A cross-sectional and prospective study found that blood samples from patients with probable systemic lupus erythematosus (pSLE) but not meeting ACR classification criteria (n = 92) are more frequently positive for cell-bound complement activation products (CB-CAPs), measured either directly (28%) or within a multi-analyte assay panel (MAP; 40%), than for low serum complement levels (9%). A MAP score >0.8 at enrolment predicted transition from pSLE to classifiable SLE within 18 months, and was a better predictor of this transition than other serum biomarkers assessed in the study, including anti-dsDNA antibody positivity and low complement levels.