Reactions of unlabeled (dashed line) and biotin-labeled (solid line), purified antibody from patient 1 with immobilized heparin: PF4 complexes prepared at various ratios of the two reactants. For each determination, the indicated quantity of heparin was incubated with 10 μg/mL PF4 to form complexes that were immobilized in microtiter trays. Complexes optimal for antibody detection were formed at a ratio of heparin 0.4 IU/mL to PF4 10 μg/mL. Brackets indicate ± 1.0 SD.