Severe obesity and the impact of medical weight loss on estimated glomerular filtration rate.

OBJECTIVE: To assess the impact of obesity, glucose tolerance, and weight loss on renal function, we measured serum creatinine and cystatin C and estimated glomerular filtration rate (GFR) indexed to 1.73m2 body surface area (BSA) and GFR indexed to actual BSA in subjects with normal and abnormal glucose tolerance before and up to 2 years after medical weight loss.

METHODS: We studied 146 subjects at baseline and 3-to-6 months after 18% reduction in weight; 43 were also studied at 2-years. GFR was estimated using the MDRD, CKD-EPICr, CKD-EPICysCr, and the CKD-EPICys equations.

RESULTS: eGFR was consistently lower when creatinine-based rather than cystatin C-based estimating equations were used. eGFR was lower when creatinine-based or cystatin C-based equations were indexed to 1.73m2 BSA than when they were indexed to actual BSA. eGFR indexed to actual BSA was more likely to demonstrate hyperfiltration (eGFR ≥135 ml/min) than eGFR indexed to 1.73m2 BSA and decreased into the normal range with weight loss. eGFR was highest in subjects with impaired fasting glucose but there was little difference in the patterns of change in eGFR across groups by glucose tolerance status.

CONCLUSIONS: With severe obesity, high fat-free mass and BSA result in low estimates of eGFR indexed to 1.73m2 BSA, especially when creatinine-based estimating equations are used. GFR indexed to actual BSA is approximately 50% higher. When eGFR is indexed to actual BSA, many subjects display evidence of renal hyperfiltration which improves with weight loss. In subjects with severe obesity undergoing medical weight loss, estimating equations that use cystatin C and are indexed to actual BSA may provide a more accurate assessment of renal function.