Proteinuria or Albuminuria for the Follow-Up of Patients with Lupus Nephritis?

Background: Albuminuria is used for the follow-up of CKD and DKD. In lupus nephritis, definitions of response are based on proteinuria or urine protein to creatinine ratio (uPCR). We aimed to evaluate the course of uACR and uPCR over the first year of follow-up of patients with LN. Methods: We included 106 patients. All received immunosuppressive therapy and were followed quarterly for ≥36 months. uACR and uPCR were measured in spot samples at flare, 3, 6, and 12 months. Response was defined according with KDIGO. Stable eGFR was defined as an eGFR slope ≤1mL/min/1.73m2. Correlation was assessed by linear regression. Predicted uACR was calculated from uPCR and agreement evaluated by intraclass correlation coefficient (ICC) and Bland-Altman plots. The course of uACR, uPCR, and their ratio was evaluated by linear mixed models for repeated measurements. The predictive value for stable kidney function of 12th-month uPCR and uACR was determined by ROC curves. Results: We included 106 patients, median age 31 years (IQR 25-38), 93 (88%) female. Creatinine was 0.9mg/dL (IQR 0.6-1.3), uPCR 3.0 g/g (IQR 1.9-5.3), uACR 1.9g/g (1.1-3.7). All patients were treated with glucocorticoids in combination with MMF, cyclophosphamide, or tacrolimus. After 12 months, 36 (34%), 27 (26%), 43 (41%) achieved a complete, partial or no response, respectively. uPCR and uACR were highly correlated at all study timepoints, with r2=0.946 and slope of 0.67 (95%CI 0.65-0.68) for the full cohort (Figure 1A). Agreement between predicted and observed uACR was excellent with ICC of 0.97 (95%CI 0.96-0.98) and P30 ≥80% at all timepoints. The uACR/uPCR ratio slightly decreased in all response groups from flare to the 12th month timepoint but was not different among response groups (Figure 1B). The best cutoff for predicting stable eGFR over follow-up were 0.71g/g and 0.41g/g for uPCR and uACR, respectively. Conclusion: Both uPCR and uACR may be used for the first year of follow-up of LN and are predictive of stable kidney function. uACR may be predicted from uPCR.