Saturday, September 29, 2007


Hi everybody. This week's my turn again to share what I've learnt with you'll.

Occult Blood Test(Blood in the stools)

Clinical Interpretation
Fecal occult blood test is performed as part of routine physical examination during examination of the rectum and it is a screening tool for colorectal cancer. Blood in the stools could indicate gastrointestinal diseases such as inflammatory bowel disease, gastric ulcers, etc.

Materials needed
-Test card(contains guaiac)
-Developing solution(contains hydrogen peroxide and denatured ethyl alsohol)
-Wooden applicator sticks

Test Principle
The test is based on the oxidation of guaiac by hydrogen peroxide to a blue compound. The heme portion of hemoglobin that is present in the stool will catalyse the oxidation of alpha-guaiaconic acid to form a blue colouration.

A small fecal sample is collected and a thin smear is applied on the test card using wooden applicatior sticks. The cover flap is closed and we have to wait for 2-3 mins for the stool to be absorbed into the test paper. The back of the test card is opened and a drop of the developer is added to the the guaiac paper directly over the smear and both positive and negative controls. Results are read after 1 minute and a blue colouration is indicative of occult blood. A brown colouration indicates negative results.

Patients should avoid red meat and NSAIDs as these substances can cause false positive results. Occult blood test must be done with screening endoscopy to detect colorectal cancer as a negative result for OB does not guarantee that a patient does not have colorectal cancer.

Enjoy reading!

Cheong Shu Hui


VASTYJ said...

Hi Shu Hui! Yeah heng tat i need not handle stool samples =p haha anyway.......

Care to elaborate further on why red meat and NSAIDs can cause false positive results? Could it be due to the presence of substances that will also catalyse the oxidation of alpha-guaiaconic acid?

Chaur Lee

ALsubs said...

Hello shu hui,
I also want to say "heng ah i no need to do OB"! hahaha
My question to u is:
Are there any limitations to the test?


first6weeks said...

Hello Shu Hui,

I would also like to know how do red meat and NSAIDs affect the test result? AND.. how do you smear STOOL? eeew.. like normal PBF? hahaha.


-Alex Tg02

J.A.M.M.Y.S said...

Hey Shu Hui,

Unlike all the others, I have had the 'honour' to experience FOBT(Faecal Occult Blood Test):).

However, my lab uses a different method where we just need to stab the stool using this small bottle-like instrument with a tube-like opening.

Anyways im not quite clear about how exactly you perform the test you mentioned. You smear the front of the test card yet add the developer to the back of the card? How does the stool come into contact with the developer then?



first6weeks said...

Hello Shuhui! =)
May i know what's the purpose of denatured ethyl alcohol in the developing solution?

June, TG02

BloodBank.MedMic.Haematology said...


Can I know is there any other tests that need to be done for the confirmation of gastrointestinal diseases and why? Thanks

Ci Liang

royal physicians said...

Hey Shu Hui

You mentioned that patients should avoid taking red-meat and NSAIDs as it would affect your results right? Does avoiding mean not consuming it within 24hrs of the test or a week's interval to ensure the test results are not affected? Thanx


Distinction in Disaster! said...

hi shu hui,

my lab is also using the same kit as yours but i think that the way we do confirmation is different. how will your lab do to confirm when the result is positive but the patient did not take any red meat or NSAID drug?

the blood can also due to the force exert when the patient is excreting (maybe suffering from constipation?)


The Lab Freaks said...
This comment has been removed by the author.
The Lab Freaks said...

Hello shuhui!
hmm may i know why red meat and NSAIDs can cause false positive results?
and erm whats the procedure of a screening endoscopy, its like a re-confirmation test issit?

Natalie (:

Vino said...

hey HI!

hey u mentioned that oxidation of guaiac by hydrogen peroxide to a blue compound.... wat is guaiac ?


ALsubs said...

Hello Chaur lee,
Funny ah u...ok. here’s your ans. Red meat contains blood that may be mistaken for human blood and NSAIDs can cause gastrointestinal bleeding. Hence, both should be avoided during OB testing to prevent false positive results.

Hey Sally
Dun bluff lah u do OB before lor. Haha Yup, the limitation is that blood, if present in the stools may not be distributed uniformly in the sample. Therefore, we may observe a negative result even if the person has the disease.

Hello Alex
Em…not like PBF. Take some of the sample using the wooden stick and just spread a bit on the test card? Yup. Just a bit will do because the test is very sensitive.

Hello Azhar
Are u talking about nagase? Sorry maybe I didn’t explain clear enough..Ok ah.. U need to open the test card to do the test. When u open the test card, u can see a thin piece of guaiac paper in it. U smear the stool here, cover the flap, then when u turn to the back of the test card, u have to tear open a part which will reveal the back of that thin piece of guaiac paper that I mentioned earlier. So since the stool will absorb the guaiac in the paper, by adding the developer to the paper, occult blood can be detected. Hope you’re clearer now..

Hello June
Im not really sure about that. I’ll try to find out more before telling u..give e a bit more time k….

Hello Ci Liang
Some tests for gastrointestinal diseases are intestinal biopsies, stool FEME (for ova, parasites, amoeba etc), endoscopy and imaging studies like magnetic resonance imaging (MRI).

Hello Johanna
They should abstain from red meat for 3 days and NSAIDs for a week.

Hello Lizzie
The lab I’m in does not do confirmation for positive OB cases. Haha..ya. I think it’s possible that a person may be tested positive if there’s blood in his stools cuz of constipation.

Hello Natalie
Yup, a screening endoscopy is done to detect for colorectal cancer if you are tested positive for OB. It is performed using an endoscope, which is a flexible fiberoptic tube with a tiny TV camera at the end. The endoscope is passed through the mouth, throat, esophagus and into the colon for examination. The camera is connected to either an eyepiece for direct viewing or a video screen that displays the images on a color TV.

Hello Vino
Guaiac is a natural resin extracted from a kind of wood that is useful in detecting occult blood.

Shu Hui

J.A.M.M.Y.S said...
This comment has been removed by the author.
J.A.M.M.Y.S said...

Hey Shuhui

So the test card contains the guaiac paper which can be accessed using from the back. The kit my lab uses (OC-light) requires the guaiac to be inserted.

Thanks for clearing that up.


ALsubs said...

Hey June, I finally got the answer for ur qn. ethyl alcohol is the used as the solvent for hydrogen peroxide.

Shu Hui