Anticardiolipin antibodies do not seem to be associated with APC resistance in vivo or in vitro.

Anticardiolipin antibodies (aCL) or lupus anticoagulants (LA) have been found to exert an inhibitory action upon the activation and function of protein C, a natural coagulation inhibitor. Recently an in vitro phenomenon called resistance to activated protein C (APC resistance) has been described as the most frequent cause of hereditary thrombophilia. In order to see whether a positive association of APC resistance with aCL exists we examined plasma of 162 consecutive outpatients referred for thrombophilia screening. Further, the IgG fraction was isolated from plasma of two aCL-positive and LA-negative patients and of two aCL-negative healthy subjects by means of protein A affinity chromatography. Each of these isolates was mixed with normal plasma, and the APC resistance was assayed; 25/162 (15.4%) patients had confirmed abnormal APC resistance. Only 1/25 (4.0%) APC resistance-positive patients and 11/137 (8.0%) APC resistance-negative patients had positive IgG- and/or IgM-aCL (p = 0.5, nonsignificant). In the in vitro test system the APC resistance ratio remained unaffected after addition of normal IgG or aCL-IgG fraction in the tested normal plasma and did not deviate from the range of buffer controls. These data do not suggest any association of aCL with abnormal APC resistance. aCL-IgG fractions from aCL-positive and LA-negative plasmas do not interfere with the APC resistance test system in vitro in low concentration.