Previous studies have established the value of measuring complement activation products (C4d) bound to platelets (PC4d) for the diagnosis and monitoring of Systemic Lupus Erythematosus (SLE). Separately, Antiphospholipid (APL) antibodies have been associated with complement activation and PC4d expression. In this study, we sought to validate the performance characteristics of PC4d, stratified by the presence or absence of APL antibodies.

This multi-centered validation cross sectional study (16 sites in the US) enrolled 402 SLE subjects fulfilling the 1982 American College of Rheumatology Criteria revised in 1997 (mean age 41 years; 91% female), 411 subjects with rheumatic and autoimmune diseases other than SLE (mean age 55, 86% female consisting of 181 rheumatoid arthritis, 90 primary fibromyalgia, 92 other rheumatic diseases, and 48 autoimmune thyroiditis or hepatitis) and 198 healthy volunteers (mean age 41 years; 66% female). PC4d densities were determined using flow cytometry (expressed as mean fluorescence intensity [MFI]). Positive PC4d consisted of PC4d levels greater than 20 net MFI. Anti-cardiolipin IgG, anti-Beta-2-glycoprotein 1 IgG, or anti-Phosphatidylserine/Prothrombin (PSPT) complex IgG antibodies were determined using ELISA (INOVA diagnostics, San Diego, CA). Presence of APL antibodies consisted of any of these antibodies above manufacturer cutoff. SLE Disease activity was assessed using the non-serological SLE Disease Activity Index SELENA modification (ns-SELENA-SLEDAI, without the complement and anti-dsDNA). Performance characteristics were established using sensitivity, specificity, and ROC Curve Area Under the Curve (AUC). Statistical evaluation was by t-test (for disease activity), by chi-squared test for equality of proportions (for sensitivities and specificities) and by the method of DeLong (for ROC Curve AUC).

PC4d was highly specific in distinguishing SLE from other rheumatic diseases (Table) and normals. Among SLE subjects, 47% (n=187) presented with at least one APL antibody as compared to 21% (n=86) of subjects with other diseases and 15% of normals. PC4d sensitivity for SLE was higher among APL positive subjects by comparison to APL negative subjects (p=0.003). Specificity was not significantly different between APL positive and negative subjects (p>0.372). ROC AUC was significantly higher among the APL positive compared to negative subjects (p=0.002). The incidence of APL antibodies among all PC4d positive subjects was 60% compared to 27% among PC4d negative subjects (p<0.001). SLE subjects presenting with positive PC4d had higher disease activity (4.1±0.5) than those presenting with negative PC4d (3.0±0.2) (p=0.03).

We confirm that PC4d is highly specific for SLE, and is associated with disease activity.