In vitro detection of mild inhibitors to factor VIII in hemophilia.

Plasma samples from patients with severe classic hemophilia, suspected of having mild inhibitors not detected in the standard Bethesda inhibitor test, were examined in modified Bethesda tests and in a screening test based on the activated partial thromboplastin time (APTT). The APTT test was more sensitive to mild inhibitors than any modification of the Bethesda method. Increasing the ratio of patient to normal plasma in either the APTT or the Bethesda test improved the sensitivity to mild inhibitors. If a patient's plasma is used as its own control in the APTT test, results are more consistent as minor fluctuations in the APTT occur over time.