Effect of large weight reductions on measured and estimated kidney function

Area of Science:

• Nephrology
• Bariatric Surgery
• Clinical Chemistry

Background:

• Weight loss impacts muscle mass and plasma creatinine (pCr).
• Creatinine-based equations for estimated glomerular filtration rate (eGFR) include pCr.
• Hypothesis: Significant weight loss reduces muscle mass and pCr, artificially increasing eGFR.

Purpose of the Study:

• To investigate the effect of major weight loss on measured GFR (mGFR) and estimated GFR (eGFR).
• To compare creatinine-based eGFR equations with cystatin C-based equations following bariatric surgery.
• To assess the influence of muscle mass changes on GFR estimation.

Main Methods:

• Prospective study of 19 patients undergoing gastric bypass surgery.
• Measured GFR (mGFR) assessed via 51Cr-EDTA plasma clearance.
• eGFR calculated using MDRD, CKD-EPI-pCr, CKD-EPI-cysC, and CKD-EPI-pCr-cysC equations.
• Dual-energy X-ray absorptiometry (DXA) scans measured lean limb mass changes.

Main Results:

• Six months post-surgery, patients lost 27 kg, and absolute mGFR decreased (p=0.024).
• Body surface area-adjusted mGFR remained unchanged (p=0.52).
• Creatinine-based eGFR (MDRD, CKD-EPI-pCr) significantly increased (p<0.001), while cystatin C-based eGFR was unchanged (p=0.51).
• Lean limb mass decreased by 3.5 kg (p<0.001), correlating with pCr changes (R²=0.28, p=0.032).

Conclusions:

• Major weight loss reduces absolute mGFR, potentially due to resolving hyperfiltration.
• Creatinine-based eGFR overestimates renal function post-weight loss due to decreased muscle mass.
• Cystatin C-based eGFR provides a more reliable measure of kidney function after significant weight loss.