Clinical significance and correlations between anti-β₂ glycoprotein I IgA assays in antiphospholipid syndrome and/or systemic lupus erythematosus

Background The objective of this investigation was to examine the clinical significance of IgA anti-β₂ glycoprotein I (anti-β₂GPI) antibodies and the inter-assay relationships between kits for their determination. Methods Serum samples from 269 patients with clinical diagnoses of systemic lupus erythematosus (SLE) and/or antiphospholipid syndrome (APS), individuals positive for antiphospholipid antibodies (aPL) with or without APS or SLE, and 182 controls were tested for anti-β₂GPI IgA antibodies using kits from four manufacturers. Results The positivity rates for the different IgA anti-β₂GPI antibody kits varied in the disease groups; 7.8–14.7% (SLE only), 12.0–15.7% (SLE and APS/aPL), 14.7–58.8% (APS only), and 17.4–52.2% (aPL only). Kappa agreements between any 2 kits within disease groups were also variable and ranged from 0.25–1.00 (SLE), 0.18–1.00 (SLE and APS/aPL), 0.22–0.94 (APS only), and 0.32–0.91 (aPL only). Univariate analyses also showed variable relative risks for specific APS clinical manifestations with the different kits evaluated. Overall, diagnostic and predictive values for IgA anti-β₂GPI antibodies are kit-dependent; therefore results are not interchangeable. While all 4 kits seem able to predict venous thrombosis tolerably well, there was a variable performance in predicting pregnancy related morbidity. Conclusions Efforts to standardize these assays are highly needed prior to their formal adoption in routine clinical evaluation.