Level Diagnosis of HIT Platelet fall (≤150×109/L, ≥30%) Clotting circuit Timing of fall (7-30 day) ELISA (IgG, IgA, IgM) Estimates of positive SRA 1 most likely + + + pos. high (~95%) 2 likely + − + pos. moderate (~75%) 3 less likely + − − pos. less moderate (~70%) 4 unlikely + + +/− neg. neg. (0%) +, clinical factor present, −, not present, SRA, 14C serotonin release assay

Table 3: Evaluating patients with three clinical factors and ELISA.