Clinical case 1: heparin induced thrombocytopenia (HIT). (A) Time course for platelet count (blue) with heparin exposure demonstrates characteristic pattern of HIT. The platelet factor 4 (PF4) immunoassay was strongly positive (OD  =  1.39; cutoff for positivity OD ≥0.4), and the serotonin release assay (SRA) was positive. Platelet count recovered after discontinuation of heparin and transition to a direct thrombin inhibitor, bivalirudin, and then rivaroxaban. (B) 4T score for pretest probability of HIT. The patient scored a high probability of HIT with a score of 8. DVT, deep vein thrombos; OD, optical density; PE, pulmonary embolism; UFH, unfractionated heparin.