Fecal occult blood testing: preparation for analysis. What can you eat for reliable results? Diet before donating feces for occult blood - a few secrets in order to do it right

blood cells under a microscope

Normally, with proper preparation of the patient, latent blood in the feces is not detected. Bleeding from the gastrointestinal tract is a problem that practitioners often face. The degree of bleeding varies greatly, and the greatest difficulty is the diagnosis of small chronic bleeding. In most cases, they are caused by cancers of the gastrointestinal tract. Tumors of the colon begin to bleed in the early (asymptomatic) stages of the disease, as a result of which blood enters the intestine.

To diagnose bleeding from the gastrointestinal tract, various screening tests are used to identify asymptomatic course of the disease in apparently healthy people, which makes it possible to achieve a positive result of treatment.

Normally, 1 ml of blood is excreted with feces per day (or 1 mg Hb per 1 g of feces). As it moves through the intestines, the blood is distributed in the feces and undergoes decay under the action of enzymes (digestive and bacterial).

To detect occult blood in the feces, most clinics use benzidine or guaiac samples. Latent blood is called, which does not change the color of feces and is not determined macro- and microscopically. Reactions for the detection of occult blood are based on the property of the blood pigment Hb to accelerate oxidative processes. Easily oxidized substance (benzidine, guaiac), oxidized, changes color. According to the rate of appearance of staining and its intensity, weakly positive (+), positive (++ and +++) and sharply positive (++++) reactions are distinguished.

When prescribing a stool test for occult blood, special preparation of the patient is necessary (in order to avoid false positive results). 3 days before the study, 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 examine the feces after 3 consecutive bowel movements, and each time samples are taken from two different places of the feces. When evaluating the results of the analysis, even one positive result should be considered as diagnostically significant (and in cases where the rules for preparing the patient were not followed).

The diagnostic value of the occult blood test in terms of early detection of colon cancer depends on the amount of bleeding from the tumor. On average, blood loss from tumors of the caecum and ascending colon is 9.3 ml/day (from 2 to 28 ml/day) [Henderson DM, 1997]. With localizations distal to the hepatic flexure of the intestine, blood loss is much less and amounts to 2 ml / day. This difference may

but, due to the large size of tumors of the proximal part of the colon. Blood loss from an adenomatous polyp averages 1.3 ml/day, regardless of its location.

The reactions used to detect fecal occult blood have different sensitivities. The reaction with benzidine makes it possible to detect only blood loss exceeding 15 ml/day, gives many false positive results and is practically not used at present. The most common test for detecting peroxidase activity in clinical practice is the guaiac test. Usually, when conducting this test, stool masses 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 was pre-applied to a plastic strip, which made it possible to standardize the conduct of studies and diagnose even minor bleeding.

The rate of positive guaiac test results depends on the amount of blood in the stool. The test is usually negative when the concentration of Hb in the feces is less than 2 mg per 1 g and becomes positive when this concentration increases. The sensitivity of the guaiac reaction at a concentration of Hb 2 mg per 1 g of feces is 20%, at a concentration of more than 25 mg per 1 g - 90%. In about 50% of cases of colon cancer, the tumor "bleeds" enough blood to be detected by a 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 much 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 and rectum) give positive results in 54% of cases, proximal - in 17%.

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

Given all these shortcomings of traditional screening tests, in recent years a completely new method for diagnosing bleeding from the gastrointestinal tract has been developed for the early diagnosis of colon cancer. We are talking about immunochemical tests (for example, Hemoselect kits), which use specific antibodies to human Hb. They allow only human Hb to be detected in feces, therefore, when using them, there is no need to restrict diet and drug intake. The tests are highly sensitive - they detect even 0.05 mg Hb 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 purposefully for the diagnosis of tumor lesions of the colon. Immunochemical tests are positive in 97% of cases

colon cancer in a single study and in 60% - with adenomatous polyps larger than 1 cm. In 3% of cases, tests can be positive in the absence of a tumor in the colon.

The experience of using immunochemical tests by foreign clinics shows that the study of feces for occult blood makes it possible to detect colon cancer in the early stages of development and leads to a decrease in mortality by 25-33%. In addition, this test is an alternative to endoscopic (colonoscopy) screening for colon cancer. Regular screening of feces for occult blood leads to a 50% reduction in cases of detection of colon cancer at the last stage of development [Henderson DM, 1997].

A positive stool test for occult blood is possible with many diseases:

peptic ulcer of the stomach and duodenum;

Primary and metastatic tumors of the esophagus, stomach, intestines, duodenal papilla;

Intestinal tuberculosis, ulcerative colitis;

Invasions by helminths that injure the intestinal wall;

Expansion of the veins of the esophagus with cirrhosis of the liver and thrombophlebitis of the splenic vein;

Randu-Osler's disease with localization of bleeding telangiectasias anywhere in the mucous membrane of the digestive tract;

Typhoid fever (in patients with typhoid fever with positive test results for occult blood in the feces, macroscopic bleeding occurs much more often than with negative ones; although severe bleeding is possible without previous latent ones);

Ingestion of blood from the oral cavity and larynx into the digestive tract, with cracked lips, with accidental or intentional (for the purpose of simulation) suction of blood from the oral cavity and when it leaks in cases of nosebleeds;

Ingestion of blood from hemorrhoids and anal fissures into the feces;

Entry into the feces of menstrual blood.

The study of feces for occult blood is one of the laboratory tests that makes it possible to diagnose internal bleeding in one of the sections of the gastrointestinal tract with a high degree of probability. Particular attention is required to prepare for the analysis of feces for occult blood, since in its absence or improper conduct, false positive results can be obtained.

Indications for the purpose of the analysis

A study of feces for hidden blood in it is prescribed for the following patient complaints:

• persistent / intermittent abdominal pain;
• frequent symptoms of dyspeptic disorder - nausea, vomiting, heartburn;
• regular diarrhea;
• unexplained weight loss.

Testing of feces for the blood present in their composition is prescribed to confirm the diagnosis, for example, peptic ulcer or erosive gastritis. The overall goal of the analysis is to identify hidden damage to the mucous membranes of the stomach or intestinal tract.

Types of tests

In the presence of bleeding areas of the mucosa, blood may be present in human excrement. If the place of the opened bleeding is the stomach or duodenum, then the feces get a dark red color. When the surface of the large intestine is damaged, they become scarlet. But it is not always possible to see blood impurities in a person's stool. Very often, small ulcers bleed only intermittently.

Laboratory testing of feces for occult blood allows you to determine even the minimum amount of hemoglobin contained in it.

Research is carried out in two ways:

• Gregersen's method (benzidine test);
• immunochemical test.

The Gregersen test is quite informative, but only if the patient conducts thorough preparation.

The Gregersen technique makes it possible to detect even the minimum concentration of hemoglobin. This is both an advantage and a disadvantage of the method. Benzodine colors iron molecules blue, but it reacts to both human and extraneous hemoglobin (present in meat).

The immunochemical method is more accurate. Its main drawback is that it is rather long in time. Test results will be received only two weeks after the submission of the material for research. Therefore, in most cases, the delivery of feces for analysis according to the Gregersen method is prescribed.

Preparation for the delivery of the analysis according to Gregersen

To get the most reliable results, it is necessary to properly prepare for the delivery of feces. Recommendations will be as follows:

1. A week before the planned delivery, it is necessary to stop taking drugs such as laxatives, bismuth- and iron-containing, anti-inflammatory nonsteroidal drugs. It is forbidden to use acetylsalicylic acid and medicines containing it.
2. It is necessary to refuse the introduction of rectal suppositories.
3. Enema is prohibited.
4. Before analyzing feces for occult blood - two days before delivery - any instrumental tests of the gastrointestinal tract should be excluded. During the study, the mucosa may be accidentally damaged. Excessive blood may cause false positive results.
5. A few days before the collection of excrement, you should stop brushing your teeth. Even a small amount of blood released when the gums are damaged can enter the stomach. And this hemoglobin can also negatively affect the final result.

Before performing the analysis, the patient will be advised to follow a special diet. Its duration is 72 hours.

The main point of preparation for passing the analysis is strict adherence to the diet

The patient's diet should completely exclude foods high in iron. The collection of feces must be preceded by a refusal:

• from apples;
• cucumbers;
• white beans;
• spinach;
• horseradish;
• cauliflower;
• meat and fish dishes;
• offal;
• green vegetables.

Ideally, in order not to make a mistake with the menu, doctors recommend sticking to a milk diet on the days of preparing for the delivery of feces. The diet may also include:

• potato;
• bread;
• cereals (exception - buckwheat, oatmeal, cells, lentils, peas).

Collection of feces for research

Even carefully conducted preparation will be completely useless if the biological material is collected incorrectly.

There are special containers for transporting feces to the laboratory - they are sterile and guarantee the biological purity of the collected excrement

How to donate feces correctly? To obtain material suitable for research, the following recommendations must be observed:

1. A sterile container must be used to collect excreta. It can be purchased at any pharmacy. The set includes a jar with a lid and a special spoon.
2. First you need to empty the bladder. Then lay an oilcloth in the toilet.
3. After emptying the bowels, it is necessary to take a collection of three servings of feces from different places.

The material must be delivered to the laboratory within the next three hours. It is not subject to storage.

False results

Failure to follow the rules of preparation before donating feces can cause erroneous results. They can be either false positives or false negatives.

The main reason for false positive tests indicating the presence of internal bleeding is improper preparation.

False positive results are quite common. The reason is the negligent attitude of a person to the preparatory stage. In this case, the test shows a high hemoglobin content in the absence of internal bleeding. Only one apple eaten a day before the delivery of biological material can distort the final results.

Properly carried out preparation for the study of feces for occult blood in many cases allows you to avoid a very unpleasant colonoscopy procedure. The technique is an examination of the intestine by introducing equipment through the anus.

The indications for an occult blood test are:

· stomach ache;

heartburn;

· nausea;

· constipation;

· weight loss;

mushy stool

Decreased appetite

Anemia

· stomach ulcer;

· nonspecific ulcerative colitis;

· polyposis;

intestinal tuberculosis.

The patient is put on a three-day diet before an occult blood test. She excludes fish and meat, liver, green vegetables, white beans, spinach, pomegranates, apples, bell peppers, beets, horseradish, tomatoes and blueberries from the diet. However, the diet before testing for occult blood includes potatoes, cereals, dairy products, boiled eggs, butter, bread, and a small amount of fresh fruit.

Before donating feces for occult blood, the diet excludes not only the consumption of certain foods, but also the use of medications containing iron. These drugs include: bismuth and barium sulfate. They can change the color of the stool.

Also, before donating feces for occult blood, the diet, in addition to restrictions in certain foods, prohibits the use of laxatives, rectal suppositories and drugs that affect intestinal motility.

Two days before the analysis, it is undesirable to brush your teeth with a toothbrush, as this can damage the gums and blood from the oral cavity will enter the stomach.

Women during menstruation do not take an occult blood test.

It is impossible to put an enema on the eve of the test, as it can damage the mucous membrane and lead to incorrect results.

The admixture of urine to feces is unacceptable.

Following a diet before an occult blood test and following all of the above recommendations can ensure accurate test results.

Fecal occult blood test procedure

The collection of material for analysis occurs in the same way as in the general analysis of feces. As a container, use a glass or plastic container, or a special container.

Fecal sampling is carried out from four places of fresh feces, eliminating the ingress of urine into them. For the most reliable results, the stool should be delivered to the laboratory within three hours of a bowel movement.

More precise recommendations that are important for the diagnosis of a particular disease can be given by the attending physician. A similar analysis can be taken at a medical institution at the place of residence. In addition, you can take a fecal occult blood test at numerous medical centers and private laboratories at any convenient time. Test results will be ready within six days. The normal value of the result of a fecal occult blood test is negative.

Can the test result be false?

It happens that the analysis gives a false result. Most often, when the occult blood diet is not followed, or there is bleeding localized in the gums, a false positive test result is observed. Thus, strict observance of the occult blood diet is a prerequisite before taking the sample.

False negative test results are also possible. This happens in cases where the bleeding is not constant, but is periodically renewed.

In general, the sample is quite reliable, but tests must be taken twice to confirm the results. If all the conditions are met before donating feces for occult blood, the diet is observed, samples are taken at intervals of 3-4 days, and the result of the studies turned out to be the same, then it is reliable.

Positive test result

When the occult blood diet is followed, all the conditions for preparing for the test are met, and the result of the analysis gave a positive result - this may indicate ulcerative, oncological or inflammatory processes. A positive test is a serious reason for the diagnosis. If the doctor has written out a referral for an occult blood test, the diet must be strictly followed before taking it.

For timely diagnosis and in order to avoid aggravation of the course of the disease, the test must be passed at the first request in compliance with the prescribed preparation. The sooner a correct diagnosis is made, the sooner effective treatment can be prescribed.

Quantitative immunochemical test

To date, there is a relatively new method for determining occult blood - a quantitative immunochemical test. It is used in the diagnosis of colorectal cancer. Preparation for the test is minimal. To determine occult blood, you just need to take a stool test, and there is no need to follow a diet.

Diet is not needed for the reason that this study uses antibodies to hemoglobin. Such antibodies do not react to myoglobin contained in the eaten meat of animals. The test is specific, characterized by high sensitivity and accuracy. The absence of the need to follow a diet makes this method very convenient for patients.

An occult blood test makes it possible to diagnose a number of diseases even before they cause irreparable harm to the body.

Gastrointestinal bleeding can be very tragic if it is not detected in a timely manner. In the case of pronounced red feces, doctors have no doubts about the presence of internal pathological processes, but some diseases are almost asymptomatic.

And only the smallest particles of blood in the feces are a sign of a disorder in the activity of the gastrointestinal tract. Conducting such a study requires compliance with a number of rules to exclude a false result.

Types of fecal occult blood tests

Damage to the digestive tract can be localized both in the upper and lower sections. If bleeding has opened in the stomach or duodenum, then the feces become dark red, and in the large intestine - bright red.

But not with every disorder such a chair appears. Often small ulcers and inflammations only occasionally bleed. There are cases of tumor development, the only symptom of which is only a small admixture of hemoglobin in the feces.

Laboratory research makes it possible to determine the presence of blood in the intestine. Analyzes are carried out in two ways:

1. Gregersen's method (benzidine test).
2. immunochemical method.

Gregersen method shows the presence of hemoglobin even at very low concentrations. This is both an advantage and a disadvantage of analysis. In the presence of benzodine, the iron of the transport molecule is oxidized by hydrogen peroxide and becomes blue.

The reaction goes to hemoglobin, both human and foreign (contained in meat products). Therefore, to clarify the results, doctors often resort to the second method.

Immunochemical analysis kala is more accurate. It is based on the specific effect of antigens on antibodies characteristic of a particular person. The method is usually used for infectious diseases, but this does not affect the primary purpose of the study - the determination of blood in the feces.

And if it is additionally possible to establish the type of pathogenic microorganisms that provoked intestinal inflammation, the diagnostic measure can even be considered overfulfilled. The disadvantage of immunochemical analysis lies in its duration: only after 2 weeks, doctors make a diagnosis.

The delivery of stool tests for occult blood is not prescribed just like that. Usually the patient complains of abdominal pain or malaise. General indications for the test are as follows:

• recurrent or persistent abdominal pain;
• regular nausea, vomiting, diarrhea or heartburn;
• chronic stool disorder;
• rapid weight loss for no apparent reason;
• to confirm the diagnosis (for ulcers, gastritis, etc.).

In fact, the purpose of the analysis is to identify damage to the walls of the intestines and stomach. And if the doctor insists on a detailed study of feces, the patient should not refuse.

The test is carried out without any internal intervention, so it will not hurt. However, the high sensitivity of the reagents requires special preparation of the patient.

Preparation for fecal occult blood testing

Any research has a margin of error. But in order for the results to be reliable, it is necessary to follow a diet before analysis. First of all, you will have to give up meat and fish, because they contain hemoglobin.

Also, the list of restrictions includes iron-containing and all red foods (tomatoes, beets, etc.), which stain the feces of even a healthy person. Diet is key, but doctors call other requirements:

1. Cancellation of medications 1 week before analysis.
2. Ban on laxatives (including folk).
3. The day before the analysis, you should not brush your teeth (there is a risk of damage to the gums and distortion of the results).
4. The analysis is taken no earlier than 3 days after the X-ray examination.
5. Enemas are excluded (emptying should be natural).
6. Women are tested on days when there is no menstruation.

How to take feces correctly?

Not every person is able to comply with such a ban as "you can not eat." Preparation for analysis requires the mandatory implementation of this paragraph, otherwise the result of the study will be very doubtful. But the process of taking feces is also important:

• It is necessary to prepare a container (sold in a pharmacy).
• Feces are needed without any liquid (water, urine, etc.). To do this, it is allowed to lay an oilcloth in the toilet.
• After emptying with a teaspoon, 3 fragments of feces are taken.
• Samples should be delivered to the laboratory within 3 hours.

A small amount of blood still enters the intestine - 1-2 ml. This is normal only if such a volume is the daily limit.

Therefore, the analysis will always show the presence of blood in the stool, but can such a small amount be considered a sign of a disorder? No. Natural processes have not been canceled, and doctors take into account only larger indicators.

False results

To get reliable fecal occult blood test results, preparation should not be ignored. And if you treat this process negligently, the results are often false.

With both false negatives and false positives. For example, doctors know for sure that a patient has polyps in the large intestine, but the analysis does not detect hemoglobin.

This is due to the fact that such neoplasms do not bleed constantly, but only periodically. But it would be foolish to deny the presence of polyps, especially if detected with an endoscope.

False positive result is also a common occurrence. If the patient neglected the rules for preparing and taking feces, then it is quite possible that the study will show the presence of a high level of hemoglobin in the absence of internal bleeding. The analysis is usually carried out according to the Gregersen method, which is sensitive specifically to iron.

Having eaten only 1 apple the day before taking feces, a person will simply distort the results. In addition, blood can enter the intestines due to trauma to the gums with a toothbrush.

Thus, non-compliance with the rules will give doctors a reason to perform a colonoscopy, the essence of which is to insert a probe through the anus. So it’s better to undergo such an unpleasant procedure not because of your own negligence!

What does a positive result mean?

And yet, the analysis can show the presence of a high level of hemoglobin in the feces. In this case, a colonoscopy is prescribed to clarify the diagnosis, because there are many causes of bleeding:

• ulcer;
• inflammation in any departments;
• Crohn's disease;
• hemorrhoids;
• tuberculosis;
• intestinal fissure;
• benign or malignant tumor (polyps, cancer, cyst, etc.).

Often, blood enters the intestines from the mouth or nose. However, vascular damage in easily accessible areas of the body is easily determined by the appropriate doctors (dentist and otolaryngologist).

Conclusion

Fecal occult blood testing is an important diagnostic tool. Sometimes only hemoglobin impurities indicate a pathological process in the intestine. And if the violations are detected in a timely manner, then the treatment will not keep you waiting.