How are Antiphospholipid Antibodies measured?
i. To test for LA, a phospholipid-dependent screening assay is performed and if prolonged, normal plasma is added in a 1:1 mix. If LA is present, addition of platelet-poor normal plasma does not correct the prolonged assay, but addition of excess phospholipid does. If a factor deficiency is present, the assay does correct with addition of platelet-poor normal plasma.
ii. aCL abs are measured by an enzyme-linked immunosorbent assay (ELISA) test for immunoglobulin G (IgG), IgM, and IgA isotypes.
iii. Anti-β2GPI antibodies are measured by an ELISA test for IgG, IgM, and IgA isotypes.
iv. Anti-PS/PT antibodies are measured by an ELISA test for IgG and IgM isotypes.
Pearl : unfortunately there is poor agreement among laboratories testing aPL abs with up to a 25% false-positive rate.