A bleeding scan is a nuclear medicine imaging test used to find the source of bleeding inside your gastrointestinal (GI) tract. It works by tagging your own red blood cells with a small amount of radioactive tracer, then tracking where those cells go using a special camera. If blood is leaking somewhere in your stomach or intestines, the tracer accumulates at that spot and shows up on the images. The scan is particularly useful for bleeding in the middle and lower portions of the GI tract, where a standard scope can’t easily reach.
How the Scan Works
A small sample of your blood is drawn, mixed with a radioactive tracer (technetium-99m), and injected back into your vein. Once the labeled red blood cells are circulating, a gamma camera positioned over your abdomen captures images showing where blood is flowing. If there’s an active bleed, the tracer escapes from the blood vessels and pools in the intestine, creating a bright spot that moves through the bowel over time.
The scan can detect bleeding rates as low as 0.1 to 0.5 milliliters per minute, making it one of the most sensitive tools available for catching slow or intermittent GI bleeds. By comparison, you’d need to be bleeding faster for other imaging methods to pick it up reliably.
What It’s Used For
The primary reason doctors order a bleeding scan is to locate the source of an active GI bleed, especially when the bleeding site isn’t obvious from an upper or lower endoscopy. It’s commonly used for:
- Mid and lower GI bleeding that colonoscopy or upper endoscopy can’t pinpoint
- Obscure but visible bleeding, where you’re passing blood but the origin is unclear
- Intermittent bleeding, since the scan can continue imaging over hours to catch bleeds that start and stop
- Planning next steps, such as guiding the timing of a more targeted procedure like angiography or surgery
A bleeding scan is not appropriate for chronic, hidden (occult) blood loss, like the kind detected through a stool test. It’s designed for situations where bleeding is actively happening or strongly suspected.
What to Expect During the Procedure
The exam typically takes about three hours, though the actual time you spend lying still under the camera varies. After the injection, imaging usually begins right away and continues at intervals. You’ll lie on your back on a table while the gamma camera takes pictures of your abdomen. The process itself is painless aside from the needle stick for the blood draw and reinjection.
One of the advantages of this test is that it requires little to no special preparation. You generally don’t need to fast or do a bowel prep, and acutely ill patients tend to tolerate it well. Your doctor may ask you to hold certain medications, so mention anything you’re taking, including supplements.
If the initial images don’t show a bleed, the labeled red blood cells remain in your circulation for hours. Doctors can bring you back to the camera for delayed images, sometimes up to 24 hours later, to catch bleeding that wasn’t active during the first round of imaging. In some cases, a second injection of labeled red blood cells is given to get clearer delayed images.
How Accurate Is It?
A large meta-analysis comparing bleeding scans to CT angiography found similar sensitivity for both: about 84% for the bleeding scan and 83% for CT angiography. CT angiography had a slightly higher specificity (90% versus 84%), meaning it produced fewer false alarms. In practice, the two tests serve somewhat different roles. CT angiography gives a fast snapshot and works well for brisk, active bleeding, while the bleeding scan’s ability to image over a longer window makes it better suited for slower or on-and-off bleeds.
The radiation exposure from a bleeding scan is lower than a three-phase CT angiography, which can be a consideration if multiple imaging studies are needed.
False Positives and Limitations
The scan isn’t perfect. Several things can mimic a GI bleed on the images, leading to false positives. The tracer can show up in places that have nothing to do with intestinal bleeding: the kidneys, bladder, uterus, liver hemangiomas (benign clusters of blood vessels), or enlarged abdominal blood vessels. Splenosis (small implants of spleen tissue scattered in the abdomen after a prior injury) and pancreatic pseudocysts can also light up on the scan.
Most of these false positives appear as fixed spots that don’t move through the bowel over time, which helps experienced readers distinguish them from true GI bleeds. Still, the possibility of a misleading result is one reason why a positive scan often leads to a confirmatory procedure rather than immediate surgery.
What Happens After a Positive Scan
If the scan identifies a likely bleeding site, the next step is usually formal angiography. In this procedure, a catheter is threaded through a blood vessel to the suspected area. Angiography serves a dual purpose: it confirms the bleeding location and, in many cases, allows treatment on the spot through embolization (blocking the leaking vessel). A positive bleeding scan significantly improves the chances that angiography will find and treat the bleed successfully.
If angiography can’t reach or treat the problem, surgery may be considered. This is more common when the bleeding comes from something that needs to be physically removed, like a Meckel’s diverticulum (a small pouch in the intestine some people are born with) or a mass. Sometimes a combination of catheter-based treatment and surgery is used.
A negative scan is also useful information. If no bleeding is detected, the likelihood that a follow-up angiography would find anything is very low, which can spare you an invasive procedure.