The influence of renal function on normalized residual activity.

OBJECTIVE: To quantify the influence of overall renal function on normalized residual activity (NORA) and to evaluate the modifying factors. METHODS: A computer simulation model generated renograms by convolution of plasma disappearance curves with artificially created retention functions. From a database, 1099mTc-MAG3 plasma curves were selected, corresponding to renal clearances ranging from 33 to 405 ml x min(-1). The retention functions had three properties: (1) until the minimal transit time (MinTT), no output and a linear increase in transit time after MinTT; (2) a ratio of MinTT to mean transit time (MTT) equal to 0.3 or 0.8; and (3) a MTT between 3 and 60 min, increasing in steps of 1 min. The model generated 1160 renograms and for each of them the NORA was calculated at 20, 40 and 60 min. For each value of MTT, the coefficient of variation (CV) of the NORA was calculated at 20, 40 and 60 min. RESULTS: For the same retention function, different clearances resulted in different values of NORA. The degree of variability of NORA depended on several factors, including the time of measurement, the MTT and the MinTT to MTT ratio. For clearances between 100 and 400 ml x min(-1), the CV of the NORA ranged from 15% to 30%, whilst for clearances below 100 ml x min(-1), the CV of the NORA ranged from 22% to 67%. Little influence of the MinTT to MTT ratio was observed. CONCLUSION: NORA is influenced by overall renal function. The influence is limited for 99mTc mercaptoacetyltriglycine (99mTc-MAG3) renal clearances over 100 ml x min(-1), whilst for clearances less than 100 ml x min(-1), the NORA is influenced much more.