Fecal occult blood test

Hidden blood in the feces

Normally, with proper preparation of the patient, the latent blood in the stool is not detected. Bleeding from the digestive tract is a problem often encountered by practical doctors. The degree of bleeding varies significantly, and the greatest difficulty is the diagnosis of small chronic bleeding. In most cases, they are caused by gastrointestinal cancers. Tumors of the large intestine begin to bleed on the early (asymptomatic) stages of the disease, as a result of which the blood enters the gut.

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

In norm with feces 1 ml of blood per day (or 1 mg of hemoglobin per 1 g of feces) is allocated. As you move through the intestines, the blood is distributed in the feces and decomposes under the action of enzymes (digestive and bacterial).

To detect the hidden blood in the feces, most clinics use benzidine or guaiac test. Hidden is the blood that does not change the color of the stool and is not detectable macro- and microscopically. Reactions for detecting hidden blood are based on the property of the blood pigment hemoglobin to accelerate the oxidative processes. The easily oxidizable substance (benzidine, guaiac), oxidizing, changes color. The speed of the appearance of coloration and its intensity distinguish between a slightly positive (+), positive (++ and +++) and a sharply positive (++++) reaction.

When the study of feces for occult blood is prescribed, special patient preparation is necessary (in order to avoid false positive results). For 3 days before the study, meat dishes, fruits and vegetables containing a lot of catalase and peroxidase (cucumbers, horseradish, cauliflower), exclude ascorbic acid, iron preparations, acetylsalicylic acids and other non-steroidal anti-inflammatory drugs are excluded from the patient's diet. To detect hidden blood, it is recommended to examine the stool after 3 consecutive defecations, each time taking samples from two different feces. When evaluating the results of an analysis, even one positive result should be considered as a diagnostic test (and in cases where the rules for preparing the patient were not observed).

The reactions used to detect latent blood in the feces have a different sensitivity. The reaction with benzidine allows us to detect only blood loss exceeding 15 ml / day, gives a lot of false positive results and is practically not used at present. The most common test for the detection of peroxidase activity in clinical practice is a guaiac test. Usually, during this test, the stools are applied to the filter paper, and then the guaiac reagent, acetic acid and hydrogen peroxide are added thereto. In this formulation, the method is very sensitive to the detection of peroxidase activity, but it is poorly standardized and often gives false-positive results. In this regard, tests have been developed in which the guaiac reagent was preliminarily deposited on a plastic strip, which allowed the standardization of studies and the diagnosis of even minor bleedings.

The frequency of positive results of a guaiac test depends on the amount of blood in the stool. The test is usually negative with a hemoglobin concentration in the feces of less than 2 mg per gram and becomes positive when the 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 gram, 90%. In approximately 50% of cases of colon cancer, the tumor "secretes" enough blood to reveal its guaiac reaction, the sensitivity of which with colorectal cancer reaches 20-30%. Guyacin test also helps in the diagnosis of colon polyps, but blood loss from polyps is much less, so the test for the diagnosis of this pathology is not sensitive enough (positive about 13% of cases). Polyps of the distal part of the large intestine (descending part of the colon, sigmoid and rectum) give positive results in 54% of cases, proximal - in 17%.

The quantitative test "Hemokvant" (based on the fluorescent detection of porphyrins in feces) has twice the sensitivity as compared with the guaiac reaction, but it can be influenced by eating meat with food and taking acetylsalicylic acid for 4 days before the analysis. Normally the content of porphyrins in stool is less than 2 mg / g of feces; 2-4 mg / g - border zone; higher than 4 mg / g - pathology.

Considering all these shortcomings of traditional screening tests, in recent years a completely new method for diagnosing bleeding from the gastrointestinal tract for the early diagnosis of colon cancer has been developed. These are immunochemical tests (for example, "Hemoselect" kits), in which specific antibodies to human hemoglobin are used. They allow us to detect only human hemoglobin in the stool, therefore, when using them, there is no need for restrictions in the nutrition and administration of medications. The tests have a high sensitivity - they even show 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 reveal bleeding from the upper sections of the digestive tract, which allows them to be used purposefully for the diagnosis of tumor lesions of the large intestine. Immunochemical tests are positive in 97% of cases of colon cancer in a single study and in 60% in adenomatous polyps larger than 1 cm. In 3% of cases, tests can be positive in the absence of a tumor in the large intestine.

The experience of using immunochemical tests by foreign clinics shows that the study of feces for latent blood 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 for screening colon cancer. Regular screening of feces for latent blood leads to a reduction in the incidence of colon cancer at the last stage of development by 50%.

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