fecal occult blood test “ fobt” hematemesis: bleeding into the gastrointestinal tract may be...
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Fecal Occult Blood Test “ FOBT”Fecal Occult Blood Test “ FOBT”
• Hematemesis: bleeding into the gastrointestinal tract may be rapid with the vomiting of blood.
• Melaena: the passage of blood through the rectum.
• When the bleeding is chronic with only small amounts of blood being passed in the feces.
• If the blood or it’s products is not recognized in the feces, it is referred to Occult blood (hidden Blood).
Raed Z. Ahmed, Medical Parasitology Lab.,2012
Principle of the Test Principle of the Test
• Chemical tests to detect occult blood are based on the principle that hemoglobin and it’s derivatives react in a similar way to peroxidase enzymes (pseudo-peroxidase activity).
• Chromogen such as guaiac, O- toluidine, 4-aminophenazone or benzidine will be used as indicator for oxidation reaction.
• Hemoglobin and its derivatives catalyze the transfer of oxygen from hydrogen peroxide to Guaiac, oxidation of the colorless chromogen produces a blue color.
Hemoglobin + H2O2 + Guaiac Oxidized guaiac+ H2O
Raed Z. Ahmed, Medical Parasitology Lab.,2012
O2
Pseudo-peroxidase
Blue color
Reagents and methodReagents and method• Reagents:
– Acetic acid 10% v/v
– Alcohol 95% v/v
– 4-aminophenazone (4-aminoantipyrine)
– Hydrogen Peroxide (H2O2) 10 vol. solution*
• Working 4-aminophenazone reagent:
– The amounts given are sufficient for 1 test with positive and negative controls.
– Prepare fresh as follows:
• Alcohol 95% v/v -------------------------------------------------- 15 ml
• Acetic acid 10% v/v ------------------------------------------------ 1ml
• 4-aminophenazone -------------------------------------------------- 0.4g
• Dissolve the 4-aminophenazone in alcohol solution and immediately before use add the acetic acid. Mix well
Raed Z. Ahmed, Medical Parasitology Lab.,2012
Procedures Procedures 1. Dispense about 7ml of distilled water into a test tube
2. Add a sample of feces about 1gm., use a glass or plastic rod to emulsify the feces.
3. Allow the fecal particles to settle or centrifuge the emulsified specimen.
4. Take 3 completely clean tubes and label them as :
– T: Patient's test.
– Neg.: Negative control.
– Pos.: Positive control.
5. Add into each tube as follow:
– T ------ 5ml supernatant fluid from emulsified feces.
– Neg. ------ 5ml distilled water.
– Pos.: ------5ml distilled water in which 5ul of whole blood has been mixed.
Raed Z. Ahmed, Medical Parasitology Lab.,2012
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6. Add 5ml of working 4-aminophenazone reagent on top of the fluid in each tube .Do Not Mix
7. Add 10 drops of the 10 vols. Hydrogen peroxide solution. Do Not Mix, allow to stand for 1 minute.
8. Look for the appearance of a blue color where 4-aminophenazone reagents meets the sample or control solutions.
– Neg. control: this should show no color change.
– Pos. control: this should show a positive reaction.
Raed Z. Ahmed, Medical Parasitology Lab.,2012
ColorResult
No color changeNegative
Pale bluePositive +
Dark bluePositive + +
Blue – blackPositive + + +
False Reactions False Reactions False positive: 1
1. Aspirin and anti-inflammatory medications.
2. Red meat (contain Myoglobin)and fish.
3. Green vegetables (Melons), and Horseradish.
4. Menstrual and hemorrhoid contamination.
5. Some intestinal bacteria that produce peroxidase enzymes. False negative: 2
1. Vitamin C greater than 250 mg/dl
2. Iron supplements containing vitamin C
Raed Z. Ahmed, Medical Parasitology Lab.,2012
Interpretation Interpretation • The commonest cause of positive occult blood tests in
tropical and other developing countries are Hookworm infections, peptic ulcer, and bleeding from esophagus or liver cirrhosis.
• Other causes include carcinoma in gastrointestinal tract, erosive gastritis duo to alcohol or drugs, or swallowed blood from nosebleeds.
• If the test is negative but there is high clinical suspicion, a further two specimen should be tested to detect bleeding which be intermittent.
Raed Z. Ahmed, Medical Parasitology Lab.,2012
Commercial MethodsCommercial Methods• Therefore, to prevent false-positive reactions, the sensitivity
of the test must be increased.
• Many commercial testing kits are available for occult blood testing with guaiac reagent.
• The kits contain guaiac impregnated filter paper, to which the fecal specimen and hydrogen peroxide are added.
• Two or three filter paper areas are provided for application of material taken from different areas of the stool, and positive and negative controls are also included.
Raed Z. Ahmed, Medical Parasitology Lab.,2012
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• Additional, more sensitive and specific methods, for the detection of occult blood have been developed.
• Hemoquant, provides a fluorometric test for hemoglobin and porphyrin.
• As hemoglobin progresses through the intestinal tract, bacterial actions degrade it to porphyrin that the guaiac test cannot detect, this can result in some false-negative results from upper gastrointestinal bleeding when using the guaiac test.
Raed Z. Ahmed, Medical Parasitology Lab.,2012
• iFOBT: The immunochemical fecal occult blood test, Hemoccult ICT, is specific for the globin portion of human hemoglobin and uses anti-human hemoglobin antibodies.
• Because Hemoccult ICT is specific for human blood in feces, it does not require dietary or drug restrictions.
• It is more sensitive to lower GI bleeding that could be an indicator of colon cancer or other gastrointestinal disease.
• Can be used for patients who are taking aspirin and other anti-inflammatory medications.
Raed Z. Ahmed, Medical Parasitology Lab.,2012
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