Fecal occult blood test: what does it show?
The study of fecal masses for the presence of hidden blood in them is an important stage in the diagnosis of inflammatory, parasitic, autoimmune and degenerative-dystrophic pathologies of the gastrointestinal tract, as well as an affordable way of timely detection of oncological diseases of the digestive system. Noticing blood in their feces, visible to the naked eye, most people rush to see a doctor, realizing how serious this signal of trouble is.
But latent blood can indicate no less formidable diagnoses, and when the disease is still at an early stage of development, and the chances of a successful recovery are much greater. That is why, in the presence of the first suspicious symptoms, it is necessary to obtain qualified medical advice and pass all the necessary tests. And people over 50 years old with a history of cases of colorectal cancer need to undergo a fecal occult blood test every year, since this simple measure can save lives in some cases.
- Latent blood in the feces - what does it mean?
- Who is a fecal occult blood test indicated for?
- Preparation, diet, collection and delivery of analysis
Research Methods: Pros and Cons
- Gregersen reaction (benzidine test)
- Weber reaction (guaiac test)
- Immunochemical analysis of feces
- Fluorescence test
- Interpretation of analysis results
Many pathologies of the gastrointestinal tract, including ulcerative colitis and malignant intestinal tumors, for the time being, do not bother the patient with pronounced discomfort. But the destruction of healthy tissues is already taking place, therefore, blood is released, which cannot yet be detected visually in the feces, but it is quite possible to do this with the help of special laboratory tests. Today we will tell you what the occult blood feces test shows, how to prepare for the study and what diet you need to follow, how to properly collect and submit the material, what the positive and negative results indicate, in which cases it can be false, and what methods of determination occult blood in the stool is most accurate.
Latent blood in the feces - what does it mean?
A certain amount of blood may normally be present in the feces - no more than 2 mg of hemoglobin per 1 g of feces, with such a maximum concentration it turns out that a healthy person loses up to 2 ml of blood with feces per day. If the bleeding becomes more intense, the color of the stool changes, and its shade indicates the localization of the problem: the darker, the higher. For example, with a bleeding ulcer of the stomach or duodenum, the feces become black, tarry (melena), if the cause lies in exacerbated ulcerative enterocolitis, the stool will acquire a burgundy color, and bleeding polyps, hemorrhoids and malignant neoplasms in the rectum lead to the appearance of scarlet veins on feces. Such signs cannot go unnoticed and should be the reason for immediate seeking medical help.
Latent blood in feces is such blood that cannot be detected either by visual or microscopic examination of feces. It can only be found by analyzing feces for occult blood - a positive result indicates the presence of latent gastrointestinal bleeding and requires a mandatory follow-up examination of the patient.
Who is a fecal occult blood test indicated for?
This kind of research is prescribed in the following cases:
- Persistent recurrent pain in any part of the abdomen, as well as in the right or left hypochondrium;
- Discomfort and soreness during or after bowel movements;
Feeling of the presence of a foreign body in the rectum;
- Any stool disorders: constipation, diarrhea, pathological increase or decrease in the volume of stool, change in their color, consistency or odor, the presence of suspicious inclusions (foam, mucus, blood, pus, parasites or their eggs);
- Loss of appetite, a sharp decrease in body weight for no apparent reason;
- Nausea, vomiting, belching, heartburn, bad taste in the mouth;
- An unexplained increase in body temperature;
- Diagnosed pathologies of the gastrointestinal tract: Crohn's disease, intestinal polyposis, ulcerative colitis, diverticulosis, esophageal varices, helminthiasis, and so on - the study is performed to determine the stage of the disease or control the course of treatment;
- Alarming results of other, previously conducted examinations, for example, coprograms, general or biochemical blood tests;
- Prevention of bowel cancer over the age of 40-50 years in patients with unfavorable heredity.
Preparation, diet, collection and delivery of analysis
It makes sense to carry out a study for occult blood in the feces only if the rules of preliminary preparation are carefully observed, otherwise the probability of obtaining a false result will be very high.
Let's start in order:
It is necessary to temporarily postpone the analysis until the following circumstances are eliminated - stomatitis, periodontal disease, nosebleeds, menstruation, dyspepsia, hematuria, exacerbation of hemorrhoids, recently conducted diagnostic manipulation with the intestines (colonoscopy, sigmoidoscopy), anal sex. All these factors are likely to lead to distortion of the research results, which means that it is necessary to solve the existing problem or wait a few days, depending on the situation;
A week before the analysis, you should stop taking any medications that undesirably affect the composition of feces, namely, NSAIDs (aspirin, ibuprofen, naproxen), barbiturates, anticoagulants, intestinal peristalsis stimulants, ascorbic acid, preparations and dietary supplements containing iron. If you constantly take any medications and are not sure that they will not affect the results of the study, it is better to consult with your doctor in advance;
Three days before the analysis, you need to start following a diet - the analysis of feces for occult blood is almost always carried out using laboratory samples that are sensitive to the composition of the food consumed by the patient. Therefore, you will have to temporarily give up any meat, poultry, fish and offal - that is, from everything that may not contain your blood. In addition, it is recommended to exclude from the menu some beans, vegetables, fruits and berries that are rich in iron and vitamin C or strongly affect intestinal motility - for example, apples, celery, cranberries, oranges, spinach, nuts, cabbage, tomatoes, carrots, beets, beans, soy and lentils;
12 hours before the analysis, it is necessary to put aside the toothbrush and refuse to eat hard food that can injure the gums and lead to the ingestion of blood from the mouth into the digestive tract.
Successful delivery of an occult blood test = preparation + diet, remember this!
How to collect and donate feces correctly?
So, the preparatory stage is over, it's time to collect material for research and take it to the laboratory:
- Early in the morning, before breakfast, make an external toilet of the intimate area using ordinary soap, rinse thoroughly and dry off;
- Defecate in a clean container prepared in advance. The process should take place naturally, without the use of enemas or laxatives. You cannot take a stool sample from the toilet. The analysis of feces for occult blood in a child is taken from a well-washed pot, from a diaper or oilcloth;
- For collection and transportation of biomaterials, special sterile containers with a lid and a spoon, which are sold in any pharmacy, are best suited. In the absence of such, you can use a small glass jar, having previously disinfected it. Collect a little feces from different parts of the stool so that the picture of the study is as objective as possible. In total, an amount is sufficient, approximately equal to 2-3 teaspoons;
- It is better to deliver the container with the sample to the laboratory immediately. However, it is permissible to store the material in the refrigerator for no more than 12 hours. This is especially useful for young children, whose bowel movements are unpredictable, or people who are accustomed to emptying their bowels before bed.
Research Methods: Pros and Cons
Currently, four main methods are used to determine occult blood in stool:
- Gregersen reaction (benzidine test);
- Weber reaction (guaiac test);
- Immunochemical analysis of feces;
- Fluorescence test.
Let's understand the essence of these techniques, their advantages and disadvantages. Looking ahead, we note that in Russia the benzidine test and immunochemical analysis are most often used, but are they the most accurate?
Gregersen reaction (benzidine test)
The benzidine test is a simple and quick laboratory test that detects blood not only in feces, but also, for example, in urine, vomit and any other body fluid of a patient. This is possible due to the oxidation of benzidine (paradiaminodiphenyl) with hydrogen peroxide or barium in the presence of hemoglobin. The benzidine test has several variations, the most commonly used of them was proposed by the doctor M. Gregersen, and therefore bears his name.
The essence of the method is as follows: take 0.025 g of benzidine, add 0.1 g of barium peroxide and 5 ml of a 50% solution of acetic acid, shake everything until completely dissolved, apply a few drops of the composition on a glass slide, on which the test feces are smeared with a thin layer, and observe the reaction. If occult blood is present, a bright blue-green coloration occurs. There is an alternative - a saturated solution of benzidine in acetic acid is prepared and mixed with 3% hydrogen peroxide solution in equal parts. The result will look about the same, and the cost of the research will change little.
- Very high sensitivity - 1: 100,000;
- The speed of obtaining the result - almost instantly;
- Availability in any laboratory;
- Low price.
The need for careful preparation for the delivery of a fecal occult blood test according to the Gregersen method, since the benzidine test gives a positive reaction to animal hemoglobin from food.
Weber reaction (guaiac test)
In addition to the two sounded, this diagnostic method has several more names: Van Deen test or Almen-Van Deen test, hemocult test. Curiously, the reaction with the use of guaiac resin is the first method in world medical practice for detecting latent blood in feces and other biological fluids of a person. Dr. Van Deen proposed it in 1864 and it is still actively used in many countries today.
The essence of the guaiac test is as follows: take 3-5 g of the test stool, dissolve in acetic acid, while the ether extract is released, hydrogen peroxide and guaiac gum tincture are added to it, and then the color change of the reagents in the test tube is observed. If the composition turns blue, then the test result for occult blood in the stool is positive. The test is repeated a total of six times: two samples from biomaterials are studied, sequentially taken for three days.
- Affordable price.
- Low sensitivity - the reaction will be positive only in case of blood loss of 30 ml or more, colorectal cancer is found in a third of cases, benign intestinal tumors - only in 15% of cases;
- Strict dietary requirements before analyzing feces for occult blood - guaiac test is sensitive not only to heme peroxidase of human hemoglobin, but also to peroxidases of other hemes contained in food.
Immunochemical analysis of feces
This test is otherwise called an immunological or immunochromatographic test. The technique is based on the reaction that occurs when specific antibodies interact with the hemoglobin of human blood. Errors are ruled out because antibodies do not respond to animal hemoglobin or to the iron found in some plant foods. Immunochemical analysis of feces for occult blood appeared in the arsenal of doctors relatively recently and has already firmly established itself there.
The essence of the technique is as follows: a sample of the investigated feces is applied to a test strip or placed in the window of the plate with reagents - monoclonal antibodies. If human hemoglobin is present in the biomaterial, it binds to antibodies, and a bright pink or purple band appears in the control area. The second bar appears in any case - it is an indicator of the good quality of the device. Taking a fecal occult blood test by this method is very similar to taking a pregnancy test - everything is very simple, quick and clear.
In the pharmacy, you can buy "Immunochrom-GEM-Express" or any other similar device for self-diagnosis of latent gastrointestinal bleeding at home, but to obtain a reliable result, it is recommended to carry out the analysis at least three times with intervals of several days.
- Speed and convenience;
- No need for preliminary preparation and diet;
- High accuracy - detects early stages of colorectal cancer in 97% of cases.
- Relatively high cost;
- Not available in every laboratory;
- The objectivity of the results of the analysis of feces for occult blood only in the case of bleeding in the lower parts of the digestive tract. Aggressive enzymes that act on hemoglobin in the stomach and small intestine make it unsuitable for reacting with monoclonal antibodies.
For the sake of completeness, it should be mentioned that occult blood can be found in feces during a chemical reaction using destructive reagents. Nitrogen-containing pigments porphyrins, contained in hemoglobin molecules and released to the outside when they die, have a super-high light absorption coefficient - about 10 to the 6th degree. Their bright fluorescent glow is registered by the laboratory assistant in the event that there is latent blood in the analyzed feces sample. However, in Russia this diagnostic method is not very popular, especially in comparison with the benzidine test.
- Affordable price;
- Relatively high accuracy - about 80%.
- The need to follow a diet, since porphyrins are present in any hemoglobin, including in an animal.
Decoding the results of the analysis for occult blood
Depending on how the test was carried out and what the internal regulations of the laboratory are, you will receive an answer in 1-6 days. From the description of the methods of this study, it is clear that its results cannot be expressed in an exact digital equivalent: in the benzidine and guaiac samples, the reagent is either colored or not - only the speed and intensity of the color differ. As for the enzyme immunoassay and fluorescent test, then everything is also unambiguous here: the second strip either appears or not, and also the glow - either there or not.
For example, consider the decoding of the results of fecal occult blood analysis according to the Gregersen method (benzidine test):
very intense reaction
Important: even a weak reaction usually alarms the doctor and prompts him to repeat the test or add additional diagnostic methods to it, since the presence of occult blood in the feces is a sign of serious trouble.
Let us now consider for what reason the results of the study may be biased:
A false-negative result of fecal occult blood analysis is almost always a consequence of a violation of the technology for conducting a sample or poor quality of reagents. It may be that the patient provided little biomaterial or was too lazy to collect it from different fragments of fecal matter. In addition, in some diseases of the gastrointestinal tract, for example, with polyposis of the large intestine, neoplasms do not bleed constantly, but sporadically. Therefore, the study is recommended to be repeated several times, especially if there are serious reasons to suspect the disease;
A false positive result of an occult blood test is, in most cases, a consequence of neglect of the preparation rules. Either the patient did not follow the diet prescribed by the doctor, or took any medications that distorted the picture of the study, or allowed injury, which led to blood entering the gastrointestinal tract. In infants, a positive fecal occult blood test result may be false if the nursing mother's nipples are cracked and bleeding - then the baby swallows blood during feeding, and it ends up in his stool. Moreover, this problem cannot be solved by expressing, blood can also get into breast milk in this case. It is better to transfer the baby to an artificial mixture for three days in order to have confidence in the objectivity of the test.
Positive result - what to do?
Anxious patients are primarily concerned with the question: what does a positive fecal occult blood test result mean? Firstly, this is definitely not a reason to panic. We have already discussed the rather high probability of errors, especially if the rules of preliminary preparation are violated. It is imperative to repeat the study at least three times before making final conclusions.
Secondly, even if a positive result turned out to be reliable, it most likely signals the most common diagnoses:
- Hemorrhoids, anal fissure;
- Intestinal polyposis, polycystic, or diverticulosis;
- Nonspecific ulcerative colitis;
- A stomach or duodenal ulcer;
- Parasitic invasion;
- Varicose veins of the esophagus.
Of course, all these diseases are extremely unpleasant and potentially dangerous, but the main concern for people who have received a positive result of a fecal occult blood test is the risk of developing cancer. To fully understand the situation, it is necessary to connect additional diagnostic methods: ultrasound of the abdominal cavity organs, radiopaque tomography, colonoscopy, sigmoidoscopy.
It is especially important to do this for those patients whose age has passed over 40 years, and who at the same time have close relatives with cancer. As for malignant intestinal tumors, early diagnosis increases the chance of survival by an average of 30%. And immunochemical analysis, for example, shows the first stage of colorectal cancer in more than 90% of cases, adenomatous polyps larger than 1 cm - in 60% of cases. That is why it is so important not to neglect the annual preventive examinations and listen to the recommendations of the attending physician. Take care of yourself and be healthy!
Article author: Mochalov Pavel Alexandrovich | d. m. n. therapist
Education: Moscow Medical Institute. IM Sechenov, specialty - "General Medicine" in 1991, in 1993 "Occupational Diseases", in 1996 "Therapy".