In Covid-19, anticoagulation with heparin is often administered to prevent or treat thromboembolic events. Heparin-induced thrombocytopenia (HIT) is a severe complication of heparin treatment, caused by heparin-dependent, platelet activating anti-platelet factor 4 (PF4)/heparin antibodies. Diagnosis of HIT is based on the combination of clinical parameters, allowing to determine the pretest probability, and laboratory testing for anti-PF4/heparin antibodies and confirmatory functional assays, such as the heparin-induced platelet activation (HIPA) test.We report the case of a patient with severe Covid-19 pneumonia requiring ECMO treatment, who developed recurrent clotting of the ECMO filter and a drop in platelet count under heparin treatment. He was therefore suspected to have HIT and the anticoagulation was switched to argatroban. Despite high clinical probability and high titres of anti-PF4/heparin antibodies, the functional HIPA test was negative. Nevertheless, argatroban was continued rather than to reinstate anticoagulation with heparin. Reevaluation 7 days later then demonstrated a strongly positive functional HIPA test and confirmed the diagnosis of HIT. Under anticoagulation with argatroban the patient gradually improved and was finally weaned off the ECMO.In conclusion, this case highlights the critical importance of clinical judgement, exploiting the 4 T score, given that Covid-19 patients may present a different pattern of routine laboratory test results in HIT diagnostics. The possibility of a false negative HIPA test has to be considered, particularly in early phases of presentation. In cases of a discrepancy with high clinical probability of HIT and/or high titre anti-PF4/heparin antibodies despite a negative HIPA test, a reevaluation within 3 to 5 days after the initial test should be considered in order to avoid precipitant reestablishment of unfractionated heparin, with potentially fatal consequences.
