RT Journal Article SR Electronic T1 Frequency of Renal Monitoring — Creatinine and Cystatin C (FORM-2C): an observational cohort study of patients with reduced eGFR in primary care JF British Journal of General Practice JO Br J Gen Pract FD British Journal of General Practice SP BJGP.2020.0940 DO 10.3399/BJGP.2020.0940 A1 Susannah Fleming A1 Rafael Perera-Salazar A1 Kathryn S Taylor A1 Louise Jones A1 FD Richard Hobbs A1 Tim James A1 Chris A O’Callaghan A1 Brian Shine A1 Jan Y Verbakel A1 Richard Stevens A1 Clare Bankhead YR 2021 UL http://bjgp.org/content/early/2021/07/07/BJGP.2020.0940.abstract AB 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 m2.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 m2 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 m2/year when estimated by serum creatinine and −2.35 ml/min/1.73 m2/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 m2. In those with eGFR <60 ml/min/1.73 m2, 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 m2.