Comparison of the Associations Between Normalized Residual Activity (NORA) and Classical vs. Area-Corrected Differential Renal Function (DRF) in Unilateral Hydronephrosis

Abstract

Objective: In unilateral hydronephrosis (HN), differential renal function (DRF) derived from DMSA scintigraphy reflects split renal function; however, its reliability in dilated kidneys remains controversial. Normalized residual activity (NORA), obtained from MAG3 scintigraphy, is considered a robust indicator of renal drainage. This study aimed to compare NORA with classical and area-corrected DRF methods to determine which better reflects renal excretory function.
Methods: A total of 63 patients with unilateral hydronephrosis who underwent both 99mTc-MAG3 and 99mTc-DMSA scintigraphy within one month were retrospectively analyzed. MAG3-derived parameters, including Tmax, T1/2, residual activity at 30 minutes (RA30), and NORA30, were calculated. Renogram patterns were classified into five categories according to diuretic response. DRF was measured using both classical and area-corrected methods based on DMSA scintigraphy. Correlations between drainage parameters, DRF measurements, hydronephrosis grade, and DMSA uptake were evaluated.
Results: MAG3-derived drainage parameters (Tmax, T1/2, RA30, and NORA30) showed significant differences across renogram patterns and progressively worsened with increasing obstruction severity (all p < 0.001). Hydronephrosis grade correlated significantly with drainage parameters but not with any DRF measurement. Neither classical nor area-corrected DRF was associated with renogram patterns or hydronephrosis severity (all p > 0.05). Area-corrected DRF demonstrated the strongest correlation with visual DMSA uptake (r = −0.773, p < 0.001). Notably, several kidneys with supranormal classical DRF were reclassified as normal after area correction, and these discordant cases exhibited abnormal NORA30/RA30 values and obstructive renogram patterns.
Conclusions: Although area correction improves DRF estimation in asymmetric hydronephrotic kidneys, DRF alone does not adequately reflect excretory function. Drainage parameters, particularly NORA and RA30, provide a more reliable assessment of renal impairment. Integrating NORA into routine evaluation may improve functional assessment and clinical decision-making in unilateral hydronephrosis.

Keywords

• Hydronephrosis
• Differential renal function
• Normalized residual activity

Project Number

decision no: E.556168

Ethical Statement

This retrospective study was initiated with the approval of the Sakarya University Medical Faculty Clinical Research Ethics Committee (decision date: 26.01.2026, decision no: E.556168).

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