Objective: This study was undertaken to determine the variability of antiphospholipid antibody (APA) assay results of cardiolipin and other frequently tested phospholipids. Study design Ten centers performing APA assays submitted samples that were positive in their assay. Twenty samples were identified to cover a broad range of APA results. Samples were distributed to the 10 participating centers for evaluation of immunoglobulin G (IgG), IgM, and IgA antibodies to phospholipids.
Results: Of 20 patients, 9 (45%) were identified as positive by all 10 centers and 2 of 20 patients (10%) were identified as positive by 8 or 9 centers. However, 9 of 20 samples (45%) returned with mixed results. The average percent of positive samples was 29%, but results from the 10 centers ranged from a low number of positives (13%) to a high number of (37%).
Conclusion: When considering a clinical diagnosis of APA syndrome, laboratory concordance of results from 20 patient samples among the 10 centers was only 55%. However, when considering a single phospholipid of a single immunoglobulin isotype, agreement of test results was 83.8%. Isolated positive APA results should be correlated with the clinical history and confirmed by repeat testing.