Stool examination:- Part 3 – Stool for Occult Blood, OB (Fecal occult blood)
- This test is done on the stool.
- The random sample can be taken and 3 mL quantity is enough.
- Avoid the outer portion and take a sample from the central area of the formed stool.
- Three consecutive stool samples are needed.
- Collect the stool in a dry, sterilized wide-mouth container.
- Instruct the patient to stop vit.C, iron-containing drugs, meat and vegetable for at least three days prior to the test.
- Fresh stool testing is recommended.
- The special diet is recommended 48 to 72 hours before the test is performed.
- Also, direct to the patient to avoid the following foods:
- No, red meat. the chicken and fish were taken.
- Peroxidase rich vegetables like turnip, spinach, and horseradish, mushrooms, broccoli, beans, cauliflower, oranges, banana, cantaloupe, and grapes.
- No raw fruits.
- Avoid Vit.C which causes a false negative reaction by inhibiting the peroxidase reaction.
- Avoid drugs like anticoagulants, aspirin, colchicine, nonsteroidal antiarthritics, iron preparation, and steroids for at least 7 days before the test.
- Take H/O of bleeding gums.
- This is a screening test for colorectal carcinoma.
- This can be used for bleeding ulcers in the gastrointestinal tract.
- This test can be included in the periodic medical checkup.
- This test is advised in older people after the age of 40 which is asymptomatic to rule out GI malignancy.
- This test is very important in the hypochromic anemias because of ulcerative or neoplastic diseases.
- Over the age of 50 years, this may be included in the annual check-ups.
- Definition: There is a presence of hemoglobin in the stool, evidenced by the chemical test but not seen by the naked eye, hidden (occult).
- Occult blood is hidden and it requires a chemical test for its detection.
- Normally 2 to 5 ml of blood passes in the stool per day but this is not detectable.
- The healthy person passes roughly 2 ml of blood in 150 grams of stool.
- This amount will not show occult blood positive.
- This test will be positive when there is 5 mL of the blood passes in the stool per day.
- Another reference says that 2 ml of blood in the stool can be detected by the occult blood test.
- Another source says >2 mg/g of the stool is positive and <2 mg/g/day is seen in healthy people and OB will be negative.
- Occult blood will be positive when the tumors grow in the lumen of the intestine and if it ulcerate and give rise to bleeding.
- Bleeding in the upper GI tract produces a black tarry stool.
- While bleeding from the lower GI tract produces just blood in the stool.
- Fecal occult blood should test for either heme or heme-derived porphyrins.
- Heme has a peroxidase-like activity that is detected by the Guaiac dye test.
- Stool with dark red to tarry black indicates a blood amount of 50 to 75 mL from the upper GI tract.
- Occult blood normally is negative.
- Advises patients to take plenty of vegetables, corn, and non-citrous fruits.
- This is a modification of the benzidine and 0-tolidine test because these are carcinogenic.
- Take 95% alcohol 15 mL.
- Dissolve 4- aminophenazone 0.4 grams.
- Add Acetic acid 10% 1 mL.
- H2O2 (Hydrogen peroxide)1 volume and 10 ml of water.
- Take 10 to 15 mL of D.water and emulsify the stool (10 mm in diameter).
- Now centrifuge and take the clear emulsified fluid.
- Take three tubes and label those as a patient, Negative control, and positive control.
- Add 5 mL of emulsified stool material to the patient tube.
- Add distle water to the negative control.
- Add one drop blood to the positive control.
- Now layer 5 mL of the mixed aminophenazone prepared reagent above the suspension and don’t mix. Just layer it over the suspension.
- Now add 10 drops of Hydrogen peroxide 10 vols and don’t mix (dilute 1 mL of H2O2 with 10 mL of D.water.).
- Check the result as follows.
- Every time you have to make a fresh sample to run this test.
- The false-positive test is seen if the patient stool has a peroxidase-like substance.
- The false-negative reaction is seen in the case of ascorbic acid excess in the stool.
- In the case of suspected cases repeat the test two more times.
- The disadvantage of the Guaiac test is that it may react with non-Hb peroxidase present in the stool like vegetables and meat.
- Vitamin C inhibits the Guaiac reaction.
- Guaiac reaction test sensitivity is 40% so many times it can not detect the tumors in the early stages.
- A more sensitive reaction developed which has sensitivity and specificity of about 80% and 94% respectively.
- In the Guaiac test, a drop of water (rehydration) is added on the slide before testing.
- This step increases the sensitivity but decreases the specificity.
- More cancers are detected after the rehydration, but a more false-positive result leading to further investigation like barium meal study or sigmoidoscopy.
- The principle of the Guaiac test:
- The stool sample is applied to the guaiac impregnated paper.
- Then add the developer solution consists of H2O2 (hydrogen peroxide) and denatured alcohol.
- Interpretation of the Guaiac test:
- Any appearance of the blue color is a positive test.
- The intensity of color development does not matter, whether it is a week or strong.
- If the paper before the test is turned blue or blue-green should be discarded.
- American cancer society recommends three consecutive samples for colorectal cancer screening.
- This method is specific for heme and porphyrins.
- Value < 2 mg/g of the stool is negative.
- A value> 2 mg/g of the stool is positive for colorectal carcinoma.
Immunological kits methods:
- These are sensitive to detect the human hemoglobin component of globin.
- A monoclonal antibody against the hemoglobin is used. These antibodies are specific for hemoglobin.
- Mostly agglutination is reported positive by the presence of coated antibodies or latex particles.
- These tests are called the fecal immunochemical tests for fecal occult blood.
- The disadvantage is that it may not detect bleeding from the upper GI tract because the globin is digested or degraded by the time it gets into feces.
- This is a new technique to detect precancerous and malignant tumors.
- The benign lesion does not bleed and these may be missed by Guaiac or other tests in occult blood screening.
- All precancerous lesions shed cells with abnormal DNA, so precancerous lesions can be removed before they transform into malignant conditions.
False-positive OB test is seen in:
- Ingested meat.
- Peroxidase rich vegetables like turnip, horseradish, mushroom, broccoli, beans, sprouts, cauliflower, oranges, bananas, cantaloupe, and grapes.
- Drugs may lead to bleeding like anticoagulants, aspirin, iron preparation, nonsteroidal antiarthritic drugs, and steroids.
- White cells and bacteria also cause a false positive test.
- Other drugs causing false-positive tests are boric acid, colchicine, bromides, and iodine.
A false-negative OB test is seen in:
- Vit C may cause a false negative when taken more than 250 mg/day.
- Foods and juices are rich in vitamin C.
- Oxidants also cause a false-negative result.
- An iron supplement containing vitamin C >250 mg.
Causes of positive OB test seen in:
- Gastrointestinal tumors.
- Rectal carcinoma.
- Gastric carcinoma.
- Inflammatory bowel disease.
- Ischemic bowel disease.
- Arteriovenous malformations of GI tract.
- Blood swallowed from the oral cavity or nasopharynx.
- Peptic ulcer.
- Kaposi’s sarcoma.
For the detection of colorectal cancer can advise:
- Occult blood in the stool.
- When screening started at the age of 50 years, it will reduce death by colorectal malignancies. In one reference this may be a 60% reduction.
- NOTE. Please see more details in another stool part.