Magnetic resonance imaging as a sole method for the morphological and functional evaluation of live kidney donors

Authors


Sir,

We read with great interest this article by El-Diasty et al.[1]. . The study included 50 consecutive kidney donors where the kidneys were imaged with Gd-enhanced dynamic MRI, which was also used for selectively determining the GFR of each kidney. All donors had a 99mTc-MAG3 renal scan as the reference standard to measure GFR. The authors concluded that Gd-enhanced dynamic MRI can provide accurate information about the anatomy of the urinary tract and vasculature of the kidney, and can be used to accurately estimate the selective GFR of each kidney. However, unfortunately there was a serious error in the study design. The authors used the MAG3 renal scan as the reference standard to measure GFR; MAG3 is a radiopharmaceutical which is taken up by the renal tubules and can only provide information about effective renal plasma flow (ERPF) [2]. It is not filtered through the glomerulus and hence GFR cannot be calculated using MAG3 [3]. To measure GFR the radiotracer should be freely filtered through the glomerulus, e.g. 99mTc-DTPA or 51chromium-labelled EDTA [4,5]. Moreover, there is no documented method of correctly predicting GFR from the ERPF values derivable from MAG3 scintigraphy. The ERPF values, from which the GFR values can be assumed to have been calculated, were again not obtained by the standard method of measuring plasma clearance of 99mTc-MAG3, a procedure that requires blood sampling [6]. Hence, the GFR values derived in the study are extremely unreliable and cannot be used as a reference in a evaluation study. The authors should have used the classical inulin clearance as the measure of true GFR. Currently radioisotope-based clearance methods have largely replaced inulin clearance and could have been used as the reference standard for GFR [7]. Unfortunately the authors did not use any of the accepted standard methods for obtaining GFR values, and hence in the presence of gross flaws in the adopted ‘gold standard’ for GFR in the current study, we express our reservations about the validity of the conclusion reached by the authors.

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