[Indexing to Body Surface Area Diminishes GFR Estimation and Increases Chronic Kidney Disease Staging in Overweight and Obese Population].

Glomerular filtration rate estimates are usually indexed to a standard body surface area (BSA) of 1.73 m2. This allows comparing values of individuals of different sizes but has the potential of affecting individuals with extremes BSA.

Aim: evaluating the differences in GFR estimates with or without indexing for BSA in a large cohort of ambulatory patients grouped by different body mass index (BMI) and how indexing affects CKD classification.

Methods: demographic and anthropometric data of 390 patients evaluated with ambulatory 24-hour creatinine clearance were registered in an anonymous database. Patients were divided in 3 groups according to BMI (18-24.9; 25-29.9; >30 kg/m2). GFR was estimated using creatinine clearance (CrCl), CKD-EPI 2009 and 2021 equations, both indexed to a standardized BSA of 1.73 m2 and using the actual BSA of the patients. CKD classification was performed with and without indexing.

Results: 224 of 390 patients were men (57.4%). 103 (26.4%) had normal BMI (group 1), 193 (49.5%) BMI of 25-29.9 (group 2) and 94 (24.1%) had BMI of 30 or more kg/m2 (group 3). Mean CrCl was 67.9 +/- 32.7 ml/min. Indexed to a BSA of 1.73 m2 CrCl was 64.8 +/- 30.5 ml/min (difference of -3.1 ml/min) (p< 0.001). The difference between real and indexed CrCl was +2.2 ml/min, -2.9 ml/min y -9.3 ml/min in groups 1, 2 and 3, respectively. Real CrCl was significantly higher in group 3 compared to group 1. Indexed ClCr was similar between the 3 groups. GFR estimation using equations was over 2 mL/min higher when removing indexation for standard BSA. Group 1 had higher indexed GFR estimates than groups 2 and 3. However, when removing indexing the 3 groups had similar GFR estimates. Classification of CKD was significantly affected by removing indexing, with almost 20% of the patients changing CKD stage. Diagnosis of GFR <60 mL/min was less frequent when removing indexing.

Conclusion: Indexing to standard BSA lowers GFR estimation in overweight and obese patients, leading to a higher prevalence of GFR < 60 mL/min and CKD diagnosis.