Background: Assessing glomerular filtration rate (GFR) is of importance in the surveillance of renal transplant recipients. As serum markers alone are inaccurate for estimating GFR, several equations have been developed with the aim of translating a serum value into a corresponding and more accurate GFR. The present study investigated the diagnostic characteristics of GFR estimates obtained by the simplified MDRD formula and the cystatin C based estimate described by Larsson et al.
Methods: Prospective study in 29 stable renal transplant recipients. GFR was assessed with (125)I-Iothalamate clearance, creatinine was measured with a modified Jaffe method on Dimension RxL (Dade-Behring, Dudingen, Switzerland), cystatin C was determined by particle enhanced turbidimetric immunassay (PETIA; Dako, Glostrup, Denmark). Bias, precision and diagnostic accuracy of the two GFR estimates were assessed with Bland-Altman method and receiver-operating characteristics (ROC) analysis. The latter was performed at a GFR cut-off of 60 ml/min/1.73 m2.
Results: The cystatin C based GFR estimate normalized to a body surface area of 1.73 m2 exhibited a bias of -4.7 ml/min/1.73 m2, the 95% limits of agreement were -25.5-16 ml/min/1.73 m2 with an AUC of 0.87. The MDRD estimates obtained from the original creatinine revealed biased results. Thus, non-constant recalibration of creatinine was done. Recalibrated creatinine gave an MDRD GFR estimate with a bias of 1.7 ml/min/1.73 m2. The limits of agreement were -23.1-26.4 ml/min/1.73 m2. ROC analysis revealed an AUC 0.8 and was not significantly different from the cystatin C based GFR estimate.
Conclusions: In renal transplant recipients, the cystatin C based GFR estimate exhibits similar diagnostic characteristics like the simplified MDRD formula. In contrast to cystatin C measurement, recalibration of creatinine might be necessary before implementing the simplified MDRD formula into clinical routine.