Simplified methods for assessment of renal function as the ratio of glomerular filtration rate to extracellular fluid volume

Background Instead of scaling glomerular filtration rate (GFR) to a body surface area of 1.73m(2), it has been suggested to scale GFR to extracellular fluid volume (ECV). The ratio GFR/ECV has physiological meaning in that it indicates how often 'that which is to be regulated' (i.e. ECV) comes into contact with the 'regulator' (i.e. the kidneys). Aim The aim of the present study was as follows: to analyse two published calculation methods for determining ECV and GFR/ECV; to develop a new simple and accurate formula for determining ECV; and to compare and evaluate these methods. Materials and methods GFR was determined as Cr-51-EDTA clearance. The study comprised 128 individuals (35 women, 66 men and 27 children) with a full Cr-51-EDTA plasma concentration curve, determined from injection until 4-5 h p.i. Reference values for GFR and ECV were calculated from the full curve. One-pool approximations Cl-1 and V-1 were calculated using only the final-slope curve. Four calculation methods were compared: simple one-pool values; GFR/ECV according to Peters and colleagues; ECV according to Brechner-Mortensen (BM); and ECV according to a new method (JBM): y=2x - 1, where x=Cl-1/Cl and y=V-1/ECV. Results The new JBM method is accurate and can be explained theoretically. BM has a slight bias for high renal function. The Peters method had bias in our data. GFR/ECV had better precision than ECV alone, especially for BM and JBM, which were within -4% to +7% of the reference values (95% limits of agreement in adults). Conclusion GFR/ECV can be precisely determined, especially with the BM and IBM methods. Expressing GFR/ECV in unit %/h gives a simple interpretation. Normal ranges for GFR/ECV need to be established. Nucl Med Commun 33:1243-1253 (C) 2012 Wolters Kluwer Health | Lippincott Williams & Wilkins.