Familial association of the lupus anticoagulant.

A lupus anticoagulant was detected in plasma from two pairs of siblings using the kaolin clotting time mixing test. Strong evidence for this inhibitor in one further sibling pair is presented. Although coagulation abnormalities are usually classified as either acquired or of genetic origin it is apparent that the lupus anticoagulant might often be an acquired coagulation defect requiring genetic predisposition.