High prevalence of antibodies to platelet factor 4 heparin in patients with antiphospholipid antibodies in absence of heparin-induced thrombocytopenia

Seventy-two patients with antiphospholipid antibodies (aPL), with or without antiphospholipid syndrome (APS), were studied for detection of heparin-PF4-induced antibodies (HPIA) using a commercial kit (Asserachrom HPIA) PF4-dependant enzyme-linked immunoassay (ELISA) test. None of the patients had a medical history of heparin induced thrombocytopenia (HIT). Eleven percent of patients were positive for HPIA. Plasma from 40 of the 72 patients (seven positive and 33 negative), was also tested with the other available HPIA ELISA (GTI) kit. Five patients were positive with both ELISA kits, two were highly positive only with Asserachrom HPIA and four only with GTI. None of the positive patients had severe thrombocytopenia. Two patients have never received heparin treatment. No relationship was found between HPIA presence and patients' age, sex, aPL levels or presence of lupus anticoagulant. No significant difference in HPIA presence was observed in patients with primary APS, secondary APS or aPL without APS. We found a poor correlation between the two commercial ELISA showing that, on the same blood sample, a patient could be highly positive with one technique and negative with the other. The PF4-dependant enzyme-linked immunoassay, which is often the first test used for the diagnosis of HIT, should be interpreted cautiously in patients with aPL since there is a danger of overdiagnosis and overtreatment.