NORA: a simple and reliable parameter for estimating renal output with or without frusemide challenge.

The aim of this study was to evaluate a simple parameter describing renal output, namely NORA (normalized residual activity). We first compared, in a simulated model, different parameters of transit to an ideal standard; we then compared, in a clinical study, NORA and output efficiency. 123I-hippurate, 99Tcm-DTPA and 99Tcm-MAG3 plasma curves, each with two levels of renal clearance, were convoluted by means of different types of simulated retention functions, with different mean transit times. On the reconstructed renograms, several parameters reflecting renal transit were determined and compared with mean transit time. In a second step, in 33 patients, we compared output efficiency and NORA (i.e. the residual renal activity), normalized by the renal activity at 2 min. These two parameters were calculated at the end of the renogram, at the end of the frusemide test and after the micturition phase. In the simulated model, both output efficiency and NORA were only slightly influenced by the level of overall renal function. In the clinical study, a good correlation was found between output efficiency and NORA, whatever part of the study considered (renogram, frusemide test, post-voiding image). NORA is a simple and reliable parameter that allows quantification of renal output; it is almost independent of the level of renal function and can be used whatever the timing of the frusemide injection.