Determination of diagnostic threshold in harmonization and comparison of clinical utility for five major antiphospholipid antibody assays used in Japan

Background: Anticardiolipin antibodies (aCL) and anti-beta(2)-glycoprotein I antibodies (a beta(2) GPI) are essential in diagnosing antiphospholipid syndrome (APS) according to the international APS guideline. Five commercial assays for aCL and a beta(2) GPI are available in Japan, but their test results are quite discordant. For harmonization of diagnosing APS, upper reference limit (URL) and diagnostic accuracy of each assay were evaluated and compared by testing common sets of specimens across all assays. Methods: We evaluated two manual and three automated assays for aCL and a beta(2) GPI of IgG- and IgM classes. 99%URL (the upper limit of reference interval: as per guideline) together with 97.5%URL were determined by testing sera from 198 to 400 well-defined healthy subjects. Both URLs were compared with the cutoff values, which were determined based on ROC analysis by testing 50 each of plasma specimens from patients with/without APS. Diagnostic accuracy was evaluated as area under curve (AUC) of the ROC curve. Results: A variable degree of discrepancy between URLs and the cutoff values was observed, which was partly attributable to between-year assay variability. 97.5%URLs were set lower and closer to the cutoff values than 99%URLs. For all assays, diagnostic accuracies of both a beta(2)GPI-IgG and aCL-IgG were generally high (AUC: 0.84-0.93); whereas those for 1gM-class assays were low (AUC: 0.57-0.67), implicating its utility is limited to rare IgG negative APS cases. Conclusion: To ensure harmonized APS diagnosis, the diagnostic thresholds of the five assays were evaluated by common procedures. Contrary to the guideline, 97.5%URL is rather recommended for diagnosing APS, which showed a closer match to the cutoff value.