Quantitation of anticephalin antibodies in a computer-assisted enzyme-linked immunosorbent assay (ELISA): relation to lupus anticoagulant.

Lupus anticoagulants (LA) are IgG or IgM antibodies which prolong phospholipid-dependent coagulation tests. For the detection and quantitation of such antibodies, we have developed an ELISA with cephalin as the coating antigen. The sensitivity of this assay was compared to the activated partial thromboplastin time (APTT). LA was defined as greater than or equal to 5 sec prolongation of the APTT with standard cephalin dilution, or greater than or equal to 10 sec prolongation with a high cephalin dilution, on a 1:1 mixture of patient and control plasma. Plasma samples from 158 healthy individuals were tested for anticephalin antibodies. The 97.5 percentile was chosen as the upper reference limit and allocated a value of 1 ELISA unit. A "four-parameter logistic" model was used for transformation of the absorbances to ELISA units. Of 314 plasma samples referred for LA screening, positive results were found in 62 by both APTT and ELISA. Twenty-three samples were ELISA positive and APTT negative; this finding may be explained by greater sensitivity of the ELISA, which gave positive results in a four-fold greater dilution than the APTT. Prolongation of the APTT without antibody activity was found in 8 samples of which 2 had an inhibitor of factor VIII:C, the remaining 6 probably had true LA. In conclusion, our computer-assisted ELISA is a sensitive and reliable test method for quantitation of anticephalin antibodies. This assay has a high concordance with LA as detected with the APTT.