The Authors Reply

Results proposed by Stehlé et al. regarding our article on the different methods to correct for the early-compartments for the determination of GFR by plasma clearances¹ is of interest because data comparing plasma versus urinary clearances are relatively rare.² In our article, we fairly stated that we had no urinary clearances, and thus our analysis was just a comparison of current equations to correct for the early-compartment.¹ Stehlé et al. perfectly confirmed what we showed: a high concordance of the results given by the Bröchner-Mortensen, Fleming, Jodal-Bröchner-Mortensen, and Ng, whereas the Chantler correction gave different results. However, we are more careful in our interpretation of the results by Stehlé et al., who suggest to consider the Chantler method in low glomerular filtration rate (GFR) ranges. First, their sample in this GFR range is relatively low. Second, as acknowledged by the authors, the difference between urinary and plasma clearances in low GFR is due to the timing of the last sample, which is (like in our own study) probably too early (270 minutes). The role of the equation to correct for the early-compartment has no role in this discrepancy. Several data suggested that concordance between plasma and urinary clearances are higher in low GFR when the last sample is later (6 to 24 hours).^3,4 We have no proof that the Chantler correction would be “better” in terms of GFR if plasma clearance would be obtained with an adequate late sampling. Last, in our opinion, it remains difficult to recommend a method that is (maybe) “better” in low GFR range, but that is “worse” in high GFR ranges.