Albuminuria or proteinuria for response assessment and follow-up in lupus nephritis

Urine albumin to creatinine ratio (uACR) is recommended for follow-up in chronic kidney disease due to its association with progressive kidney disease, cardiovascular events, and mortality. In lupus nephritis (LN), response criteria and follow-up are based on urine protein to creatinine ratio (uPCR). This study aimed to assess the clinical course and prognostic associations of uACR and uPCR. uACR and uPCR were evaluated at LN flare, 3, 6, and 12 months after initial therapy. We followed the estimated glomerular filtration rate (eGFR) for 36 months. Correlation between uACR and uPCR was evaluated in all samples and at each timepoint. Predicted uACR was estimated from observed uPCR and agreement between predicted and observed uACR evaluated. The 12-month course of uPCR and uACR was assessed by linear mixed models for repeated measurements and compared over response groups. The ability of uACR and uPCR to predict the stability of kidney function (eGFR decline ≤1mL/min/1.73m2/year) was determined by receiver-operating-characteristic curves. We included 106 patients, 93 (88%) female, with a median age of 31 years (IQR 21-48). Serum creatinine, uPCR, and uACR at LN flare were 0.9mg/dL (IQR 0.6-1.3), 3.0g/g (IQR 1.9-5.3), and 1.9g/g (IQR 1.1-3.7), respectively. uPCR and uACR were highly correlated (r2=0.946) in all samples and at each study timepoint. Predicted and observed uACR had an intraclass correlation coefficient of 0.972, and 81% of values of predicted uACR were within 30% of the observed uACR. The best cutoff for predicting a stable kidney function was 0.71g/g and 0.41g/g for uPCR and uACR, respectively. There is a high correlation and agreement between uACR and uPCR during the first 12 months of therapy in LN. There is no difference in the uACR/uPCR ratio between response groups. By 12 months, a uACR ≤0.41g/g and uPCR≤0.71g/g are associated with stable kidney function.