Role of Cystatin C in Predicting Disease Activity and Flare-Up in Systemic Lupus Erythematosus: A Longitudinal Follow-Up Study

BACKGROUND: We aimed to determine the sensitivity of serum cystatin C (Cys-C) in predicting lupus flare-up. METHODS: In a longitudinal study, 77 patients were followed-up for up to 15 months. Cys-C, physician global assessment (PGA), and lupus activity index (SLEDAI) were recorded during each visit. Flare-up was defined as an increase ≥4 scores in SLEDAI compared to the last visit. The predictability of flare-up by Cys-C was evaluated by generalized linear-mixed effect model (GLMM) and generalized estimating equation (GEE). Predictive power of Cys-C, SLEDAI, and PGA was compared by the area under the curves (AUC) and application of receiver operating characteristic (ROC) curves. RESULTS: Lupus flare-up was observed in 14 out of 77 patients on the 1(st) visit, 3 out of 41 patients on the 2(nd) visit, 2 out of 26 patients on the 3(rd) visit, 1 out of 14 patients on the 4(th) visit, and 1 out of 3 patients on the 5(th) visit. Mean Cys-C levels in patients with flare-up vs. those with no flare-up in the 1(st), 2(nd), and 3(rd) visits were 1769 vs. 1603 (P = 0.6), 5701 vs. 2117 (p = 0.2) and 1409 vs. 1731 (p = 0.9), respectively. Cys-C had lower predictive power than PGA and SLEDAI for either flare-up, active nephritis or SLEDAI in GLMM/GEE models. Cys-C also showed lower sensitivity (AUC = 0.701, 95%CI = 0.579-0.823, P = 0.003) than PGA and SLEDAI, to distinguish patients prone to flare-ups. CONCLUSIONS: Although Cys-C had some sensitivity for predicting flare-up, active nephritis or SLEDAI, its sensitivity was lower than that in PGA and SLEDAI.