Day of first onset of immune response against PF4/heparin. (A) The day at which the immune response against PF4/heparin complexes was first detected (first day of heparin use = day 0). There was a narrow time period between day 4 and day 12 during which the immune response occurred for combined IgG/IgA/IgM EIA; 100% represents the 244 patients who showed an increase in OD. (B) This pattern was the same whether the immune response was very weak (< 0.1 OD), weak (0.1- < 0.5 OD), or intermediate (0.5- < 1.0 OD), as indicated by change in the OD in the PF4/heparin EIA. There was a trend toward a slightly earlier onset in case of a strong (> 0.1 OD) immune response (median day 5 vs median day 6; P = .068).