GFR estimation in the morbidly obese pre- and postbariatric surgery: one size does not fit all.

BACKGROUND: Glomerular hyperfiltration commonly associated with obesity is expected to improve postbariatric surgery. However, formula-based glomerular filtration rate (GFR) estimation in these patients is limited by body size confounders necessitating use of modified equations, the reliability of which remains uncertain.
METHODS: In this study, various GFR-estimating formulae were compared in morbidly obese patients at baseline and postbariatric surgery. Through a retrospective chart review, we identified 220 patients who had undergone this procedure, with over 6-month follow-up, during which major weight reduction was achieved.
RESULTS: A significant decrease in BP and glomerular hyperfiltration was observed, though there was large variability in GFR estimation using the different formulae. Gross over and underestimation was observed which improved with correction for body size confounders especially lean body weight (LBW). Postoperatively, significant attenuation in estimated GFR was demonstrated when LBW or body surface area-adjusted versions were used. In a subgroup of patients with chronic kidney disease, a significant improvement in GFR was seen postoperatively with the LBW-modified formula but there were again inconsistencies when using other equations.
CONCLUSION: Though clinicians must be critical in the application of GFR estimates to patient care, LBW adjustment appears to be the most practical solution to its estimation in the obese patients. This is particularly true for patients with normal renal function but appears to be also applicable to those with compromised kidney function. Future studies are needed to compare these equations with a gold standard GFR measure as well as to explore whether the renal benefits from bariatric surgery are sustained or seen in more advanced CKD stages.