Dynamic of anti-PF4 antibody response. (A) Kaplan-Meier Analysis of the proportion of patients with an anti-PF4/heparin IgG EIA optical density (OD) >0.5 and patients with a positive platelet activation assay after VITT (n = 65). The time (in weeks) to a negative anti-PF4/heparin IgG EIA (OD <0.5) and to a negative test by the platelet activation assay is shown. All patients initially had a positive assay for PF4-dependent platelet-activating antibodies and were repeatedly tested (median follow-up of 25 weeks; range, 3-36 weeks). Patients at risk are shown under the graph. Antibody levels in the anti-PF4/heparin IgG EIA decreased over the time of follow-up, but only in 14 patients the OD fell below the cut off OD <0.5. (B) ODs of the anti-PF4/heparin IgG EIA (antigen assay) of 4 VITT patients with persistently high ODs over time of undiluted and diluted serial samples. The OD of the anti-PF4/heparin IgG EIA remained high with values of >2.0 over 11 weeks. After dilution of the samples (1:4000 in patients 1-3; 1:2000 in patient 4), ODs again declined, showing that the persistently high OD values were caused by the low dynamic range of the EIA at high antibody concentrations. The dotted line (...) shows the cutoff OD <0.5.