Evaluation of the anti-cardiolipin antibody test: report of an international workshop held 4 April 1986.

Thirty laboratories from institutions in Britain, France, Italy, The Netherlands, New Zealand, Sweden and the USA participated in a workshop to evaluate the anti-cardiolipin (aCL) test. Participants were asked to measure IgG and IgM aCL in seven samples on each of three separate days. The seven samples were prepared so that IgG and IgM aCL concentrations were known before distribution. Twenty-three of 30 laboratories measuring IgG aCL had significant regression slopes (P less than 0.001) when optical absorbance readings or counts per minute were compared with IgG aCL concentration. Twenty-four of 28 laboratories measuring IgM aCL had significant regression slopes (P less than 0.001). Coefficient of determination (R2) ranged from 81.1% to 98.7% for laboratories with valid IgG aCL assays and from 48.0% to 96.7% for valid IgM aCL assays. Valid assays had in common the use of 10% fetal calf or 10% adult bovine serum in PBS. Assays that were not valid had in common the use of PBS, PBS-Tween, or 0.3% gelatin as diluents. All laboratories with valid assays defined samples with high and moderate aCL levels as positive but there was no consensus about low positive samples. This study shows that properly performed ELISA or SRIA assays can be used to provide an accurate, reproducible, and quantitative measure of IgG and IgM aCL concentration in serum samples.