Is normalized residual activity a good marker of renal output efficiency?

Background: Output efficiency (OE) and normalized residual activity (NORA) are two parameters that allow quantifying the renal drainage at any moment of renographic acquisition. Although OE is theoretically more accurate than NORA in case of a decreased overall renal function, both parameters present some weaknesses.

Objectives: To compare both parameters and to evaluate whether the clinical information provided by both parameters is identical.

Methods: From a large database of Tc-99m mercaptoacetyltriglycine 3 renographic studies, 450 kidneys were selected covering a large range of ages, overall function, split function, and quality of drainage. NORA and OE were calculated at the end of the 20-min renogram, as well as on the late post erect postmicturition (PM) views.

Results: An inverse correlation was observed between NORA 20 and OE 20 (r=-0.926), as well as between NORA PM and OE PM (r=-0.936). Discrepancies were noted in approximately 10% of the kidneys, but main discrepancies, which would result in a different estimation of the quality of drainage, were only observed in 2% of the kidneys. There was no bias in the discordances; OE could reveal a better as well as a worse quality of drainage than NORA. It is likely therefore that imperfections of both parameters might be the cause of the divergences. The stratification of the kidneys according to age, overall renal function, split function, or quality of drainage did not modify the results.

Conclusion: NORA, being much easier to program, can replace the output efficiency in the evaluation of renal drainage.