Frequency of Renal Monitoring — Creatinine and Cystatin C (FORM-2C): an observational cohort study of patients with reduced eGFR in primary care

BACKGROUND: Monitoring is the mainstay of chronic kidney disease management in primary care; however, there is little evidence about the best way to do this. AIM: To compare the effectiveness of estimated glomerular filtration rate (eGFR) derived from serum creatinine and serum cystatin C to predict renal function decline among those with a recent eGFR of 30–89 ml/min/1.73 m(2). DESIGN AND SETTING: Observational cohort study in UK primary care. METHOD: Serum creatinine and serum cystatin C were both measured at seven study visits over 2 years in 750 patients aged ≥18 years with an eGFR of 30–89 ml/min/1.73 m(2) within the previous year. The primary outcome was change in eGFR derived from serum creatinine or serum cystatin C between 6 and 24 months. RESULTS: Average change in eGFR was 0.51 ml/min/1.73 m(2)/year when estimated by serum creatinine and −2.35 ml/min/1.73 m(2)/year when estimated by serum cystatin C. The c-statistic for predicting renal decline using serum creatininederived eGFR was 0.495 (95% confidence interval [CI] = 0.471 to 0.519). The equivalent c-statistic using serum cystatin C-derived eGFR was 0.497 (95% CI = 0.468 to 0.525). Similar results were obtained when restricting analyses to those aged ≥75 or <75 years, or with eGFR ≥60 ml/min/1.73 m(2). In those with eGFR <60 ml/min/1.73 m(2), serum cystatin C-derived eGFR was more predictive than serum creatinine-derived eGFR for future decline in kidney function. CONCLUSION: In the primary analysis neither eGFR estimated from serum creatinine nor from serum cystatin C predicted future change in kidney function, partly due to small changes during 2 years. In some secondary analyses there was a suggestion that serum cystatin C was a more useful biomarker to estimate eGFR, especially in those with a baseline eGFR <60 ml/min/1.73 m(2).