Occult blood (clinical analyzes)

Content

Introduction

The examination (test) for the search for occult blood in the faeces (called SOF or FIT, from the English Fecal Immunochemical Test) research, through different chemical methods or specific laboratory analyzes, for the presence of tiny quantities of blood (called, in fact, occult blood) in a small stool sample.

The causes of bleeding are numerous and not always obvious. The test is usually prescribed by the attending physician to investigate the origin of anemia, the presence of an inflammatory disease or cancer of the small intestine or colon.

Highlighting the presence of blood not visible to the naked eye favors the discovery, from the initial stages (early) of their development, of a colorectal cancer or so-called intestinal polyps benign adenomatosis (quite common in people over the age of 50) which could give rise to colon cancer in 7-15 years. Since the presence of a small amount of blood in the stool is often the first, and sometimes the only, sign that allows us to suspect the presence of precancerous or cancerous lesions at an early stage, the test for the detection of occult blood is recommended for colon cancer prevention. Unless otherwise prescribed by a doctor, the Ministry of Health advises people between the ages of 50 and 69 to perform this test every two years, even in the absence of symptoms (symptoms).

Gods have been launched in various regions colorectal cancer screening programs which provide for the execution of the test every two years in people aged between 50 and 69-74 years.

The test

There are different types of tests for the detection of occult blood in the stool. They include the so-called guaiac test, some colorimetric tests "Over the counter" (that is to say they can be purchased in pharmacies without a doctor's prescription) and the immunochemical test which is the most widely used method for colorectal cancer screening in the European Union.

These types of analyzes differ in the method of collection and in the analysis of stool samples.

You can perform them in the privacy of your own home. The container allows, in fact, to collect in a very simple way small quantities (samples) of feces which will then have to be analyzed in the laboratory.

In relation to the sensitivity of the test, one or more (generally three) samples are required to be collected in subsequent days. The ways to do this vary according to the method used to carry out the exam:

  • guaiac test, detects traces of blood from any part of the intestine (stomach, small intestine and colon) and does not distinguish blood of human origin from blood of food origin. It involves the use of small strips of absorbent paper (card), previously treated with a chemical substance (hydrogen peroxidase) capable of producing a color change if the test is positive, that is, if it shows occult blood. The collection must be carried out at three different times (usually three consecutive days). Stool should be placed in a clean container avoiding contamination with urine and water. Your name and the date of collection must be written on each card. Then, using the applicator stick, a thin layer of feces must be spread in a specific area of ​​the card.When it has dried and become dry, the preparation remains stable for a few weeks even at room temperature. As soon as the collection of all consecutive samples is completed, the "cards" must be sent to the laboratory.
    There are various packs (Kits) of guaiac test commercially available, classified as low or high sensitivity.
    For the correct execution of the test, specific limitations in the diet and drugs taken are required because the test is able to detect the presence of blood of any origin. It is therefore necessary, in order not to compromise the reliability of the examination, to observe some precautions to avoid the introduction of blood into the digestive canal.
    To improve the quality of the test result and to avoid traces of blood in the stomach, it is advisable, where possible, to avoid or discontinue the use of some medicines (such as iodine or boric acid) or non-steroidal anti-inflammatory drugs (NSAIDs, such as aspirin and ibuprofen). They could, in fact, damage the lining of the stomach causing bleeding that would be detected by the test giving rise to a result positive. Regarding the diet, you should avoid drinking alcohol and eating certain foods (such as red meat, salami, fish, turnips, broccoli, cauliflower, apples, oranges, mushrooms and horseradish) which could be "mistaken" for blood by the chemicals present in the test, thus altering the result of the analysis (false positive). Before and during the examination, it is also advisable to avoid the consumption of vitamin C supplements and fruit juices rich in this vitamin, as they can interfere with the chemical reaction of the test by preventing the formation of color even in the presence of traces of blood in the stool (false negative).
    It is also advisable not to undergo dental treatment for up to three days before starting to collect stool samples, as blood from the gums (following dental procedures or gum disease) can also be detected by this test. .
    It is preferable to avoid collection of stool samples during the menstrual cycle, in the presence of bleeding hemorrhoids or when blood is lost in the urine
  • immunochemical tests (occult blood in the stool by immunological method or fecal immunochemical test, FIT). They represent an alternative and more recent type of test to detect the presence of occult blood in the stool. These tests use specific antibodies to detect human blood (anti-globin antibodies). They are very sensitive in detecting colon bleeding, but not that coming from the stomach and small intestine because globin is broken down as it passes through it. For this reason they are the recommended tests in the prevention of colon cancer and are prescribed when the your doctor suspects bleeding in this part of the intestine.
    The collection is carried out using a specific container, to be purchased at the pharmacy, accompanied by useful instructions for using it correctly. The container, sterile, is provided with a stick-teaspoon, fixed on the inside of a screw cap, to be used for the collection of feces. The stick, by rubbing it in 3-4 different points on the surface of the stool, allows the withdrawal of the quantity necessary for the execution of the test which will be delivered to the laboratory.
    In addition to having a greater diagnostic capacity, this type of analysis allows you to collect a single sample of stool and not three as, instead, required in the other tests.
    Furthermore, no specific diet or preparation is required; however, it is always best to avoid collecting stool samples during the menstrual cycle.
    To ensure that the test result is reliable, it is advisable, when collecting feces, to carefully follow the instructions included in the package, those provided by the doctor or by the healthcare staff of the reference analysis laboratory.

Results

The result of the occult blood test is, normally, negative. This means that there is no blood in the stool. On average, for every 100 people who take the test, five turns out positive, in other words, their faeces contain traces of blood or an "interfering substance" found in the gastrointestinal tract.

The guaiac test highlights the presence of bleeding from any part of the intestine and, therefore, from esophageal varices, ulcer, diverticulitis, polyps, hemorrhoids, chronic inflammatory bowel diseases, small anal wounds caused by constipation, blood loss from the gums following dental interventions or as a consequence of taking drugs that can cause gastrointestinal bleeding such as, for example, anticoagulants, aspirin, steroids. Eating certain foods such as red meat, fish or turnips in the three days before the test can also affect the test result as these foods can produce the same chemical reaction as human blood and cause the test to appear positive even if it is not. it is.

So-called results false negatives on the other hand, they can be caused by the consumption of large doses of vitamin C or by the collection of a few stool samples because in many conditions the bleeding of intestinal polyps and tumor lesions is not continuous and the blood, therefore, is not evenly distributed throughout the " Therefore, it is advisable to collect three different stool samples on three different days, in order to increase the chances of detecting bleeding.

The immunochemical test (FIT) shows only blood present in the intestinal tract called the colon. This type of test can also give false negative results due to intermittent bleeding.

If, through the examination of occult blood in the faeces, the presence of blood is detected, further investigations are necessary to determine its origin. The search for occult blood in the stool, in fact, can only highlight the presence or absence of blood, but does not indicate the causes of the bleeding.

The family doctor or specialist doctor will suggest the necessary investigations to ascertain the origin of the bleeding. In general, the execution of an endoscopic examination (gastroscopy and / or colonoscopy depending on the type of test used and the clinical suspicion) represents the first choice investigation (elective investigation) to ascertain the causes of the bleeding.

Bibliography

Italian Association for Cancer Research (AIRC). Colorectal Cancer Screening

Agostino Gemelli University Polyclinic Foundation - IRCCS. Catholic University of Sacred Heart. The examination of research of occult blood in the feces

Italian Auxological Institute - IRCCS. Search for occult blood in the stool

Bambino Gesù Pediatric Hospital. Occult blood in the stool

MedLine Plus. Fecal Immunochemical test (FIT) (English)

Humanitas Research Hospital. Blood in the stool

In-depth link

International Agency for Research on Cancer (IARC). Is it better to do the guaiac test for fecal occult blood (gFOBT) or the fecal immunochemical test (FIT)?

Ministry of Health. FAQ - Colorectal cancer

Emilia Romagna region. Emilia Romagna Regional Health Service. The right line is to prevent. Colorectal Cancer Prevention Screening Program. 2011

Robertson DJ, Selby K. Fecal Immunochemical Test: The World's Colorectal Cancer Screening Test. [Summary]. Gastrointestinal Endoscopy Clinics of North America. 2020; 30: 511-526 

Centers for Disease Control and Prevention (CDC). Colorectal Cancer Screening Tests

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