Is the 99th Percentile Cutoff Still Relevant? A Single-Center Assessment of Different Thresholds for Diagnosing Antiphospholipid Syndrome.

Background: The 2023 American College of Rheumatology and modified Sapporo criteria for antiphospholipid syndrome (APS) recommend ELISA to detect anticardiolipin (aCL) and anti-β2-glycoprotein I (aβ2GP1) IgG/IgM antibodies, focusing on moderate to high levels or exceeding the 99th percentile. This study aims to establish the 99th percentile threshold for anti-phospholipid (aPL) antibodies and compare the diagnostic accuracies of these thresholds with manufacturer cutoffs using 2 methodologies.

Methods: The 99th percentile cutoffs for aPL antibodies from 305 healthy donors were established using Aptiva, Particle-Based Multi-Analyte Technology (PMAT), and QUANTA Lite (QL) ELISA, following nonparametric reference interval estimation. Sera from 34 APS patients and 190 APS controls were tested. Diagnostic performances were compared at the 99th percentile-, manufacturer-, receiver operating characteristic (ROC) derived optimal-, and 95% specificity-optimized cutoffs. An expanded cohort of 61 APS patients and 1299 APS controls from a 2-year retrospective review was also included.

Results: For ELISA, the 99th percentile cutoffs for aCL (IgG/IgM) and aβ2GP1 (IgG) were at the assay limit of quantification. Optimal cutoffs from the ROC curves, 95% specificity-matched and manufacturer cutoffs, showed better diagnostic accuracy than the 99th percentile. On the Aptiva PMAT platform, the 99th percentile cutoffs were lower but provided comparable diagnostic accuracies to manufacturer and optimal cutoffs, although specificity was below 95%.

Conclusions: The clinical utility of 99th percentile cutoffs is assay dependent. For QL, these cutoffs were unsuitable, while Aptiva showed better alignment with clinical thresholds. Manufacturer-recommended cutoffs, supported by extensive validation, offer a reliable alternative when clinical studies are infeasible.