Gastrointestinal bleeding localisation

Gastrointestinal bleeding scintigraphy (GIBS) detects and localises active bleeding into the bowel lumen using the patient's own red blood cells labelled with Tc-99m, exploiting the tracer's long intravascular half-life to allow continuous imaging of the entire gastrointestinal tract for many hours, unlike single-timepoint techniques such as CT angiography. It is used for both overt bleeding (melena, haematochezia) and, less often, occult bleeding, and is particularly valuable because gastrointestinal bleeding is frequently intermittent, so a technique that can watch continuously for extended periods has a real advantage over a snapshot study.

The SNMMI/ACNM (and previously SNMMI/EANM) procedure standard specifies an administered activity of 555-1,110 MBq of Tc-99m-labelled red blood cells (labelled in vitro, in vivo, or by a modified in vivo technique), dynamic imaging beginning immediately after injection at no more than one frame per 60 seconds, and a minimum of 60 minutes of initial imaging if no bleeding is seen, extendable up to 24 hours by rescanning if there is ongoing clinical suspicion. The diagnostic criteria require all three of: focal activity appearing outside expected blood-pool structures, a change in the intensity of that activity between images, and a pattern of movement consistent with peristalsis through the bowel - meeting only one or two of these criteria is not sufficient to call a positive study.

Because the test can detect bleeding rates as low as roughly 0.1 mL per minute in clinical use, it is generally more sensitive than catheter angiography (which typically needs faster bleeding, around 0.5-1 mL/min, to show extravasation) and is often used to decide whether, and where, to proceed to angiography and embolisation or, alternatively, direct the endoscopist or surgeon to the likely segment of bowel. It requires no patient preparation, has minimal radiation burden relative to repeated CT, and can be safely performed in haemodynamically unstable patients provided appropriate monitoring is in place.