Anti-PF4 VITT antibodies are oligoclonal and variably inhibited by heparin.

BACKGROUND: COVID-19 vaccines have been associated with a rare thrombotic and thrombocytopenic reaction, Vaccine-induced immune thrombotic thrombocytopenia (VITT) characterized by platelet-activating anti-PF4 antibodies. This study sought to assess clonality of VITT antibodies and evaluate their characteristics in antigen-based and functional platelet studies. METHODS: Anti-PF4 antibodies were isolated from five patients with VITT secondary to ChAdOx1 nCoV-19 (n=1) or Ad26.COV2.S (n=4) vaccination. For comparative studies with heparin-induced thrombocytopenia (HIT), anti-PF4 antibodies were isolated from one patient with spontaneous HIT, another with "classical" HIT, and two patients with non-pathogenic (non-platelet activating) anti-PF4 antibodies. Isolated antibodies were subject to ELISA and functional testing, and mass spectrometric evaluation for clonality determination. RESULTS: All five VITT patients had oligoclonal anti-PF4 antibodies (3 monoclonal, one bi- and one tri-clonal antibodies), while HIT anti-PF4 antibodies were polyclonal. Notably, like VITT antibodies, anti-PF4 antibodies from a spontaneous HIT patient were monoclonal. The techniques employed did not detect non-pathogenic anti-PF4 antibodies. The ChAdOx1 nCoV-19-associated VITT patient made an excellent recovery with heparin treatment. In vitro studies demonstrated strong inhibition of VITT antibody-induced platelet activation with therapeutic concentrations of heparin in this and one Ad26.COV2.S-associated VITT patient. Oligoclonal VITT antibodies with persistent platelet-activating potential were detected at 6 and 10 weeks after acute presentation in two patients tested. Two of the 5 VITT patients had recurrence of thrombocytopenia and one patient had focal seizures several weeks after acute presentation. CONCLUSION: Oligoclonal anti-PF4 antibodies mediate VITT. Heparin use in VITT needs to be further studied.

Evaluation of a new commercial method for von Willebrand factor multimeric analysis.

INTRODUCTION: Evaluation of von Willebrand factor (VWF) multimeric distribution is useful for subclassification of von Willebrand disease (VWD). Multimer analysis has historically been a manual, labor-intensive laboratory-developed test. The first commercial method for multimeric analysis was recently developed that utilizes a single instrument for gel electrophoresis, staining, and densitometry. The current study was undertaken to evaluate the performance characteristics of the new commercial method. METHODS: Studies performed with the commercial method included evaluation of accuracy (method comparison), reference intervals (establishment of normal migration patterns in normal donor specimens), precision (multimer pattern reproducibility), and analytical sensitivity. RESULTS: In the method comparison studies, concordant interpretations were obtained in 19 of 24 comparisons, including normal and abnormal specimens. The 5 specimens with discordant interpretations all involved slight differences and none were considered clinically significant. Thirty-eight normal donor specimens demonstrated normal multimer patterns. Multimer pattern reproducibility was demonstrated in normal and abnormal controls tested on each gel. In the sensitivity studies, adequate visualization of multimers was determined to require VWF protein concentrations of approximately 5%-10% of normal. CONCLUSION: The commercial multimer method is a streamlined test that demonstrates comparable performance characteristics to our current laboratory-developed method and that provides the advantage of both electrophoresis gels and densitometry scans to aid interpretation.

A high-performance liquid chromatography method for the serotonin release assay is equivalent to the radioactive method.

INTRODUCTION: The serotonin release assay (SRA) is considered the gold standard laboratory test for heparin-induced thrombocytopenia (HIT). The historic SRA method uses platelets loaded with radiolabeled serotonin to evaluate platelet activation by HIT immune complexes. However, a nonradioactive method is desirable. We report the performance characteristics of a high-performance liquid chromatography (HPLC) SRA method. METHODS: We validated the performance characteristics of an HPLC-SRA method, including correlation with a reference laboratory using the radioactive method. Serotonin released from reagent platelets was quantified by HPLC using fluorescent detection. Results were expressed as % release and classified as positive, negative, or indeterminate based on previously published cutoffs. RESULTS: Serum samples from 250 subjects with suspected HIT were tested in the HPLC-SRA and with the radioactive method. Concordant classifications were observed in 230 samples (92%). Sera from 41 healthy individuals tested negative. Between-run imprecision studies showed standard deviation of <6 (% release) for positive, weak positive, and negative serum pools. Stability studies demonstrated stability after two freeze-thaw cycles or up to a week of refrigeration. CONCLUSION: The HPLC-SRA has robust performance characteristics, equivalent to the historic radioactive method, but avoids the complexities of working with radioactivity.

Quality of factor XI activity testing in North American Specialized Coagulation Laboratories.

INTRODUCTION: The performance of factor XI activity (FXI) by laboratories in the North American Specialized Coagulation Laboratory Association proficiency testing program was analyzed. METHODS: Over 10 years (2003-2013), 80 samples were distributed; 33-55 laboratories participated per exercise providing 3833 total responses. Analysis was performed on numeric results and qualitative classification of results. RESULTS: The sample FXI levels ranged from 3.8 to 154.0 IU/dL. The overall interlaboratory average coefficient of variation (CV%) was 17.5%; the CV was higher for a sample with low (3.8 IU/dL) FXI. Results were correctly classified as abnormal (100%) for a sample with 3.8 IU/dL FXI and normal/borderline normal (97.7%) for 45 samples with 80 to < 140 IU/dL FXI. The classification was heterogeneous for samples with FXI of 50 to < 80 IU/dL. Six specimens were repeat-tested from 2007 to 2013. The mean FXI was not significantly different in laboratories using the same method on both exercises, suggesting good intralaboratory precision over time. Univariate analysis of data from 2011 to 2012 did not find a consistent significant difference among the activators, analyzers, calibrators, and FXI-deficient plasmas. CONCLUSION: Laboratories generally performed well in assessment of FXI based on interlaboratory precision when FXI >30 IU/dL and on classification of samples with very low or normal FXI.

Thrombocytopenia: an update.

Thrombocytopenia is a common clinical problem with numerous potential causes including decreased bone marrow platelet production, increased peripheral platelet destruction, increased splenic sequestration, and dilution. Investigation of the etiology of thrombocytopenia requires careful consideration of clinical history and laboratory features. A complete blood count and peripheral smear review are essential components of the diagnostic work-up, and physicians should be knowledgeable about appropriate selection and interpretation of more specialized tests, including bone marrow examination, to assist with diagnosis. This review article aims to summarize and address appropriate work-up of the major and/or life-threatening causes of thrombocytopenia and some of the better-characterized congenital thrombocytopenias.

Prolonged prothrombin time correlates with serum monoclonal protein concentration in patients with plasma cell dyscrasia.

INTRODUCTION: Abnormal screening coagulation tests are frequently observed in asymptomatic patients with multiple myeloma and other plasma cell neoplasms. METHODS: Prothrombin time (PT), activated partial thromboplastin time (APTT) and fibrinogen activity were correlated with clinical history and disease parameters in patients referred to the Myeloma Institute for Research and Therapy. RESULTS: An isolated prolonged PT was the most common abnormal coagulation test (25%). Prolonged PT was more frequently observed in patients with multiple myeloma (n = 157) compared to MGUS patients (n = 34) or other diagnostic categories of plasma cell dyscrasia. There were no differences in age, gender, previous chemotherapy, or immunoglobulin isotype in patients with isolated prolonged PT (n = 62) compared to those with normal screening coagulation tests (n = 173). Fibrinogen activity was significantly lower in patients with prolonged PT; however, there was no correlation between fibrinogen activity and PT. Serum M protein concentrations were significantly greater in patients with prolonged PT and were positively correlated with PT. CONCLUSION: An association between disease severity and prolonged PT is suggested by our finding that patients with multiple myeloma were more likely to have prolonged PT than patients with other plasma cell neoplasms. Of the factors examined, the monoclonal protein level was significantly higher in patients with isolated prolonged PT and correlated with PT.