Faecal Occult Blood Tests (FOBTs) : Significance, Methods, Interpretation, Interference

fecal occult blood test

Faecal Occult Blood Tests (FOBTs)

Fecal occult blood test is a simple, low-cost and supreme commonly performed screening test on stool specimen. It is used for the finding of blood in the faeces specimen which is not noticeable on gross examination. When very small amounts/trace of blood being delivered in the stool, the blood or its (broken-down products) is not seen and is called as occult blood. The trace amount is frequently less than 50mg of hemoglobin/gram of stool specimen.

SIGNIFICANCE OF FAECAL OCCULT BLOOD:

Normal adult, generally passes upto 2ml of blood/150 gram of faeces (2 – 3mg hemoglobin/gram of faeces) into the GI-tract every day. More than 2ml of blood in the faeces in 24 hours is the sign of pathologically condition. The raised quantity is associate with a different variety/type of benign and malignant GI-tract diseases, particularly colon cancers, blood-loss anemia, hook-worm infection, diverticulitis,colitis, polyps, and fissures.

METHOD FOR FAECAL OCCULT BLOOD:

Types Of Faecal Occult Blood
Types Of Faecal Occult Blood

For the detection of occult blood in stool specimen following techniques are used in clinical setup:

  • Chemical Methods:

In this Test, Guaiac based reagents set in the laboratory setup, e.g. Amino-phenazone test, or ready made reagent in kit is available.

  • Immunochemical Methods:

In this Test hemoglobin specific cassette or strip test is used.

1- CHEMICAL METHODS:

These are the old-fashioned technique using guaiac based reagents which were organized in the Laboratory, The conventional /standard tubes test have been changed by kit method that are very precise and easy to use and are very low cost.

PRINCIPLE OF TEST:

Principle is based on chemical use to identify occult blood,based on fact that hemoglobin and its byproducts react in a parallel way to per-oxidase enzymes which further catalyzes the transfer of an oxygen atom from the per-oxide to a chromogenic such as benzidine, O-toludine, Guaiac or Amino-phenazone. Oxidation/Corrosion of the chromogenic is shown by the indication of a blue to blue-greenish or pink color.

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Aneasy reaction equation is displayed below:

Guaiac Colorless + H2O  Hemoglobin Oxidized Guaiac       (Blue Color)

Trace amount of blood will give a positive results. If it is accessible ask the patient or give instructions which he/she should follow at least 7 days before the test and must endure over the test period. Patient guidelines contain both drug and diet plans.

PROCEDURE OF TEST:

A paper slide is used for the test is which consists of paper squares coated/smeared with guaiac reagent a chemical brought from tree resin. A small part of stool specimen is smeared to the paper.  Solution having hydrogen peroxide (H2O2) is applied to the paper. After the reaction,if the blood is present in the stool specimen, the iron (Fe) in the hemoglobin catalyzes between guaiac chemical in the paper and the H2O2. Blue color production will give positive results and indicating the presence of stool in the specimen.

2- IMMUNO-CHEMICAL METHODS:

Modern technique for stool occult blood testing has been replaced to an immuno-chemical testing that is specific for human hemoglobin protein.

PRINCIPLE:

Test uses a qualitative technique, sandwich dye conjugate immun-oassay to specifically detect the globulin component of human hemoglobin protein in stool specimens. The immun-oassay utilizes a mixture of mono-clonal and poly-clonal antibodies, consumes an immuno-chemical chromate-graphic technique for the identification and has a high amount of analytical sensitivity.

PREPARTION OF PATIENT:

Because test is specific for human blood, no unusual drug or dietary boundaries are compulsory. Though, women patients may not save specimen for 3 days before, during or 3 days after their menstrual cycle, if they have bleeding hemorrhoids, presence of blood in urine or exposed cuts on their hands.

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PROCEDURE:

In these kinds of technique, a specimen of the patient’s stool specimen is positioned on a special type of collection card and send back to the laboratory. The small part of the collection card comprising the patient’s specimen has taken from the collection card, and the specimen is mixed with buffer solution. The buffer is then presented into a test device that comprises poly-clonal antibodies. The buffer will drift over the test device for a specific quantity of time, generally for 5 minutes, and a colored line will indicate at the “T” if the specimen having human hemoglobin.

Immuno - Chemical Methods
Immuno – Chemical Methods

OTHER METHODS FOR OCCULT BLOOD:

Some other techniques such as Over The Counter(OTC), Flushable Reagent pad or Tissue Technique may also help, in the presence of blood in stool will produces a color changes. At least promising, and practically quantitative method is to measure stool heme by the specific Fluorescence of their Porphyrin Byproducts, after extraction of the porphyrins from the sample to remove intrusive constituents.

INTERPRETATION:

Positive result shows that abnormal bleeding is arising some place in the GI-tract/Digestive tract. The most common sources of occult blood in tropical/hot and in other developing countries are Hook-Worm infestation, Colitis,Peptic Ulcer, and Bleeding from esophageal varices because of cirrhosis of the liver organ. Other reason comprise carcinoma/cancer in the GI-Tract, Erosive Gastritis Because of too much alcohol consumption, drugs or inflammatory bowel diseases in patients.

INTERFERENCE FOR TEST:

In chemical methods, non-hemoglobin elements with per-oxidase action maycon sequently create false positive results. Other constituents may restrict with per-oxidase activity indication in the form of false negative reactions.

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ELEMENTS THAT SOURCE FALSE POSITIVE REACTION:

Drugs: Such as Boric acid, Colchicine, Iodine and Bromides.

Animal Foods: Foods such as processed meats and Liver, in diet which consists of Hemoglobin, Myoglobin and definite Enzymes.

Fruits and Vegetables: They have per-oxidase action for example; Radishes, Mushrooms, Turnips, horse-radish, Apple, Bananas, And Broccoli.

ELEMENTS THAT SOURCE FALSE NEGATIVE REACTION:

Ascorbic acid having rich amount of (Vitamin C), Iron supplements which consists of Vitamin C.

OTHERS SUBSTANCES AFFECTING TEST RESULTS:

Bleeding hemorrhoids, Collection of specimen in women during menstrual cycle, Hematuria and some long distance runners.

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