Fecal occult blood tests help to determine the presence of occult blood (hidden GI bleeding) and to distinguish between true melena and melena-like stools. Certain medications, such as iron supplements and bismuth compounds, can darken stools so that they resemble melena.

The guaiac fecal occult blood test Hemoccult SENSA has greater sensitivity for cancer and advanced adenomas than does the Hemoccult II. Both tests produce a blue reaction in a fecal smear if occult blood loss exceeds 2 to 3 mL in 24 hours. A newer test called the fecal immunochemical test (FIT) detects human globin, a protein that—along with heme—constitutes human hemoglobin. This makes the FIT more specific for human blood than guaiac-based tests, such as the Hemoccult SENSA. The FIT is more specific for lower GI-type bleeding, improving its specificity for detecting colorectal cancer. Some FITs also require fewer samples and less direct handling of stool.

Occult blood tests are particularly important for early detection of colorectal cancer because 80% of patients with this disorder test positive. However, a single positive test result doesn’t necessarily confirm GI bleeding or indicate colorectal cancer. To confirm a positive result, the test must be repeated at least three times while the patient follows a special diet, according to the manufacturer’s recommendations for the particular occult blood test. Even then, a confirmed positive test doesn’t necessarily indicate colorectal cancer. However, it does indicate the need for further diagnostic studies because GI bleeding can also result from noncancerous causes, such as ulcers, colon polyps, and diverticula.

These tests are easily performed on collected specimens or smears from digital rectal examination.