Quick summary
Renal scintigraphy evaluates differential renal function, obstruction, renovascular hypertension, and transplant complications. Tc-99m MAG3 (tubular secretion) is preferred — works well even at reduced GFR. Tc-99m DTPA (glomerular filtration) is used for GFR calculation but requires adequate function.
Standard protocol: 3-phase (flow 1–2 min, function 20 min, excretion) + Lasix 0.3–1 mg/kg IV.
- F+20 protocol: Lasix at 20 min post-injection (standard)
- F−15 protocol: Lasix 15 min before injection — preferred when a dilated collecting system is suspected
Lasix Washout T½ Thresholds
| T½ After Lasix |
Interpretation |
Action |
| <10 min |
No obstruction — normal washout |
No further workup |
| 10–20 min |
Equivocal / indeterminate |
Ensure adequate hydration; consider F−15 protocol; clinical correlation |
| >20 min |
Obstructed |
Urology referral; correlate with symptoms and ultrasound |
Clinical Scenarios
| Scenario |
Key Scintigraphic Findings |
| UPJ obstruction |
Activity accumulates in dilated renal pelvis; little/no ureteral activity; delayed cortical transit time; T½ >20 min after Lasix |
| Renovascular hypertension (captopril renogram) |
Baseline MAG3 + repeat 1h after captopril 25–50 mg PO. Positive: asymmetric ↓ in function (>10%) or prolonged time-to-peak on captopril scan → significant renal artery stenosis |
| Renal transplant — ATN |
Good perfusion (flow phase normal) + poor tubular function (absent/markedly delayed excretion); improves over days |
| Renal transplant — Rejection |
Decreased perfusion AND decreased function; may have photopenic halo; worse prognosis than ATN |
| Urine leak (post-transplant) |
Focal accumulation of activity outside collecting system; pools dependently in pelvis |
| Split renal function |
Normal ~45–55% each kidney; <40/60 split = significant asymmetry; <35% = poor function → urology evaluation |
ATN vs. rejection on transplant scan: Both impair function, but ATN preserves flow — the flow phase is normal. Rejection impairs both flow AND function. This distinction guides management before renal biopsy.
MAG3 Pitfalls
| Pitfall |
Explanation |
| Dehydration |
Reduces Lasix response → falsely prolonged T½; hydrate 10 mL/kg IV NS before and during scan |
| Markedly dilated pelvis (reservoir effect) |
Large extrarenal pelvis retains tracer even without true obstruction; correlate with degree of hydronephrosis on ultrasound |
| Severely impaired GFR (<15 mL/min) |
Inadequate Lasix response; equivocal results common; MR urography may provide better anatomic and functional data |
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