We determined the prevalence of and risk factors for British Isles Lupus Activity Group (BILAG) flare in patients with systemic lupus erythematosus (SLE).

We followed 299 patients for 1 year with the BILAG scores calculated using British Lupus Integrated Prospective System software and confirmed with manual calculation.

“A” flares occurred at a rate of 0.254/year, “B” flares 1.637/year, and A or B flares 1.765/year. The most common A flares were renal and mucocutaneous. The most common B flares were hematologic, renal, mucocutaneous, and musculoskeletal. Risk factors for a later A or B flare in the hematological system included: low C3 (p < 0.0001), low C4 (p = 0.0004), and positive anti-double-stranded (ds)DNA (p = 0.003); in the mucocutaneous system: low C3 (p = 0.02) and low C4 (p = 0.0004); and in the renal system: low C3 (p = 0.02) and low C4 (p = 0.02). In a stepwise regression model, only ethnicity (p = 0.02) and low C4 (p = 0.0002) remained as independent predictors of later A or 2B flares.

The organ system distribution of A and B flares is very different, with A flares more common in renal and mucocutaneous, and B flares more common in hematologic and renal systems. A or 2B flares are significantly more common in African Americans and in patients with abnormal serologies (low C3, low C4, or high anti-dsDNA). If flare is an outcome in an SLE clinical trial, these factors must be balanced by taking them into account at baseline in terms of randomization, or by statistical adjustment in final analyses.