Clinical significance of cystatin C based renal function assessment, proteinuria and albuminuria in patients with rheumatoid arthritis

Aim : This study aimed to determine the relationship between renal function markers and clinical background factors in patients with rheumatoid arthritis (RA) as well as propose effective strategies for accurate renal assessment and management of RA, especially in the context of modern treatment options. Methods : A retrospective, observational, cross-sectional study was conducted in Hashima municipal hospital and Kindai University Nara Hospital involving 140patients with RA who met the 1987 American college of rheumatology (ACR) or 2010 ACR/European league against rheumatism criteria. Clinical background, comorbidities, disease activity, and serum C reactive protein (CRP). creatinine, cystatin C, and urinary protein levels, and albuminuria were assessed. eGFR was obtained from both creatinine and cystatin C levels. Data of 38 treatment-naïve patients were also analyzed longitudinally for changes in C reactive protein (CRP) and cystatin C levels. Statistical analyses included multivariate regression and correlation analysis. Results : Renal function and proteinuria: 20.7% of patients had proteinuria (urinary protein–creatinine ratio ≥0.15), and 26.4% had albuminuria (urinary albumin–creatinine ratio ≥30). An eGFR of <60 mL/min was noted in approximately 23% of cases based on both creatinine and cystatin C levels. Associations: CRP levels were significantly associated with proteinuria, cystatin C, and discordance between eGFR based on creatinine and cystatin C levels. Hypertension was related to albuminuria. Longitudinal analysis: In treatment-naïve patients, cystatin C levels decreased in parallel with the subsequent reduction in CRP following treatment. Conclusion : In patients with RA, elevated CRP levels are associated with increased cystatin C levels, proteinuria, and eGFR discordance even without glomerular disease. These markers may reflect inflammatory activity rather than intrinsic kidney damage and may improve with treatment. Due to implications for drug selection and risk management particularly with methotrexate and biologics, accurate and multifactorial assessment of renal function remains critical in RA.