Fecal occult blood test

Hidden blood in stool

Normally, with proper preparation of the patient, occult blood in the stool is not detected. Gastrointestinal bleeding is a problem that practicing doctors often encounter. The degree of bleeding varies significantly, and the greatest difficulty is diagnosing minor chronic bleeding. In most cases, they are caused by cancerous diseases of the gastrointestinal tract. Colon tumors begin to bleed in the early (asymptomatic) stages of the disease, as a result of which blood enters the intestine.

To diagnose gastrointestinal bleeding, various screening tests are used to identify asymptomatic progression of the disease in apparently healthy people, which allows achieving a positive treatment result.

Normally, 1 ml of blood is excreted with feces per day (or 1 mg of hemoglobin per 1 g of feces). As it moves through the intestines, the blood is distributed in the feces and is broken down by enzymes (digestive and bacterial).

To detect hidden blood in feces, most clinics use benzidine or guaiac tests. Hidden blood is blood that does not change the color of feces and is not detected macro- or microscopically. Reactions for detecting hidden blood are based on the property of the blood pigment hemoglobin to accelerate oxidative processes. An easily oxidizable substance (benzidine, guaiac), when oxidized, changes color. Based on the rate of color appearance and its intensity, a weakly positive (+), positive (++ and +++) and strongly positive (++++) reaction are distinguished.

When prescribing a stool test for occult blood, special preparation of the patient is necessary (to avoid false positive results). Three days before the test, meat dishes, fruits and vegetables containing a lot of catalase and peroxidase (cucumbers, horseradish, cauliflower) are excluded from the patient's diet, ascorbic acid, iron preparations, acetylsalicylic acid and other non-steroidal anti-inflammatory drugs are canceled. To detect occult blood, it is recommended to test the feces after 3 consecutive bowel movements, with samples taken from two different places of the feces each time. When evaluating the results of the analysis, even one positive result should be considered diagnostically significant (even in cases where the rules for preparing the patient were not followed).

Reactions used to detect occult blood in feces have varying sensitivity. The benzidine reaction can only detect blood loss exceeding 15 ml/day, gives many false-positive results, and is currently almost never used. The most common test for detecting peroxidase activity in clinical practice is the guaiac test. Typically, when conducting this test, feces are applied to filter paper, and then guaiac reagent, acetic acid, and hydrogen peroxide are added to them. In this formulation, the method is very sensitive for detecting peroxidase activity, but is poorly standardized and often gives false-positive results. In this regard, tests were developed in which the guaiac reagent is pre-applied to a plastic strip, which made it possible to standardize the conduct of studies and diagnose even minor bleeding.

The frequency of positive guaiac test results depends on the amount of blood in the stool. The test is usually negative when the hemoglobin concentration in the stool is less than 2 mg per 1 g and becomes positive when this concentration increases. The sensitivity of the guaiac reaction at a hemoglobin concentration of 2 mg per 1 g of stool is 20%, at a concentration of more than 25 mg per 1 g - 90%. In approximately 50% of cases of colon cancer, the tumor "releases" enough blood to be detected by the guaiac reaction, the sensitivity of which in colorectal cancer reaches 20-30%. The guaiac test also helps in the diagnosis of colon polyps, but blood loss from polyps is significantly less, so the test for diagnosing this pathology is not sensitive enough (positive in about 13% of cases). Polyps of the distal colon (descending colon, sigmoid colon and rectum) give positive results in 54% of cases, proximal - in 17%.

The quantitative test "Hemoquant" (based on fluorescent detection of porphyrins in feces) has twice the sensitivity of the guaiac reaction, but it can be affected by eating meat and taking acetylsalicylic acid for 4 days before the test. Normally, the content of porphyrins in feces is less than 2 mg / g of feces; 2-4 mg / g - borderline zone; above 4 mg / g - pathology.

Given all these shortcomings of traditional screening tests, a completely new method for diagnosing gastrointestinal bleeding for early detection of colon cancer has been developed in recent years. We are talking about immunochemical tests (for example, the Hemoselect kits), which use specific antibodies to human hemoglobin. They detect only human hemoglobin in feces, so there is no need for dietary or medication restrictions when using them. The tests are highly sensitive - they detect even 0.05 mg of hemoglobin per 1 g of feces (usually values above 0.2 mg/g of feces are considered a positive test result). They do not detect bleeding from the upper gastrointestinal tract, which allows them to be used specifically for diagnosing tumor lesions of the colon. Immunochemical tests are positive in 97% of cases of colon cancer in a single study and in 60% of cases of adenomatous polyps larger than 1 cm. In 3% of cases, the tests can be positive in the absence of a tumor in the colon.

Experience of using immunochemical tests in foreign clinics shows that stool occult blood testing allows detecting colon cancer at early stages of development and leads to a 25-33% reduction in mortality. In addition, this test is an alternative to the endoscopic (colonoscopy) method of colon cancer screening. Regular stool occult blood screening leads to a 50% reduction in cases of colon cancer detection at the last stage of development.

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