Finally, it’s my third entry. For this entry, I decided to post about Occult Blood Test.
As many of you would have already know that Occult Blood Test is to test for occult blood in the fecal and I believe it is a very common test to be carried out in many labs. However, different labs may be using different kit or materials and methods so I decided to share this method that I learned from my lab.
My lab uses this slide call Hema-Screen Slides which is a guaiac slide test for the qualitative detection of occult blood in fecal. It can be used to diagnose some gastrointestinal disorders and is usually used in routine physical examinations, routine hospital test, and mass screening for colorectal cancer. The occult blood detection is very important in many gastrointestinal diseases. The existence of occult blood can mean that there’s gastrointestinal pathology like hemorrhoids, diverticulitis, fissures, colitis or even colorectal cancer. Hema-Screen provides an easy, cheap and visual test designed for use in the collection and preparation of stool samples.
Principle of the test:
Hema-Sreen is made up of guaiac impregnated paper together in a cardboard frame which allows a maximum of two samples to be applied on one side of the paper and develop the result on the reverse side of the paper. The guaiac paper tests for occult blood through the oxidation of phenolic compounds (guauaconic acids) present in the guaiac to quinines causing the production of blue colour. Due to its similarity to the prosthetic group of peroxidase, the hematin portion of the hemoglobin molecule catalyzes the oxidation of guaiac by acting in a pseudoenzymatic way.
When the stool containing the occult blood is being applied to the test paper, the contact made between the hemoglobin and the guaiac will cause a pseudoperoxidase reaction to take place when the developer solution is added. A blue chromagen will form proportionally to the concentration of hemoglobins. The reaction usually takes about thirty seconds.
Specimen collection and handling:
It is suggested to have patient to go on a high residue diet starting two days before the test and continue throughout the test.
After stool samples are taken from the patient, use the applicator provided to spread a very thin smear of stool to the HemaScreen slides. Allow the smears to dry. The slide smears may be prepared and developed immediately or stored up to 12 days prior to development. Care should be taken to prevent any contact with blood to the specimen. Patient specimens and all materials in contact with them should be handled as potentially infectious materials and should be disposed in proper precautions.
Procedure
Materials provided:
-Hema-Screen slide with On-slide monitors (quality control)
-Hema-Screen developer
-Specimen applicators
Methods:
Write down the information to the empty lines of the front flap of Hema-Screen slide
Open the front flap
Use the applicator stick provided to collect a small amount of stool samples from the container of the patient stool and apply a very thin smear to the box.
Allow samples to dry and close the cover
Open perforated window on the back of the slide
Drop 2 drops of developer to the back of the area where the samples are being applied
Read results after 30 seconds
Any blue colour traced from the stool is considered positive for occult blood.
Limitations:
Results aabtained with the Hema-Sccreen cannot be considered conclusive evidence for presence or absence of gastrointestinal bleeding or pathology. It is used only for preliminary testing which cannot replace any other diagnostic procedures.it will only detect hemoglobin released upon hemolysis of the red cell. If whole blood is applied, it is required to hemolyse the red cells by addition of a drop of water before adding the developer. Positive result can be due to a couple of reasons such as red meat in the diet, diverticulitis, hemorrhoids, colitis to colorectal cancer.
Sharon
Tg01
8 comments:
Hi Sharon,
What is it meant by a high residue diet? And why must the patient remain on it throughout the test? The test will be conducted more than once on the same paitent? And ya, if positive result is obtained, what is the confirmatory test for it?
Thanks
Ying Chee
TG01
Hi Sharon,
Why there is a need to develop the the slide smears immediately?
How you ensure that the slide and developer are working correctly? By a control maybe?
Since you mentioned that this test is a preliminary test, what are the confirmatory tests?
Hardina =)
hi sharon,
may I know what is present in the developer solution that causes the pseudoperoxidase reaction to take place when haemoglobin and guaiac come into contact?
Why is there a need to add a drop of water before developer solution is added to hemolyse the red cells for whole blood? is it to dilute the blood? thanks.
Malerie
TG02
To Ying Chee:
High residue diet include only small amount of well-cooked chicken, turkey and tuna, a lot of raw and cooked vegetables such as lettuce, corn, spinach, carrots and celery, plenty of fruits, cereals and moderate amount of peanuts and popcorn daily. And this diet should exclude red or rare meat and raw fruits and vegetables containing high peroxidase activity as these foods can cause false positive result to the test. This test would not be done on more than once on the same patient. Once positive result is obtained, a follow-up additional diagnostic test is required so the patient have to remain on the diet during the test so that their diet won’t affect the result of other tests. One of the confirmatory tests is X-ray.
To Hardina:
The slide smear can also be stored up to 12 days prior to development. Yes, every kit or machine needs control to determine whether it is in functional state. For this kit, it has on-slide monitors just at the reverse side of the slide smear where the developer is being drop. By placing one to two drops of developer the on-slide monitor, the positive control should turn blue while the negative control should remain colourless. .
To Malerie:
The developer contains <6% hydrogen peroxide and denatured ethanol. The purpose of adding water to whole blood samples is to destruct the red cells and release the hemoglobin as the kit shows reaction to hemoglobin but not red cells.
hello sharon, if heama-screen is not used to conclude the result of whether there is occult blood loss, then what can be used to confirm the test
To unknown:
Like what I've replied to Ying Chee, one of the confirmatory test to confirm whether there's gastrointestinal pathology is X-ray. Others include barium enema and proctosigmoidoscopy examination.
Hi Sharon
Is there a reason why cant this test be done more than once on the same patient? Are there any advantages of using this method? coz it seems quite inconvenient as there is a need for patients to go on a high residue diet.
Thanks!
LeeJin
TG02
To LeeJin:
It's not cant but usually they don't because like i said it cannot be used to replace other more accurate test. The advantage of this test is that it's inexpensive and easy to carry out. Once positive result is seen, follow up test should be done instead of repeating the same test.
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