Optimal time window for measurement of renal output parameters.

Although normalised residual activity (NORA) and output efficiency (OE) are usually measured at a fixed time t, their dependency on t may affect the prediction of mean transit time (MTT). This study aimed to evaluate their degree of dependency on t and to determine an optimal time of measurement by assessment of their relationship with MTT for various times t. A simulation model generated 232 cortical renograms by convolving one plasma disappearance curve with 232 created cortical retention functions. The results show that considerable changes are observed for NORA and OE, depending on the time of measurement t. The choice of this time significantly influences the predictive value of these parameters for estimating MTT. The optimal time for measurement of NORA and OE should be close to the MTT, at the moment when emptying takes place. In the clinical practice, it should be adapted to the clinical problem under investigation.