Studies on the relationship between 'antiphospholipid' antibodies and the lupus anticoagulant

The correlation between lupus anticoagulant (LA) potency and anticardiolipin antibody (ACA) ELISA was found to be poor (r = 0.40) in a group of 56 patients accumulated by a haematology department mainly for studies of LA. This correlation was similar whether LAs were assessed by kaolin clotting time or activated partial thromboplastin time increments. When the more procoagulant phospholipid phosphatidyl serine, used in a calcium-containing buffer, was substituted for cardiolipin in the ELISA, the correlation with LA was only slightly improved (r = 0.58). In fact, binding of antibody from patient plasmas to blank wells, although quantitatively reduced, was found to correlate equally well with LA activity. LAs are not necessarily phospholipid-binding antibodies but may interfere more generally with other surface-dependent processes in the clotting mechanism.