Glomerular filtration rate (GFR) normalization by body surface area (BSA) or extracellular fluid volume (ECV): Importance in relation to sex in adult patients with reduced kidney function

Objectives GFR is commonly but unreasonably normalized by BSA. GFR normalization by ECV is physiologically meaningful and can ameliorate GFR differences between sexes. Exogenous tracer plasma clearance is the gold standard for precise GFR assessment. Assuming a 2-compartment model, sequential plasma sampling biexponentially approximated allows for simultaneous GFR and ECV calculation. We aim to show the importance of GFR indexation by ECV in clarifying GFR differences between men and women with chronic kidney disease (CKD).

Methods In 186 pts (38.2% females) with CKD and GFR category G2-G4 (KDIGO 2013) aged 61.1±13.5 years, GFR was measured for clinical purposes. After i.v. injection of 3.7 MBq Cr-51-EDTA, 10 blood samples were drawn at 5 min - 4 hours p.i. and GFR and ECV were calculated. After indexation by BSA and ECV, GFR was expressed in ml/min/1.73 m2 (GFR/BSA) and ml/min/l (GFR/ECV) respectively. On sex basis, the patients were allocated in two groups M (male, 115 pts) and F (female, 71 pts).

Results Age (yrs), BSA (m2), ECV (l), GFR/BSA (ml/min/1.73 m2) and GFR/ECV (ml/min/l) were 61.0±13.2, 1.99±0.21, 16.2±2.8, 61.0±13.2, 3.99±1.34 and 58.8±14.1, 1.77±0.21, 12.8±2.7, 59.1±17.9, 4.92±1.89 for the M and F groups respectively. Significant differences between the groups were found for BSA, ECV and GFR/ECV but not for age or GFR/BSA. The between M and F groups relative difference [100x(M-F)/M] of the GFR/BSA and GFR/ECV means was 3.1% and -23.3% respectively.

Conclusions Normalization of GFR by ECV significantly changes kidney function assessment obtained by GFR normalized by BSA in relation to the sex of the patients with CKD. ECV normalized GFR should be adopted as the standard GFR expression.