Misdiagnosis of antiphospholipid syndrome can occur owing to the wide diversity of antiphospholipid (aPL) assays and a lack of international calibrators and harmonized reference intervals.


To assess laboratory practices regarding reporting and establishing reference intervals for immunoglobulin (Ig) G/IgM anti-cardiolipin (aCL) and anti–beta-2 glycoprotein I (anti-β2GPI) assays.


Supplemental questions related to reporting and establishing reference ranges for aPL assays were sent as part of the Antiphospholipid Antibody (ACL)-B 2019 College of American Pathologists (CAP) proficiency testing survey. The response rate and methods assessment details were determined, as well as qualitative and quantitative results for 3 test samples.


The number of participants reporting results for IgG aCL (n = 489), IgM aCL (n = 476), IgG anti-β2GPI (n = 354), and IgM anti-β2GPI (n = 331) varied by antibody type. The enzyme-linked immunosorbent assay (ELISA) (up to 58.6%, 260 of 444) was the most used method; others included multiplex (from 18.9% to 23.9%), fluorescence enzyme immunoassay (14.4%–17.6%), and chemiluminescence immunoassay (6.5%–9.0%). More respondents reported quantitative than qualitative results and manufacturer cutoff ranges were used by 92.9% and 94.2% of respondents for aCL and anti-β2GPI, respectively. Despite variation in the use of semiquantitative ranges, qualitative negative/positive reporting of the test samples achieved almost 100% consensus. Qualitative consensus was met in contrast to the wide range of quantitative results obtained for each analyte across different kits.


ELISA remains the most used method for detecting aPL antibodies with most laboratories reporting quantitative results based on manufacturers' suggested reference ranges. The categorization of quantitative results as equivocal, weak positive, or positive for responders using kits from the same manufacturer was variable.

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The authors have no relevant financial interest in the products or companies described in this article.

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