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 1.73m², 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 ⁵¹Cr-EDTA clearance. The study comprised 128 individuals (35 women, 66 men and 27 children) with a full ⁵¹Cr-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 Cl₁ and V₁ 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 Brøchner-Mortensen (BM); and ECV according to a new method (JBM): y=2x – 1, where x = Cl₁/Cl and y = V₁/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 JBM methods. Expressing GFR/ECV in unit %/h gives a simple interpretation. Normal
ranges for GFR/ECV need to be established.