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Red cell scan

The radionuclide labelled red cell scan is a highly sensitive investigation and can detect bleeding rates as low as 0.1 ml/min.
What are the indications of red cell scan?

  • Upper GI bleed: limited role due to the widespread use of endoscopy as a first-line modality for such bleeding.
  • Lower GI bleed: Tc-99m RBC scintigraphy plays a larger role in the evaluation of lower GI bleeding due to the limited sensitivity of endoscopy within the lower GIT and has typically been used as a screening examination to identify patients who require angiography or surgery.
  • Obscure GI bleed

How is red cell scan performed?

  • Blood is taken from the patient. The red blood cells are separated from the rest of the blood sample and then mixed with the radioactive material. The red cells are tagged with 99m Technetium and injected back in the patient
  • A second method involves injection of the radioactive material directly in the vein. This attaches to the red cells in the circulation.
  • Images are subsequently obtained using the gamma camera.

What are the advantages and disadvantages of red cell scan?
Advantages

  • Non-invasive
  • More sensitive than angiography (detects bleeding at a rate more than 0.5ml/min). Tc-99m RBC scintigraphy is 93% sensitive and 95% specific for detecting a bleeding site with active arterial or venous bleeding rates as low as 0.04 mL/min (Zuckier LS. Semin Nucl Med 2003) anywhere within the gastrointestinal tract.
  • Patients can be scanned up to 24 hrs after injection to detect intermittent bleeding.

Disadvantages

  • Anatomical localisation is insufficiently accurate. Peristalsis can cause antegrade or retrograde movement of the radioisotope within the bowel, further increasing the difficulties in localisation.

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