BACKGROUND: Estimation of GFR (eGFR) using formulae based on serum creatinine concentrations are commonly used to assess kidney function. Physical exercise can increase creatinine turnover and lean mass; therefore, this method may not be suitable for use in exercising individuals. Cystatin-c based eGFR formulae may be a more accurate measure of kidney function when examining the impact of exercise on kidney function. The aim of this study was to assess the agreement of four creatinine and cystatin-c based estimates of GFR before and after a 12-month exercise intervention. METHODS: One hundred forty-two participants with stage 3 to 4 chronic kidney disease (CKD) (eGFR 25 to 60 mL/min/1.73 m2) were included. Subjects were randomised to either a control group (standard nephrological care (n = 68)) or a lifestyle intervention group (12 months of primarily aerobic based exercise training (n = 74)). Four eGFR formulae were compared at baseline and after 12 months: (1) MDRDcr, (2) CKD-EPIcr, (3) CKD-EPIcys and (4) CKD-EPIcr-cys. RESULTS: Control participants were aged 63.5 (9.4) years, 60.3% were male, 42.2% had diabetes, and had an eGFR of 40.5 +/- 8.9 ml/min/1.73m2. Lifestyle Intervention participants were aged 60.5 (14.2) years, 59.5% were male, 43.8% had diabetes, and had an eGFR of 38.9 +/- 8.5 ml/min/1.73m2. There were no significant baseline differences between the two groups. Lean mass (r = 0.319, p < 0.01) and grip strength (r = 0.391, p < 0.001) were associated with serum creatinine at baseline. However, there were no significant correlations between cystatin-c and the same measures. The lifestyle intervention resulted in significant improvements in exercise capacity (+1.9 +/- 1.8 METs, p < 0.001). There were no changes in lean mass in both control and lifestyle intervention groups during the 12 months. CKD-EPIcys was considerably lower in both groups at both baseline and 12 months than CKD-EPIcr (control -10.5 +/- 9.1 and -13.1 +/- 11.8, and lifestyle intervention -7.9 +/- 8.6 and -8.4 +/- 12.3 ml/min/1.73 m2), CKD-EPIcr-cys (control -3.6 +/- 3.7 and -4.5 +/- 4.5, and lifestyle intervention -3.6 +/- 3.7 and -2.5 +/- 5.5 ml/min/1.73 m2) and MDRDcr (control -9.3 +/- 8.4 and -12.0 +/- 10.7, lifestyle intervention -6.4 +/- 8.4 and -6.9 +/- 11.2 ml/min/1.73 m2). CONCLUSIONS: In CKD patients participating in a primarily aerobic based exercise training, without improvements in lean mass, cystatin-c and creatinine based eGFR provided similar estimates of kidney function at both baseline and after 12 months of exercise training. TRIAL REGISTRATION: The trial was registered at www.anzctr.org.au (registration number ANZCTR12608000337370) on the 17/07/2008 (retrospectively registered).

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