Gastrointestinal bleeding scan (GIBS) is a non-invasive diagnostic radionuclide imaging study to evaluate patients with a suspected overt GI bleed, especially involving mid and lower gastrointestinal (GI) tract. It is performed with 99mTc-RBCs and helps determine the bleeding status (active or intermittent), gross localization, and estimation of the amount.[1]  Gastrointestinal bleeding (GIB) can be occult, overt, or obscure. Occult GIB has a positive guaiac test or iron deficiency anemia with no visible signs of bleeding. Overt GIB has signs of active bleeding such as melena, hematemesis, or hematochezia. Obscure GIB has persistent or recurrent bleed with no known source of bleeding after negative bidirectional endoscopy.

For purposes of geographical localization and potential intervention, GI bleeding typically classifies as upper, mid, and lower. Upper gastrointestinal bleed (UGIB) includes bleeding up to the level of the ampulla of Vater, which is within reach of esophagogastroscopy; this can identify major causes such as gastric and duodenal ulcers, esophageal varices, esophagitis, and gastritis. Mid GIB includes up to the level of the terminal ileum, which undergoes an evaluation with capsule endoscopy; this can diagnose etiologies such as Meckel diverticulum, angiodysplasia, and Crohn disease.[2] Most common causes of lower GIB within reach of colonoscopy are angiodysplasia, polyps, diverticulosis, inflammatory, and infectious colitis.  The clinical signs and symptoms of overt GI bleeding are often unreliable and can manifest late, especially if it is intermittent. Prompt and timely identification of GI bleed is essential for the next step in patient management, which includes CT angiography, Catheter angiography, surgical intervention, or observation.

GI bleeding scan uses 99mTc-RBCs and 99mTc-sulfur colloids.[3][4] Sulfur colloid has a short half-life of 3 minutes and is taken up by spleen, liver, and bone marrow. The sensitivity of exam is decreased, especially for an intermittent bleed due to background activity in the reticuloendothelial system and short imaging time (20 to 30 minutes). Multiple studies have shown that 99mTc-RBCs is more superior due to long intravascular half-life, which allows longer imaging duration.[5][6][7]

There are three ways of erythrocyte labeling with technetium 99m with different labeling efficiency.[4]

In vivo method:

Involves injecting 1 mg of stannous pyrophosphate followed by intravenous injection of 555-1, 110 MBq of 99m Tc-pertechnetate. Labeling efficiency ranges from 75 to 80%. It is not recommended but can be helpful in those patients who do not want to receive blood products.

Modified in vivo method:

Similar to in vivo method with the exception that it entails mixing a vile of blood with 555-1, 110 MBq of 99m Tc-pertechnetate, wait for 10 minutes and then injecting into the patient. Labeling efficiency is around 85 to 90%.

In vitro method:

This method involves drawing blood from the patient and added to a vile containing stannous pyrophosphate. After 5 minutes, sodium hypochlorite is added and then mixed with citrate buffer. 555-1, 110 MBq of 99m Tc-pertechnetate is admitted to the mixture and incubated and injected into the patient; this is a method of choice with labeling efficiency greater than 97%. This method uses a commercially available kit. 

Recommended radionuclide dosage[8][9][1][10]:

Adults: 15-30 mCiChildren[1]: 2.16-21.2 mCi (per EANM pediatric dosage card)

Image acquisition:

After injecting the patient with radiolabeled erythrocytes, imaging is acquired using a gamma camera with a 128 x 128 matrix. Dynamic images are obtained usually at 10 to 20 seconds/frame. A dual-head gamma camera increases sensitivity for localization. The imaging duration is variable but usually lasts 1 to 4 over. Initial imaging of a minimum of 60 minutes is the recommendation if no bleeding is detected.[11][12][13][14] The exam terminates upon identification of bleeding. Sometimes, the exam can be continued up to approximately 24 hours.[1] Also, SPECT or SPECT-CT images can be obtained to further localize bleeding site, especially in cases of equivocal planar image findings.[15][16][17]

The most common indication of GIBS is for a GI bleed that especially involving mid and lower GI tracts. It can detect GIB at a rate as low as 0.1mL/ml.[18][19] GIBS should not be used for chronic occult bleeds. Other indications are;

• To help identify the source of the obscure overt bleed
• Risk identification in GI bleeding
• Directing timely diagnostic angiography
• Assisting in angiographic interventional or surgical planning

• Active and intermittent small bowel bleeding
• Active and intermittent colonic bleeding
• Meckel diverticular bleeding
• Obscured overt GI bleeding
• Angiodysplasia
• AV malformation
• Intra-abdominal vascular tumors