The use of absolute values improves performance of estimation formulae: a retrospective cross sectional study

BACKGROUND Estimation of Glomerular Filtration Rate (GFR) by equations such as Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) or Modification of Diet in Renal Disease (MDRD) is usually expressed as a Body Surface Area (BSA) indexed value (ml/min per 1.73 m²). This can have severe clinical consequences in patients with extreme body sizes, resulting in an underestimation in the case of obesity or an overestimation of GFR in the case of underweight patients. The aim of this study was to compare the performance of both estimation formula expressed in ml/min, instead of ml/min per 1.73 m², with a reference method. METHODS Retrospective single centre cross sectional study of 185 patients. GFR was measured with 51Cr-EDTA and estimated with CKD-EPI and MDRD. Bias, precision and accuracy of absolute estimated GFR was calculated. RESULTS Bias of CKD-EPI and MDRD formulae expressed as an absolute value was 0.49 and 0.27 ml/min respectively, which is lower than previously reported. Precision was 12.95 and 16.33 and accuracy expressed as P30 was over 92.43% for CKD-EPI. There were no significant differences in GFR between the reference method and the estimation formulae. CONCLUSIONS The performance of CKD-EPI and MDRD formulae can be significantly improved in the individual patient if the absolute values are used by removing the BSA normalization factor. Absolute estimated GFR by CKD-EPI is comparable to measured GFR, improving the performance of this formula in the assessment of individual kidney function, thus providing clinicians with an alternative to reference methods.