Performance of Glomerular Filtration Rate Estimating Equations Before and After Bariatric Surgery

Abstract

Rationale & Objective Evaluation of glomerular filtration rate (GFR) is challenging in adults undergoing bariatric surgery because creatinine and cystatin C levels are influenced by changes in muscle and fat mass. Additionally, indexing of GFR by body surface area (BSA) may by affected by decreases in BSA. Study Design Prospective observational study. Setting & Participants 27 adults with body mass index (BMI) ≥ 35 kg/m 2 who underwent measurement of GFR before and after bariatric surgery. Outcomes Indexed and nonindexed GFRs measured (mGFRs) using plasma iohexol clearance, indexed and nonindexed estimated GFR (eGFR) based on levels of creatinine, cystatin C, or both from Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations. Analytic Approach Bias and percent of estimates within 20% and 30% of mGFR (P 20 and P 30 ) for estimating equations were examined. Results Mean presurgery BMI was 49.5 (SD, 9.4) kg/m 2 , BSA was 2.42 (SD, 0.27) m 2 , nonindexed mGFR was 117.3 (SD, 34.1) mL/min, and indexed mGFR was 84.1 (SD, 22.0) mL/min/1.73 m 2 . After 6 months, mean BMI changed by –13.8 (95% CI, −15.9 to −11.8) kg/m 2 , BSA by −0.30 (95% CI, −0.33 to −0.27) m 2 , and nonindexed mGFR by −9.2 (95% CI, −17.2 to −1.1) mL/min, while indexed mGFR was unchanged at 5.1 (95% CI, −0.1 to 10.4) mL/min/1.73 m 2 . Nonindexed eGFR cr was unbiased (median bias, 5.0 [95% CI, −4.3 to 11.6] mL/min) before surgery, but overestimated mGFR (8.8 [95% CI, 1.8 to 16.9] mL/min) after surgery. Nonindexed eGFR cys underestimated mGFR before (median bias, −12.1 [95% CI, −21.4 to −1.2] mL/min) and after surgery (−11.2 [95% CI, −21.8 to −7.3] mL/min). Nonindexed eGFR cr-cys was unbiased before (median bias, −6.0 [95% CI, −11.0 to 1.0] mL/min) and after surgery (−2.0 [95% CI, −8.8 to 4.9] mL/min). Findings were similar for indexed eGFR compared with indexed mGFR. Limitations Small, mostly white sample. Conclusions Changes in indexed and nonindexed GFRs may be discordant after bariatric surgery in adults because of decreases in BSA. Indexed and nonindexed eGFR cr-cys may be less biased than indexed or nonindexed eGFR cr or eGFR cys because of opposite biases in estimating mGFR.

Assessment of kidney function: clinical indications for measured GFR

“…Recently it has been shown that in severely obese subjects enrolled in medical weight loss programmes, cystatin C–based eGFR and indexation to actual body surface area (BSA) instead of 1.73 m 2 may provide the most accurate estimate of kidney function [ 42 ]. Also, Chang et al [ 43 ] assessed kidney function before and after bariatric surgery and observed a decrease in absolute iohexol clearance, while the creatinine-based eGFR overestimated and the cystatin C-based eGFR underestimated mGFR. Due to the discordant bias of both equations, the combined creatinine and cystatin C–based Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation yielded the least bias and was suggested as the most suitable equation in this setting.…”

Obesity Related Glomerulopathy: weighing in the effect of Body Surface Area

“…5 Lifestyle modification and bariatric surgery can reverse hyperfiltration, reducing GFR for patients with eGFR >90 mL/min following the intervention. 41,42 43…”

Expert opinion on the preoperative medical optimization of adults with diabetes undergoing metabolic surgery

“…The errors in using the standard equations in severely obese subjects arise from changes in the body surface area (error of indexing), alteration in serum creatinine and cystatin C levels, and obesity-induced glomerular hyperfiltration[ 177 ]. The combined equation CKD-EPIcreat-cyst using serum creatinine and cystatin C values reliably predicted eGFR in severe obesity both before and after surgery but further validation in larger cohorts is needed[ 176 , 178 , 179 ].…”