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Background. In organ transplanted patients, impaired renal function is of major prognostic importance and influences therapeutic decisions. Therefore, monitoring of renal function with glomerular filtration rate (GFR) is of importance, both before and after heart transplantation (HTx). The GFR can be measured directly (mGFR) or estimated (eGFR) with equations based on circulating creatinine or cystatin C levels. However, these equations have not been thoroughly validated in the HTx population. Methods. We investigated the correlation, agreement and accuracy between mGFR (using 51Cr-ethylenediaminetetraacetic acid or iohexol clearance) and three commonly used eGFR equations (Modification of Diet in Renal Disease, Cockcroft-Gault and Chronic Kidney Disease Epidemiology Collaboration) in a retrospective analysis of 416 HTx recipients followed between 1988 and 2012. Comparisons were performed prior to transplantation and at 1, 5 and 10 years of follow-up. Results. The correlations between eGFR and mGFR were only moderate, with r-values ranging from 0.55 preoperatively to 0.82 during follow-up. Most importantly, the level of agreement between eGFR and mGFR was very low for all three estimates, with percentage errors ranging from 93.3 to 157.3%. Also, the percentage of patients with eGFR within 30% of mGFR (P30) rarely reached the National Kidney Foundation recommended minimum level of 75%. Conclusion. We argue that the accuracy and the precision of the most commonly used estimation equations for assessment of kidney function are unacceptably low and we believe that mGFR should be used liberally as the basis for clinical decision-making both before and after HTx when eGFR is subnormal. [ABSTRACT FROM AUTHOR]