Accuracy of a GFR Estimating Equation Over Time in People With a Wide Range of Kidney Function

BACKGROUND: Change in glomerular filtration rate (GFR) is important for clinical decision making. GFR estimates from serum creatinine provide an unbiased but imprecise estimate of GFR at single time points. However, the accuracy of estimated GFR over time is not well known. STUDY DESIGN: Longitudinal study of diagnostic test accuracy SETTINGS AND PARTICIPANTS: Four clinical trials with longitudinal measures of GFR and serum creatinine on the same day, including subjects with and without kidney disease, with a wide range of kidney function, diverse racial backgrounds and varied clinical characteristics. INDEX TEST: GFR estimated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. REFERENCE TEST: GFR measured using urinary clearance of (125)I-iothalamate RESULTS: Data included 19,735 GFR measurements in 3531 subjects over mean follow up of 2.6 years. Mean at baseline for measured and estimated GFR and error (measured GFR – estimated GFR) were 73.1 (95% CI, 71.6 to 74.5), 72.7 (95% CI, 71.5 to 74.0) and 0.14 (95% CI, −0.35 to 0.63) ml/min/1.73 m(2), respectively. The mean rate of change in measured and estimated GFR and error was −2.3 (95% CI, −2.4 to −2.1), −2.2 (95% CI, −2.4 to −2.1) and −0.09 (95% CI, −0.24 to 0.05) ml/min/1.73 m(2) per year (p <.001, p <.001, and p = 0.2 respectively). The variability (ie, SD) among subjects in rate of change in measured GFR, estimated GFR and error was 4.3, 3.4 and 3.3 ml/min/1.73 m(2) per year, respectively. Only 15% of subjects had a rate of change in error of more than 3 ml/min/1.73 m(2) per year, and only 2% had a rate of change in error larger than 5% per year. LIMITATIONS: Subject characteristics were not available over time. CONCLUSION: Accuracy of GFR estimates did not change over time. Clinicians should interpret changes in estimated GFR over time as reflecting changes in measured GFR rather than changes in errors in the GFR estimates in most individuals