tokens: PATIENTS AND METHODS : Patients with more than 50 % decrease in platelet count or thrombocytopenia ( < 150 x 10 ( 9 ) / L ) after exposure to heparin , who had a positive two - step antigen assay [ optical density ( OD ) > 0 . 4 and > 50 inhibition with high concentration of heparin ] were included in the study .