Gastrointestinal (GI) bleeding is separated into two categories based upon where the bleeding occurs in relationship to the Ligament of Treitz. Upper GI bleeding occurs above this ligament and lower GI bleeding occurs below. Methods employed in diagnosing the location of GI bleeding differ based upon the location of the bleeding. A patient's symptom history usually guides the clinician to the most probable source of bleeding. If a clinician has determined that the site of bleeding originates above the Ligament of Treitz, he/she will place a nasogastric tube. If that clinician aspirates reddish or coffee ground material, a diagnosis of upper GI bleeding is confirmed and is 90% accurate. Diagnosing lower GI bleeds is not always as straight forward.

Lower GI bleeds have different characteristic findings based on their origins below the Ligament of Treitz. Diagnostic procedures such as colonoscopy, anoscopy and sigmoidoscopy can provide information as to the location and etiology of these bleeds. These methods require direct visualization of the affected bowel. Diagnosis becomes problematic when the patient is briskly bleeding because the bleeding site may be obscured due to the blood. Other means of detecting active bleeding site include angiography and GI bleeding scans. Angiography is less sensitive in detecting the bleeding site than GI bleeding scan, and it requires higher rate of bleeding (0.5-1.0 mL/min) to detect than GI bleeding scan. GI bleeding scan, using Tc-99m sulfur colloid or Tc-99m-UltraTag kit, only requires approximately 0.05-0.1 mL/min. Poor spatial resolution limits the diagnostic ability of GI bleeding scan to establish the exact location of lower GI bleeding. GI bleeding scan will be negative unless there is active bleeding in the patient. Observing that the GI bleeding scan is positive is not sufficient information for the referring clinician (and angiographer). It is imperative for the nuclear medicine physician to localize the initial site of accumulating intraluminal radiotracer activity as precisely as possible since it represents the origin of hemorrhage.