The absence of standardization in antiphospholipid antibody testing may favor the use of 99th percentile cutoffs in antiphospholipid syndrome classification.

Background: Classification criteria for antiphospholipid syndrome (APS) issued by the American College of Rheumatology/European Alliance of Associations for Rhuematology necessitate a positivity for any of the 3 molecular targets: lupus anticoagulant, anticardiolipin (aCL) immunoglobulin G, or anti-β2 glycoprotein I (aβ2GPI) immunoglobulin G, with the latter 2 requiring concentrations > 40 units. This specification implies having standardized and comparable calibration strategies to achieve proper patient classification. In the past, calibrator tests suffered from poor standardization; thus, the 99th percentile was established as the cutoff point. Objectives: We aimed to find a balance between sensitivity and specificity in the laboratory criteria for patient enrollment in APS studies by harmonizing the 99th percentile and 40-unit threshold. Methods: In a cohort of 250 healthy individuals, we tested aCL and aβ2GPI concentrations by 4 different methods: 3 colorimetric, standardized ELISA platforms and 1 chemiluminescence assay, to define the 99th percentile. We tested cross-reactivity of standardized calibrators between kits and how to implement better accuracy for patient enrollment in a cohort of 80 APS patients. Results: We found that the 99th percentile was substantially <40-unit cutoff and observed considerable interkit variability in the determined cutoffs, which originated from the inadequate standardization of kit calibrators. In a second cohort of 80 APS patients, we estimated the accuracy of these different methods by comparing the 99th percentile and 40-unit cutoffs. For certain ELISA kits, using a fixed cutoff of 40 units instead of the 99th percentile decreased their sensitivity without increasing specificity, which affected patient classification and thus the number of patients eligible for APS studies. Testing with 2 ELISA platforms at the 99th percentile cutoff would improve patient eligibility. Conclusion: Our survey suggests that in the absence of standardized calibrators for testing aCL or aβ2GPI, a cutoff point at the 99th percentile of 2 different ELISA kits should be adopted.