Anti-PF4/heparin antibodies (by EIA) per significant events in the evolution of HIT: comparison of 12 patients with HIT and 36 seropositive non-HIT controls. (A) Individual results of anti-PF4/heparin antibodies detected using the commercial immunoassay (EIA-GTI) at 3 sequential time points for 12 patients with HIT: (1) early postoperative platelet count nadir (ie, preceding the platelet count fall indicating HIT), (2) earliest platelet count decrease associated with HIT, and (3) platelet count fall of 50% or greater. (4) Maximum OD value for each patient is also shown. connected by lines indicate data from the 12 patients with HIT; □ represent 36 seropositive non-HIT controls (maximum OD values). Asterisks (*) indicate the median OD values, with the bars representing the 25% and 75% IQR values. For the 12 patients with HIT, the maximum OD values were significantly greater than the maximum OD results observed for the 36 seropositive non-HIT control patients (median, 2.77 vs 1.59, respectively; P < .001). (B) Individual results of anti-PF4/heparin antibodies detected using an in-house immunoassay that detects IgG class antibodies (EIA-IgG) at 3 time points for 12 patients with HIT ( indicate patients with HIT; ◯ indicate seropositive non-HIT controls; time point designations are as in panel A). For the patients with HIT, the maximum OD values were significantly greater than for the maximum OD results observed for the 36 seropositive non-HIT controls (median, 1.63 vs 0.94; P < .001). EIA indicates enzyme immunoassay; and HIT, heparin-induced thrombocytopenia.