Radiopharmaceutical: Tc-99m labeled autologous RBCs, 20–25 mCi. In vitro labeling (UltraTag) preferred — highest labeling efficiency (~98%). In vivo labeling risks free Tc-99m pertechnetate causing gastric, thyroid, and renal activity that mimics bleeding.
Findings and Interpretation
| Finding | Interpretation |
|---|---|
| Focal activity appearing at or before 5 min, distinct from blood-pool structures, increasing over time and moving in direction of peristalsis | Active GI bleeding — localize to time of first appearance, not subsequent frames |
| Activity confined to liver, spleen, heart, great vessels, kidneys at all time points | No active bleeding at time of imaging; delayed images at 2h, 4h, 24h may detect intermittent bleed |
| Activity in renal collecting system and bladder on early images | Free Tc-99m pertechnetate (poor in vivo labeling) — non-diagnostic; repeat with in vitro UltraTag |
| Persistent focal activity that does not move or increase over time | Normal variant: penile blood pool, accessory spleen, hemangioma, uterus — not GI bleed; obtain multiple views |
Localize to earliest appearance — not later frames. Tagged blood travels rapidly antegrade and retrograde through bowel. Activity first seen in the cecum at 5 min may appear to be in the sigmoid on later images. Always record the time of first appearance for anatomic localization.
GI Bleed Scan Pitfalls
| Pitfall | Explanation |
|---|---|
| Intermittent / slow bleeding | Scan may be negative if bleeding has stopped; tagged RBCs have 24h half-life — delayed imaging at 2h, 4h, 24h can detect intermittent bleeds |
| Poor labeling (in vivo method) | Free Tc-99m → gastric, thyroid, renal activity → mimics bleed; always use in vitro UltraTag |
| Penile / midline pelvic blood pool | In men: midline pelvic RBC pool can mimic rectal bleed; obtain lateral views; void and re-image |
| Upper GI bleed | Rapid transit can mimic lower GI; upper GI bleed activity appears in stomach first, then moves distally within minutes |
| Liver / spleen overlapping bowel | Hepatic and splenic flexure bleeds may be obscured; SPECT/CT adds anatomic localization |