Evolution of the HIT immune response in relation to clinical manifestations. Twelve patients with HIT (■) and 36 control patients who were PF4/H seropositive but did not have HIT (□) were monitored after orthopedic surgery for PF4/H antibodies, thrombocytopenia, and thrombosis. The time course of PF4/H seroconversions are shown on the x-axis and OD levels are shown on the y-axis. The difference in OD between the HIT patients and the seropositive non-HIT controls was statistically significant (P < .05 by unpaired t test). Four key events for the 12 patients with HIT are shown: first day PF4/H antibody was detectable, onset of HIT-related decrease in platelet count, decrease in platelet count ≥ 50%, and thrombotic event. These events are shown as medians (small black squares within rectangles), interquartile ranges (length of open rectangles), and ranges (ends of thin black lines). Adapted with permission from Warkentin et al.¹⁴