Percent of Tracer Clearance at 40 Minutes in MAG3 Renal Scans Is More Sensitive Than T1/2 for Symptomatic Ureteropelvic Junction Obstruction

Abstract

To increase the diagnostic sensitivity of standard MAG3 diuretic renal scans for ureteropelvic junction obstruction (UPJO) by exploring the utility of an alternative measurement P40, the percentage of maximal tracer counts present at 40 minutes. Patients with strong clinical and anatomic evidence for UPJO may have a normal T1/2, making definitive diagnosis difficult. We reviewed the charts of 142 consecutive patients who underwent successful laparoscopic or robotic-assisted laparoscopic pyeloplasty for UPJO between 2005 and 2015. Both pre- and postoperative renal scan images were available for 37 symptomatic patients with primary unilateral UPJO and 2 kidneys. We defined P40 as the percentage of maximal tracer counts present at 40 minutes. We identified the upper limit of normal (97.5th percentile, +2SD) for P40 using the preoperative renal scans from the unaffected kidney. We compared the sensitivity of P40 to T1/2 to identify symptomatic UPJO. In our cohort, 51% of symptomatic patients (n = 19) had a normal T1/2 (median 8.9 minutes; interquartile range: 7.5 minutes) and 49% (n = 18) had an abnormal T1/2 (median: 40 minutes; interquartile range: 0 minute). None of the patients had an abnormal P40 on their unaffected kidney. All patients with an abnormal T1/2 also had an abnormal P40. P40 increased the sensitivity of the renal scan from 49% (n = 18 of 37) to 73% (n = 27 of 37) when compared to T1/2. The majority of patients (95%) demonstrated an improvement in P40 after pyeloplasty. P40 markedly increases the sensitivity of a renal scan for diagnosing symptomatic UPJO and may be another valuable marker in addition to T1/2 to document functional improvement in drainage after pyeloplasty.