Extracellular fluid volume: A suitable indexation variable to assess impact of bariatric surgery on glomerular filtration rate in patients with chronic kidney disease

Background Bariatric surgery (BS) might be a nephroprotective treatment in obese patients with chronic kidney disease (CKD), and the non-linear relation between body surface area (BSA) and extracellular fluid volume (ECFV) in obese people raises the question of the most relevant way to scale glomerular filtration rate (GFR) for assessing renal function changes after BS. Methods We screened 1774 BS candidates and analysed 10 consecutive participants with CKD stage 3. True GFR (mGFR), measured by the renal clearance of Cr-51-ethylenediaminetetraacetic acid (EDTA), was scaled either to BSA (mGFR(BSA)) or to ECFV measured by Cr-51-EDTA distribution volume (mGFR(ECFV)) before and one year after BS. Results The 10 candidates for BS had a mean body mass index of 43.3 +/- 3.6 kg/m(2) and a mean GFR of 48 +/- 8 mL/min/1.73 m(2). Six participants had a sleeve gastrectomy and four had a Roux-en-Y gastric bypass. One year after BS, ECFV decreased (23.2 +/- 6.2 to 17.9 +/- 4.3 L, p = 0.001), absolute mGFR was not significantly modified (74 +/- 23 versus 68 +/- 19 mL/min), mGFR(BSA) did not change significantly (53 +/- 18 versus 56 +/- 17 mL/min/1.73 m(2)) whereas mGFR(ECFV) significantly increased (42 +/- 13 versus 50 +/- 14 mL/min/12.9 L, p = 0.037). The relation between mGFR(ECFV) and mGFR(BSA) was different from the identity line before (p = 0.014) but not after BS (p = 0.09). Conclusion There is a difference between mGFR(BSA) and mGFR(ECFV) following BS and the latter might better reflect the adequacy between renal function and corpulence.