Measuring glomerular filtration rate using ⁵¹Cr-EDTA: Body surface area normalization before or after Bröchner-Mortensen correction?

Background Guidelines for measuring glomerular filtration rate (GFR) using ⁵¹Cr-EDTA require normalizing of GFR for body surface area (BSA) before applying the Bröchner-Mortensen (BM) correction.

Materials and methods We theoretically showed that the order of applying BM correction and BSA indexing is indeed important for patient populations having a low BSA and a high slow GFR. We then compared the exact GFR, obtained from the double-exponential concentration-time curve in Duchenne muscular dystrophy (DMD) patients, with the GFR obtained from the slow compartment method using the BM correction.

Results The median GFR for the 20 DMD patients obtained from the BSA-BM order deviates 5.40% from the exact GFR (P=0.0006), whereas the median GFR obtained from the BM-BSA order deviates only -0.05% (P>0.05) from the exact GFR, resulting in a median of differences of 5.50% between the two methods (P<0.0001).

Conclusion The correct order of application in this DMD population should be BM correction first, followed by BSA indexing, and not vice versa. In general, the order of applying the BM correction and BSA normalization becomes more important with increasing slow GFR and extreme low BSA. The order of application is of less importance for people with normal BSA and/or normal GFR.