A rapid test (STic Expert®) for the diagnosis of heparin-induced thrombocytopenia

Management of heparin-induced thrombocytopenia

PURPOSE OF REVIEW

The purpose of this review is to summarize recent findings on heparin-induced thrombocytopenia (HIT), a prothrombotic disorder caused by platelet-activating IgG targeting platelet factor 4 (PF4)/polyanion complexes.

RECENT FINDINGS

HIT can explain unusual clinical events, including adrenal hemorrhages, arterial/intracardiac thrombosis, skin necrosis, anaphylactoid reactions, and disseminated intravascular coagulation. Sometimes, HIT begins/worsens after stopping heparin ('delayed-onset' HIT). Various HIT-mimicking disorders are recognized (e.g., acute disseminated intravascular coagulation/'shock liver' with limb ischemia). HIT has features of both B-cell and T-cell immune responses; uptake of PF4/heparin complexes into macrophages ('macropinocytosis') facilitates the anti-PF4/heparin immune response. Antibody-induced activation of monocytes and platelets via their FcγIIA receptors triggers an intense procoagulant response. Sometimes, HIT antibodies recognize PF4 bound to (platelet-associated) chondroitin sulfate, explaining how HIT might occur without concurrent or recent heparin (delayed-onset HIT, 'spontaneous HIT syndrome'). The molecular structure of HIT antigen(s) has been characterized, providing a rationale for future drug design to avoid HIT and improve its treatment. The poor correlation between partial thromboplastin time and plasma argatroban levels (risking subtherapeutic anticoagulation) and need for intravenous administration of argatroban have led to increasing 'off-label' treatment with fondaparinux or one of the direct oral anticoagulants.

SUMMARY

Understanding the molecular mechanisms and unusual clinical features of HIT will improve its management.

Diagnostic accuracy of rapid immunoassays for heparin-induced thrombocytopenia. A systematic review and meta-analysis

The platelet factor 4/heparin ELISA has limited specificity for heparin-induced thrombocytopenia (HIT) and frequently does not provide same-day results. Rapid immunoassays (RIs) have been developed which provide results in 30 minutes or less. We conducted a systematic review and meta-analysis to evaluate the diagnostic accuracy of RIs for HIT. We searched the literature for studies in which samples from patients with suspected HIT were tested using a RI and a functional assay against which the performance of the RI could be measured. We performed sensitivity analyses of studies that directly compared different RIs with each other and with ELISAs. Estimates of sensitivity and specificity for each RI were calculated. Twenty-three articles, collectively involving six different RIs, met eligibility criteria. All RIs exhibited high sensitivity (0.96 to 1.00); there was wider variability in specificity (0.68 to 0.94). Specificity of the IgG-specific chemiluminescent assay (IgG-CA) was greater than the polyspecific chemiluminescent assay [0.94 (95 %CI 0.89-0.99) vs 0.82 (0.77-0.87)]. The particle gel immunoassay demonstrated greater specificity than the polyspecific ELISA [0.96 (0.95-0.97) vs 0.91 (0.89-0.92)]. The IgG-CA and lateral flow immunoassay [0.94 (0.91-0.97)] exhibited greater specificity than the IgG-specific ELISA [0.86 (0.82-0.90)]. Given their high sensitivity and rapid turnaround time, RIs are a reliable means of excluding HIT at the point-of-care in patients with low or intermediate clinical probability. Additionally, some RIs have greater specificity than HIT ELISAs. In summary, IgG-specific RIs appear to have improved diagnostic accuracy compared with ELISAs in patients with suspected HIT and may reduce misdiagnosis and overtreatment.