Longitudinal analysis of performance of estimated glomerular filtration rate as renal function declines in chronic kidney disease.

BACKGROUND Numerous studies have assessed the accuracy of equations estimating glomerular filtration rate (eGFR) from serum creatinine in individuals with chronic kidney disease (CKD) in cross-sectional studies. Limited literature exists, however, on the consistency of performance of these equations in longitudinal studies as renal function declines. METHODS Radionucleotide-measured GFR from 155 predialysis patients with stage 3-5 CKD was compared with eGFR derived from four equations [6-variable Modification of Diet in Renal Disease (6-MDRD), 4-variable MDRD (4-MDRD), Cockcroft-Gault (CG) and Cockcroft-Gault equations corrected for body surface area (CGC)] at baseline, 12 and 24 months. Bias (difference between eGFR and measured GFR) was used as a measure of performance. Restricted Maximum Likelihood (REML) models were used to identify variables potentially affecting the performance of estimating equations across time. RESULTS Mean measured GFR (+/-SD) at baseline, 12 and 24 months was 25.9 +/- 10.7, 23.1 +/- 10.6 and 20.3 +/- 10.1 mL/min/1.73 m(2), respectively. There was a statistically significant negative association between bias and GFR for all four estimates (range: -0.76 to -0.71, P < 0.001 for all), indicating worsening underestimation and overestimation at higher and lower GFR, respectively. This negative association significantly reduced over the 24 months (P < 0.001); however, this was largely due to persistent underestimation of eGFR from individuals with GFR >50 mL/min/1.73 m(2). For those with a baseline GFR <50 mL/min/1.73 m(2), the change in bias for any of the four equations over 24 months was <or=1.1 mL/min/1.73 m(2), suggesting relatively preserved performance with time. The MDRD equations showed a sustained advantage in estimating renal function that was more evident as GFR declined. CONCLUSION GFR estimates are inexpensive and show an acceptable longitudinal performance for monitoring CKD patients with GFR <50 mL/min/1.73 m(2). Inaccuracies appear more substantial above this level of GFR, and care with interpretation is required.