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.