Timing of GFR Scan After Urinary Diversion
Renal functional imaging with MAG3 scan should be performed at 6 to 12 weeks after urinary diversion, with 6 weeks representing the earliest acceptable timepoint when renal perfusion has adequately recovered to provide meaningful functional assessment. 1
Why 6 Weeks is the Minimum Timeframe
Immediate post-diversion imaging is unreliable because kidneys demonstrate low uptake of Tc-99m MAG3 and slower cortical transit times during the first 6 weeks after relief of obstruction, making earlier studies unable to predict long-term outcomes. 1
Renal immaturity and suboptimal blood flow in the immediate post-obstruction period lead to inaccurate results that either overestimate or underestimate true kidney function. 1
The 6-week timepoint specifically represents when renal perfusion has adequately recovered to provide meaningful functional data for clinical decision-making. 1
What the Scan Should Assess
Differential renal function parameters:
- <40% differential function in the affected kidney suggests significant impairment and may indicate need for surgical intervention. 1
- >5% decline in differential function on consecutive scans indicates deteriorating function requiring intervention. 1
Drainage assessment:
- T½ >20 minutes on time activity curves indicates persistent obstruction requiring potential surgical intervention. 1
Critical Pitfalls to Avoid
Do not perform functional imaging before 6 weeks, as premature studies will underestimate recovery potential and may lead to unnecessary interventions. 1
Do not rely solely on ultrasound at this timepoint, as it cannot adequately assess differential function or quantify obstruction. 1
Ensure adequate hydration before the study, as dehydration can mask true drainage patterns and functional capacity. 1
Obtain at least two baseline measurements if using absolute GFR values to reduce inherent measurement variation. 2, 1
Clinical Context: Expected GFR Changes After Diversion
Significant GFR decline occurs after urinary diversion:
Median measured GFR decreased from 84.1 to 69.9 mL/min/1.73 m² at 6 months post-cystectomy, with 74% of patients experiencing significant decline. 3
In long-term follow-up (≥10 years), median GFR decreased from 65.5 to 57 mL/min/1.73 m² in ileal conduit patients and from 68 to 66 mL/min/1.73 m² in orthotopic bladder substitution patients. 4
Risk factors for greater GFR decline:
- Diabetes mellitus was more frequent in patients with highest tertile of relative GFR decline (44% vs. 11%). 3
- Urinary tract obstruction (ureteroileal stricture, stomal stenosis) was the leading independent risk factor for renal function deterioration in both ileal conduit (58% with obstruction had deterioration) and bladder substitution patients (37% with obstruction had deterioration). 4
- Pre-operative weight was independently and negatively associated with post-operative measured GFR. 3
Follow-Up Strategy After Initial 6-12 Week Scan
If the scan shows stable or improving function with adequate drainage:
- Repeat functional imaging should be performed at 3-6 month intervals to monitor trends. 1
Serial functional studies are superior to single timepoint assessments because they can detect stabilization of previously declining GFR, which represents treatment success even without absolute improvement. 2, 1
Continue surveillance until function stabilizes or clinical decision regarding definitive surgical management is made. 1
Worsening drainage on serial imaging despite adequate diversion warrants surgical correction. 1
Measurement Considerations
Early assessments of functional outcome should be performed with creatinine values obtained 1 week after intervention because measurements obtained immediately after procedures may be transiently affected by radiocontrast or periprocedural hydration. 2
Radioisotope renal scans can provide differential renal function to estimate GFR, though actual GFR may be approximately 12% higher than predicted by renal scan. 2