Serotonin Release Assay: Functional Assay for Heparin- and Vaccine-Induced (Immune) Thrombotic Thrombocytopenia.

The serotonin release assay (SRA) has been the gold-standard assay for detection of heparin-dependent platelet-activating antibodies and integral for the diagnosis for heparin-induced thrombotic thrombocytopenia (HIT). In 2021, a thrombotic thrombocytopenic syndrome was reported after adenoviral vector COVID-19 vaccination. This vaccine-induced thrombotic thrombocytopenic syndrome (VITT) proved to be a severe immune platelet activation syndrome manifested by unusual thrombosis, thrombocytopenia, very elevated plasma D-dimer, and a high mortality even with aggressive therapy (anticoagulation and plasma exchange). While the platelet-activating antibodies in both HIT and VITT are directed toward platelet factor 4 (PF4), important differences have been found. These differences have required modifications to the SRA to improve detection of functional VITT antibodies. Functional platelet activation assays remain essential in the diagnostic workup of HIT and VITT. Here we detail the application of SRA for the assessment of HIT and VITT antibodies.

A multicenter evaluation of the Technoscreen ADAMTS13 activity semi-quantitative screening test for thrombotic thrombocytopenic purpura diagnosis and exclusion.

INTRODUCTION: Thrombotic thrombocytopenic purpura (TTP) is a rare but potentially fatal microangiopathy, with an untreated mortality rate of around 90%. TTP is caused by severe deficiency in ADAMTS13, which results in accumulation of ultra large von Willebrand factor multimers, triggering a consumptive thrombocytopenia, microangiopathic hemolytic anemia and end-organ dysfunction and damage. Demonstration of severe ADAMTS13 deficiency is diagnostic for TTP, but long turnaround times for quantitative activity testing often necessitates empirical plasma exchange and/or caplacizumab treatment. METHODS: Multisite (n = 4) assessment of the Technoscreen ADAMTS13 activity assay (semi-quantitative flow through screening assay) for diagnosis/exclusion of TTP compared to current standard practice of quantitative assays (ELISA or chemiluminescence AcuStar). RESULTS: A total of 128 patient samples were analyzed, with quantitative ADAMTS13 values ranging from 0% to 150%. The Technoscreen assay demonstrated high sensitivity and negative predictive value (NPV) for ADAMTS13 deficiency, but low specificity and positive predictive value (PPV), especially with one lot of reagent. Good inter-observer reliability was demonstrated. Excluding one possibly compromised batch and other test failures, results of 80 samples yielded sensitivity of 100% (95% CI = 84-100), specificity of 90% (80-95), PPV 77% (58-89) and NPV 100% (93-100). CONCLUSION: The Technoscreen assay appears to be a reliable screening test for ADAMTS13 activity to exclude TTP in routine clinical practice. However, the assay falsely identified ADAMTS13 deficiency in many cases, partially batch related, which mandates confirmation with a quantitative assay, as well as initial assessment of kits as 'fit for purpose' prior to use for patient testing.

A pilot study assessing the implementation of 96-well plate-based aggregometry (Optimul) in Australia.

Identification of disordered platelet function is important to guide peri-operative bleeding management as well as long term treatment and prognostic strategies in individuals with platelet bleeding disorders. Light transmission aggregometry (LTA), the current gold standard diagnostic test of platelet function is a time consuming technique almost exclusively performed in specialised laboratories and almost universally unavailable in regional centres in Australia, where there is an unmet need for access to specialised platelet function diagnostic services. 96-well plate-based aggregometry (Optimul, UK), has been utilised in research laboratories as a novel platform to investigate platelet function. We evaluated the Optimul assay at two centres in Australia, one regional and one tertiary metropolitan, to assess its feasibility as a screening test applicable to remote regional centres. Concentration-response curves were established from 45 healthy volunteers at the participating regional hospital and from 31 healthy volunteers at the tertiary institution. Optimul successfully detected anti-platelet effects in individuals taking aspirin (n=4), NSAID (n=2), clopidogrel (n=2) and dual therapy with aspirin and clopidogrel (n=1). When tested in parallel to LTA in individuals referred for the evaluation of abnormal bleeding symptoms there was overall a very good level of agreement between Optimul and LTA [Cohen's kappa (k2)=0.84], supporting its role as a useful screening tool in the assessment of platelet function. Optimul assay performance was quick and the methodology simple, requiring no specialised training or resources to be implemented at either the regional or metropolitan laboratory. Widespread implementation, particularly in regional laboratories within Australia where specialised platelet function testing is unavailable, has the potential to drastically improve the inequity of access to such services.

A novel flow cytometry procoagulant assay for diagnosis of vaccine-induced immune thrombotic thrombocytopenia.

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a severe prothrombotic complication of adenoviral vaccines, including the ChAdOx1 nCoV-19 (Vaxzevria) vaccine. The putative mechanism involves formation of pathological anti-platelet factor 4 (PF4) antibodies that activate platelets via the low-affinity immunoglobulin G receptor FcγRIIa to drive thrombosis and thrombocytopenia. Functional assays are important for VITT diagnosis, as not all detectable anti-PF4 antibodies are pathogenic, and immunoassays have varying sensitivity. Combination of ligand binding of G protein-coupled receptors (protease-activated receptor-1) and immunoreceptor tyrosine-based activation motif-linked receptors (FcγRIIa) synergistically induce procoagulant platelet formation, which supports thrombin generation. Here, we describe a flow cytometry-based procoagulant platelet assay using cell death marker GSAO and P-selectin to diagnose VITT by exposing donor whole blood to patient plasma in the presence of a protease-activated receptor-1 agonist. Consecutive patients triaged for confirmatory functional VITT testing after screening using PF4/heparin ELISA were evaluated. In a development cohort of 47 patients with suspected VITT, plasma from ELISA-positive patients (n = 23), but not healthy donors (n = 32) or individuals exposed to the ChAdOx1 nCov-19 vaccine without VITT (n = 24), significantly increased the procoagulant platelet response. In a validation cohort of 99 VITT patients identified according to clinicopathologic adjudication, procoagulant flow cytometry identified 93% of VITT cases, including ELISA-negative and serotonin release assay-negative patients. The in vitro effect of intravenous immunoglobulin (IVIg) and fondaparinux trended with the clinical response seen in patients. Induction of FcγRIIa-dependent procoagulant response by patient plasma, suppressible by heparin and IVIg, is highly indicative of VITT, resulting in a sensitive and specific assay that has been adopted as part of a national diagnostic algorithm to identify vaccinated patients with platelet-activating antibodies.

The utility of flow cytometric platelet forward scatter as an alternative to mean platelet volume.

The use of mean platelet diameter (MPD) to classify inherited thrombocytopenia (IT) has been demonstrated in several studies. Alternatively, the mean platelet volume (MPV) may be used, but in macrothrombocytopenia this may not be available. We hypothesized that platelet forward scatter (FSC) measurements using flow cytometry may be used for the size-based classification of IT. The study aimed to assess the ability of platelet FSC to measure platelet size and whether it could be used as an alternative to the MPD or MPV.Blood samples were obtained from individuals undergoing investigation for inherited platelet function disorders (IPFD, n = 40) or platelet number disorders (IPND, n = 46). A hematology analyzer was used to obtain MPV and platelet counts, flow cytometry to measure platelet FSC and ImageJ software to measure MPD from stained blood smears. The International Society of Thrombosis and Hemostasis (ISTH) Bleeding Assessment Tool (BAT) was used to calculate bleeding scores.Twenty-nine(63%) of IPND patients had an MPV that could not be reported. A significant correlation to platelet FSC was found to the MPD (p < .0001) and MPV (p < .0001) and an inverse correlation with platelet count (p < .0001). No significant correlation was found between FSC and bleeding history. In conclusion, platelet FSC is an alternative to MPV and may be used in macrothrombocytopenia where the MPV is not recorded.

Building platelet phenotypes: Diaphanous-related formin 1 (DIAPH1)-related disorder.

Variants of the Diaphanous-Related Formin 1 (DIAPH-1) gene have recently been reported causing inherited macrothrombocytopenia. The essential/"diagnostic" characteristics associated with the disorder are emerging; however, robust and complete criteria are not established. Here, we report the first cases of DIAPH1-related disorder in Australia caused by the autosomal dominant gain-of-function DIAPH1 R1213X variant formed by truncation of the protein within the diaphanous auto-regulatory domain (DAD) with loss of regulatory motifs responsible for autoinhibitory interactions within the DIAPH1 protein. We affirm phenotypic changes induced by the DIAPH1 R1213X variant to include macrothrombocytopenia, early-onset progressive sensorineural hearing loss, and mild asymptomatic neutropenia. High-resolution microscopy confirms perturbations of cytoskeletal dynamics caused by the DIAPH1 variant and we extend the repertoire of changes generated by this variant to include alteration of procoagulant platelet formation and possible dental anomalies.

A multicentre assessment of contemporary laboratory assays for heparin induced thrombocytopenia.

Heparin induced thrombocytopenia (HIT) is a rare but potentially fatal complication of heparin therapy. In some patients, HIT causes platelet activation and thrombosis (sometimes abbreviated HITT), which leads to adverse clinical sequalae ('pathological HIT'). The likelihood of HIT is initially assessed clinically, typically using a scoring system, of which the 4T score is that most utilised. Subsequent laboratory testing to confirm or exclude HIT facilitates exclusion or diagnosis and management. The current investigation comprises a multicentre (n=9) assessment of contemporary laboratory testing for HIT, as performed over the past 1-3 years in each site and comprising testing of over 1200 samples. The primary laboratory test used by study participants (n=8) comprised a chemiluminescence procedure (HIT-IgG(PF4-H)) performed on an AcuStar instrument. Additional immunological testing performed by study sites included lateral flow (STiC, Stago), enzyme linked immunosorbent assay (ELISA), Asserachrom (HPIA IgG), PaGIA (BioRad), plus functional assays, primarily serotonin release assay (SRA) or platelet aggregation methods. The chemiluminescence procedure yielded a highly sensitive screening method for identifying functional HIT, given high area under the curve (AUC, generally ≥0.9) in a receiver operator characteristic (ROC) analysis against SRA as gold standard. ELISA testing resulted in lower ROC AUC scores (<0.8) and higher levels of false positives. Although there is clear association with the likelihood of HIT, the 4T score had less utility than literature suggests, and was comparable to a previous study reported by some of the authors.

A multicenter laboratory assessment of a new automated chemiluminescent assay for ADAMTS13 activity.

BACKGROUND: Thrombotic thrombocytopenic purpura (TTP) is a rare but potentially fatal disorder caused by ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) deficiency. Prompt identification/exclusion of TTP can thus be facilitated by rapid ADAMTS13 testing. The most commonly utilized (enzyme-linked immunosorbent assay [ELISA]-based) assay takes several hours to perform and so does not generally permit rapid testing. OBJECTIVES: To evaluate the utility of a new automated test for ADAMTS13 activity, the HemosIL AcuStar ADAMTS13 Activity assay, based on chemiluminescence and able to be performed on an ACL AcuStar instrument within 33 minutes. PATIENTS/METHODS: This multicenter (n = 8) assessment included testing of more than 700 test samples, with similar numbers of prospective (n = 348) and retrospective (n = 385) samples. The main comparator was the Technozym ADAMTS13 Activity ELISA. We also assessed comparative performance for detection of ADAMTS13 inhibitors using a Bethesda assay. RESULTS: Overall, the chemiluminescent assay yielded similar results to the comparator ELISA, albeit with slight negative bias. ADAMTS13 inhibitor detection was also comparable, albeit with slight positive bias with the AcuStar assay. Assay precision was similar with both assays, and we also verified assay normal reference ranges. CONCLUSIONS: The HemosIL AcuStar ADAMTS13 Activity assay provided results rapidly, which were largely comparable with the Technozym ADAMTS13 Activity ELISA assay, albeit lower on average. Conversely, inhibitor levels tended to be identified at a higher level on average. Thus, the HemosIL AcuStar ADAMTS13 Activity assay provides a fast and accurate means to quantitate plasma levels of ADAMTS13 for TTP/ADAMTS13 identification/exclusion, and potentially also for other applications.

An integrated approach to inherited platelet disorders: results from a research collaborative, the Sydney Platelet Group.

Inherited disorders of platelet function (IPFD) and/or number (IPND) are heterogeneous conditions that result in variable mucocutaneous bleeding symptoms as a result of deranged primary haemostasis caused by platelet dysfunction or thrombocytopenia. Diagnosis is important to guide post-operative bleeding prophylactic strategies, to avoid treatment with inappropriate medications, and inform prognosis. Achieving an accurate diagnosis has traditionally been hampered by the requirement of multiple, often complex, laboratory tests that are not always available at single centres. To improve the diagnosis of these disorders a research collaborative was established, the Sydney Platelet Group, that explored an integrated approach combining traditional and contemporary platelet phenotypic and genetic diagnostic platforms available at four Sydney tertiary hospitals. Herein we report the outcomes of the first 50 patients evaluated using this approach. The cohort included 22 individuals with suspected IPFD and 28 with thrombocytopenia. Bleeding scores were higher in individuals with IPFD (mean 5.75; SD 4.83) than those with IPNDs (mean 2.14; SD 2.45). In cases with suspected IPFD, diagnosis to the level of the defective pathway was achieved in 71% and four individuals were found not to have a definitive platelet function defect. Dense granule secretion disorders were the most common platelet pathway abnormality detected (n=5). Mean bleeding scores in these individuals were not significantly different to individuals with defects in other commonly detected platelet pathways (dense granules, signal transduction and 'undetermined'). A molecular diagnosis was achieved in 52% of individuals with IPNDs and 5% with IPFD. Likely pathogenic and pathogenic variants detected included variants associated with extra-haematological complications (DIAPH1, MYH9) and potential for malignancy (ANKRD26 and RUNX1). The level of platelet investigation undertaken by this initiative is currently not available elsewhere in Australia and initial results confirm the utility of this integrated phenotypic-genetic approach.

HIT or miss? A comprehensive contemporary investigation of laboratory tests for heparin induced thrombocytopenia.

Heparin induced thrombocytopenia (HIT) is a rare but potentially fatal complication of heparin therapy, which in a proportion of patients causes platelet activation and thrombosis. Initial clinical assessment of the likelihood of HIT is facilitated by laboratory testing to confirm or exclude HIT. This prospective investigation was performed over an 18-month period, and has involved testing of over 300 test samples from over 100 consecutive patients. Clinical assessment by 4T score was supplemented by laboratory tests that comprised both immunological [lateral flow ('STiC'), chemiluminescence (AcuStar; HIT-IgG(PF4-H)), ELISA (Asserachrom HPIA IgG)] and functional assays [SRA, platelet aggregation using whole blood ('Multiplate') and platelet rich plasma ('LTA')]. We observed both false positive and false negative test findings with most assays. Overall, the whole blood aggregation method provided a reasonable alternative to SRA for identifying functional HIT. STiC, AcuStar and ELISA procedures were fairly comparable in terms of screening for HIT, although STiC and AcuStar both yielded false negatives, albeit also resulting in fewer false positives than ELISA. The 4T score had less utility in our patient cohort than we were expecting, although there was an association with the likelihood of HIT. Nevertheless, we accept that our observations are based on limited test numbers. In conclusion, no single approach (clinical or laboratory) was associated with optimal sensitivity or specificity of HIT exclusion or identification, and thus, a combination of clinical evaluation and laboratory testing will best ensure the accuracy of diagnosis.