Can modifications of the MDRD formula improve the estimation of glomerular filtration rate in renal allograft recipients?

Background: Two modifications of the MDRD equation [the Mayo Clinic (MC) equation and Rule's refitted (RR) MDRD formula] were proposed to overcome disadvantages of the original MDRD formula to calculate glomerular filtration rate (GFR). Additionally, a correction factor for the original MDRD formula has been introduced to adapt this formula to creatinine values measured by the isotope-dilution mass spectrometry (IDMS) method. Although precise determination of GFR is of central importance in renal transplant recipients, these equations have not been tested in these patients so far.

Methods: Considering the impact of different creatinine calibrations, we analysed the MC equation and the RR-MDRD formula in comparison with the old as well as the re-expressed (IDMS traceable) MDRD equation and the Cockcroft-Gault (C-G) formula in 126 consecutive patients after kidney transplantation with respect to correlation, bias, precision, accuracy and ROC analysis. GFR was determined as technetium-diethylenetriamine pentaacetic acid ((99m)Tc-DTPA-clearance).

Results: After adjustment to IDMS creatinine determination, the performance of the re-expressed MDRD formula improved considerably in comparison to the original MDRD equation. In comparison with the re-expressed MDRD formula bias of the MC formula and the RR-MDRD formula were significantly smaller (2.31 and -0.35 vs 3.82 ml/min/1.73 m(2)). However, precision and correlation of these formulae did not differ significantly from one another, but all equations showed a higher precision than the C-G formula (P < or = 0.006 each). The accuracies within 30% of true GFR of the MC (79.4%) and the RR-MDRD equation (84.9%) were significantly higher than those of the re-expressed MDRD formula (72.2%; P < 0.03).

Conclusion: In comparison to the original and the re-expressed MDRD formula, calculation of GFR by the MC equation and the RR-MDRD formula led to improved diagnostic performance in renal transplant recipients after adjustment of creatinine. In quotidian work both formulae can be applied to these patients. Nonetheless, to determine GFR exactly, gold standard techniques are mandatory.