pain vs 8/93 (8.6%) with other thrombosis (P = .64). Of 22 seropositive patients retested, 8 (7 recently hospitalized) had platelet-activating antibodies.

CONCLUSION

Heparin-PF4 antibody prevalence is 7.4% in ED patients with chest pain or thrombosis, with approximately 1 in 3 seropositive patients having platelet-activating antibodies. Alternative, nonheparin anticoagulation would be prudent in these at-risk patients.

Heparin-induced Thrombocytopenia, a Prothrombotic Disease

Heparin-induced thrombocytopenia (HIT) is a serious, yet treatable prothrombotic disease that develops in approximately 0.5% to 5% of heparin-treated patients and dramatically increases their risk of thrombosis (odds ratio, 37). The antibodies that mediate HIT (ie, heparin-platelet factor 4 antibodies) occur more frequently than the overt disease itself, and, even in the absence of thrombocytopenia, are associated with increased thrombotic morbidity and mortality. HIT should be suspected whenever the platelet count drops more than 50% from baseline (or to <150 x 10(9)/L) beginning 5 to 14 days after starting heparin (or sooner if there was prior heparin exposure) or new thrombosis occurs during, or soon after heparin treatment, with other causes excluded. When HIT is strongly suspected, with or without complicating thrombosis, heparins should be discontinued, and a fast-acting, nonheparin alternative anticoagulant such as argatroban should be initiated immediately. With prompt recognition, diagnosis, and treatment of HIT, the clinical outcomes and health economic burdens of this prothrombotic disease are improved significantly.

The prevalence of antibodies to the platelet factor 4 -heparin complex and association with access thrombosis in patients on chronic hemodialysis

INTRODUCTION

Heparin-induced thrombocytopenia is a serious complication that can lead to thrombocytopenia, venous and arterial thrombosis. Patients with this disorder develop antibodies to the platelet factor 4-heparin (PF4-H) complex. Hemodialysis patients are repeatedly exposed to heparin and are at risk for developing PF4-H antibodies. We sought to determine the prevalence of PF4-H antibodies in a large cohort of patients on chronic hemodialysis and to evaluate the relationship between PF4-H antibodies and hemodialysis vascular access thrombosis in a case-control study.

MATERIAL AND METHODS

Pre-dialysis blood samples were drawn on 419 patients; 107 cases with access thrombosis and 312 controls that never had access thrombosis. All samples were screened for PF4-H antibodies using an ELISA assay (GTI PF4 Enhanced, GTI Diagnostics). All positive and indeterminate samples were then tested using an IgG-specific PF4-H ELISA assay and a platelet serotonin-release assay.

RESULTS

Antibodies to PF4-H were positive in 54 (12.9%) patients using the screening ELISA assay. Nine (2.1%) patients had IgG-specific PF4-H antibodies. None of the patient's had a positive platelet serotonin-release assay. No relationship between hemodialysis access thrombosis and PF4-H antibodies was noted using the screening ELISA assay (unadjusted odds ratio 0.63; 95% CI 0.30-1.30; P = 0.21), the IgG-specific ELISA assay (unadjusted odds ratio 0.83; 95% CI 0.17-4.06; P = 0.82) or indeterminate platelet serotonin-release assay results (unadjusted odds ratio 0.97;95% CI 0.10-9.44;P = 0.98).

CONCLUSIONS

Hemodialysis with repeated exposure to unfractionated heparin was associated with a moderately elevated prevalence of PF4-H antibodies. However, our results do not support a relationship between PF4-H antibodies and hemodialysis vascular access thrombosis.

Expression, crystallization, and three-dimensional structure of the catalytic domain of human plasma kallikrein

Plasma kallikrein is a serine protease that has many important functions, including modulation of blood pressure, complement activation, and mediation and maintenance of inflammatory responses. Although plasma kallikrein has been purified for 40 years, its structure has not been elucidated. In this report, we described two systems (Pichia pastoris and baculovirus/Sf9 cells) for expression of the protease domain of plasma kallikrein, along with the purification and high resolution crystal structures of the two recombinant forms. In the Pichia pastoris system, the protease domain was expressed as a heterogeneously glycosylated zymogen that was activated by limited trypsin digestion and treated with endoglycosidase H deglycosidase to reduce heterogeneity from the glycosylation. The resulting protein was chromatographically resolved into four components, one of which was crystallized. In the baculovirus/Sf9 system, homogeneous, crystallizable, and nonglycosylated protein was expressed after mutagenizing three asparagines (the glycosylation sites) to glutamates. When assayed against the peptide substrates, pefachrome-PK and oxidized insulin B chain, both forms of the protease domain were found to have catalytic activity similar to that of the full-length protein. Crystallization and x-ray crystal structure determination of both forms have yielded the first three-dimensional views of the catalytic domain of plasma kallikrein. The structures, determined at 1.85 A for the endoglycosidase H-deglycosylated protease domain produced from P. pastoris and at 1.40 A for the mutagenically deglycosylated form produced from Sf9 cells, show that the protease domain adopts a typical chymotrypsin-like serine protease conformation. The structural information provides insights into the biochemical and enzymatic properties of plasma kallikrein and paves the way for structure-based design of protease inhibitors that are selective either for or against plasma kallikrein.