Body Surface Area Indexing Attenuates Apparent Early eGFR Decline After Sleeve Gastrectomy: A Retrospective Cohort...

Background: Early postoperative changes in creatinine-based estimated glomerular filtration rate (eGFR) after bariatric surgery can be misread as a kidney injury. During rapid weight loss, indexing eGFR to a fixed body surface area (BSA) of 1.73 m² may alter apparent trajectories. We compared absolute (mL/min) and BSA-indexed (mL/min/1.73 m²) eGFR changes after sleeve gastrectomy, stratified by baseline glomerular hyperfiltration (GH). Methods: In this retrospective cohort of 145 adults undergoing laparoscopic sleeve gastrectomy, serum creatinine was obtained at baseline (≤30 days pre-op) and 3 months (post-op days 75-105). Indexed eGFR was calculated with the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) 2021 creatinine equation; BSA with the Mosteller formula; and absolute eGFR as indexed eGFR × (BSA/1.73). GH was defined as indexed eGFR ≥ 120 mL/min/1.73 m². A REML mixed-effects model (Group, Time, Group × Time) with patient-cluster bootstrap inference was used. An age-adjusted sensitivity model including Age and Age × Time was also fitted. Results: Fifty-four participants (37%) met the GH criteria. Absolute eGFR declined by -26.6 mL/min in GH versus -17.3 mL/min in non-GH (difference-in-differences [DiD] -9.3 mL/min; 95% CI -13.9 to -4.7; p < 0.001). The indexed eGFR changes were smaller (-4.2 vs. -0.5 mL/min/1.73 m²; DiD -3.7; 95% CI -7.3 to -0.03; p = 0.048; bootstrap p_sign = 0.052). In the age-adjusted sensitivity model, the Group × Time interaction for absolute eGFR attenuated but remained statistically significant (-6.57 mL/min; 95% CI, -13.09 to -0.06; p = 0.048), whereas the corresponding interaction for indexed eGFR was attenuated and no longer statistically significant (-3.99 mL/min/1.73 m²; 95% CI -9.15 to 1.16; p = 0.129). Conclusions: Within three months after sleeve gastrectomy, participants with higher baseline indexed filtration showed a larger decline in absolute eGFR but only a small change in indexed eGFR. These results show that early postoperative creatinine-based eGFR trajectories are scale dependent and should be interpreted cautiously during rapid weight loss. Because postoperative acute kidney injury (AKI) was not adjudicated and direct kidney function markers were unavailable, this study does not distinguish physiological hemodynamic change from structural kidney injury. Reporting both absolute and indexed eGFR may improve early postoperative interpretation and help align dosing decisions with rapid changes in body size.