Background: Autoimmune heparin-induced thrombocytopenia (aHIT) is increasingly recognized. It is a life-threatening highly prothrombotic condition with prolonged thrombocytopenia. Patient sera cause strong platelet activation even in the absence of heparin (buffer reactivity), which can be inhibited by high concentrations of heparin.

Aims: To assess the prevalence of aHIT among samples referred to a reference laboratory with clinically suspected HIT and further characterization of the aHIT antibodies.

Methods: All patients referred to our laboratory with clinically suspected HIT between 2017 and 2019 were enrolled. Data were retrieved from the laboratory files. For further characterization the HIPA test (washed platelet activation test) was performed. In brief, patient sera were incubated with washed platelets of healthy donors under different conditions: buffer (control) and low dose heparin (reviparin 0.2 aFXaU/mL). Platelet aggregation was visually detected every 5 min. Confirmed aHIT sera were step-wise diluted with AB-serum of a healthy donor and retested.

Results: Of 7,510 referred sera, 5,226 (69.6%) were negative; 1,196 (15.9%) ELISA+ & HIPA negative; and 998 (13.3%) ELISA+ & HIPA+. 86 sera (1.15% of all referred sera; 3.9% of all ELISA+ sera; 8.6% of all HIPA+ sera) showed the typical aHIT pattern. After retesting, 40 sera still showed the typical aHIT pattern. Upon serial dilution, different patterns of platelet activation were observed: 25 sera contained low titer antibodies (Fig. 1a); 8 sera contained high titer antibodies (Fig. 1b). 7 sera contained only autoantibodies, as heparin did not increase reactivity (Fig. 1c).

Conclusions: The prevalence of aHIT reactivity pattern in sera is higher than currently anticipated, with 3.9% of all ELISA reactive sera and 8.6% of all HIPA positive sera. Most sera contain a mixture of heparin-independent and heparin-dependent platelet activating antibodies. The antibody titers differ considerably, which may impact the efficacy of treatment.

[Mean HIPA reaction time of serially diluted autoimmune HIT samples]

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/insights-into-autoimmune-heparin-induced-thrombocytopenia/