Does a fecal occult blood (FOB) stool test only show positive for blood from the upper gastrointestinal (GI) tract?

From the Guidelines

A fecal occult blood stool test, specifically the immunochemical test (FIT), is more specific for detecting blood from the lower gastrointestinal tract, but it can still detect blood from the upper GI tract, although with less sensitivity due to the degradation of globin by digestive enzymes in the upper GI tract. The concept of FIT detecting human globin, a protein that constitutes human hemoglobin, makes it more specific for human blood than guaiac-based tests 1. This specificity, combined with the fact that globin is degraded by digestive enzymes in the upper gastrointestinal tract, suggests that FIT is more specific for lower gastrointestinal bleeding, thus improving its specificity for colorectal cancer (CRC) 1. However, this does not mean that FIT cannot detect blood from the upper GI tract, but rather that its sensitivity for upper GI bleeding may be lower. Some of the key advantages of FIT over guaiac-based tests include its higher specificity for human blood, lack of false-negative results in the presence of high-dose vitamin C supplements, and less demanding sample collection procedures 1. In clinical practice, a positive FOBT result, including FIT, indicates the presence of blood somewhere in the GI tract, but further diagnostic procedures such as colonoscopy, upper endoscopy, or other imaging studies are typically needed to determine the specific source of bleeding. Key points to consider when interpreting FOBT results include:

• The test's sensitivity and specificity for detecting blood from different parts of the GI tract
• The potential for false-negative or false-positive results
• The need for further diagnostic testing to determine the source of bleeding
• The importance of considering the patient's medical history, symptoms, and other diagnostic findings when interpreting FOBT results.

From the Research

Fecal Occult Blood Test Sensitivity

• The sensitivity of fecal occult blood tests (FOBT) in detecting upper gastrointestinal (GI) bleeding has been studied, with results indicating that these tests can detect small amounts of blood from the upper GI tract 2.
• A study published in 1999 found that FOBTs, including Hemoccult II SENSA, were able to detect upper GI blood in healthy volunteers who ingested small amounts of their own blood mixed with tomato juice 2.
• The study found that the proportion of positive tests during and immediately after the period of blood ingestion was greatest in the group ingesting 20 ml of blood per day, with 64% of SENSA tests and 67% of HemoQuant tests being positive 2.

Upper GI Bleeding Detection

• The ability of FOBTs to detect upper GI bleeding is important, as many upper GI tract lesions can bleed small quantities of blood that may not be detected by other means 2.
• A systematic review published in 2010 found that a significant number of individuals with a positive FOBT and negative colonoscopy did not have an identifiable lesion in the lower GI tract, suggesting that upper GI bleeding may be a possible cause of the positive FOBT result 3.
• However, the review found that the current evidence is insufficient to recommend routine esophagogastroduodenoscopy (EGD) as a means of detecting gastric or esophageal cancers in patients who are FOBT positive and colonoscopy negative 3.

Test Specificity

• Immunochemical tests, on the other hand, did not detect upper GI blood in any of the blood ingestion groups in the 1999 study, suggesting that these tests may be more specific for lower GI bleeding 2.
• The use of a combination of a highly sensitive guaiac-based FOBT and an immunochemical test may help differentiate occult upper from lower GI bleeding 2.