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Guaiac-based FOBT & Immunochemical-based FOBT

Introduction to Fecal Occult Blood Tests


Colorectal cancer is the third most common cancer diagnosed in Canada and the second leading cause of cancer death. An effective population based screening program will decrease colorectal cancer mortality through earlier detection and decrease colorectal cancer incidence by removing colorectal adenomas, the precursor to most colorectal cancers. Several screening strategies are recommended. 1,2,3 Presently, the guaiac fecal occult blood test (gFOBT) and the immunochemical-based fecal occult blood test (iFOBT) are either in use or being considered for use throughout the various provinces of Canada. Either one or both of the tests are offered provincially through a pilot program or as part of a permanent population-based screening program or in those cases where there is neither through the family physician. Please refer to the Colorectal Cancer Association of Canada’s “Screening & Treatment Map” to determine what screening program is available in your respective province by clicking http://www.colorectal-cancer.ca/en/just-the-facts/canada-map2/ .

Fecal Occult Blood Tests (FOBT) have an important place in screening for colorectal cancer as they serve to identify people who are more likely to have early stage colorectal cancers and so can direct them to colonoscopy. There are two main types of FOBT: guaiac-based FOBT (gFOBT) and immunochemical-based FOBT (iFOBT), both of which are discussed below. They are quite different from each other in their biological, behavioral, clinical and technological characteristics, all of which are discussed below. Ideally, sampling for gFOBT/iFOBT is done at home at the convenience of the individual.



The guaiac-based Fecal Occult Blood Test

A guaiac-based Fecal Occult Blood Test (gFOBT) uses the chemical guaiac (reagent derived from wood resin of Guajacum trees) to detect heme in stool. Heme is the iron-containing component of the blood protein hemoglobin. Heme contains pseudoperoxidase, a chemical which converts guaiac to the colour blue. Hydrogen peroxide is dropped onto the paper containing the smear of stool; and if trace amounts of blood are present, the paper will change color. This method works as hemoglobin has a peroxidase-like effect (pseudoperoxidase), rapidly breaking down hydrogen peroxide. The idea behind the gFOBT is that blood vessels at the surface of larger colorectal polyps or cancers are often fragile and easily damaged by the passage of feces. The damaged vessels usually release a small amount of blood into the feces, but only rarely is there enough bleeding to be noticeable in the stool. This test, however, cannot determine whether the blood is from the colon or from other portions of the digestive tract (such as the stomach). Therefore, if the test is positive, a colonoscopy is required to determine if there is a cancer, polyp, or other cause of bleeding such as ulcers, hemorrhoids, diverticulosis (tiny pouches that form at weak spots in the colon wall) or inflammatory bowel disease (colitis).

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gFOBT is done with a take home kit that can be used in the privacy of the patient’s home. People having this test will receive a kit with instructions explaining how to take a stool or feces sample at home (usually specimens from 3 consecutive bowel movements that are smeared onto small squares of paper). Some foods or drugs can affect the test, so the doctor may suggest that the following foods be avoided before the test to promote as much accuracy as possible as well recommend cessation of certain medications:

Foods associated with false positives, therefore suggested to avoid for 3 days prior to testing:



Medications & Supplements To Avoid:

A Word About Sensitivity & Specificity

Sensitivity represents the proportion of truly diseased persons in a screened population who are identified as being diseased by the test. It is a measure of the probability of correctly diagnosing a condition.   Specificity is the proportion of truly non-diseased persons who are so identified by the screening test. It is a measure of the probability of correctly identifying a non-diseased person.

The guaiac-based fecal occult blood test usually picks up a daily blood loss of approximately 10 ml (about two teaspoonfuls).  And the sensitivity of a single FOBT has been quoted at approximately 30%, but if 3 tests are performed (as is standard), the sensitivity rises to 92%.  Normally, there is only about 0.5 – 1.5 ml of blood that escapes blood vessels into the stool each day.  The gFOBT represents traditional testing and is commonly used. 

As an example, the gFOBT currently used in BC has a sensitivity of 36% and a specificity of 94% for the detection of colorectal cancer when colonoscopy is used as the gold standard for cancer detection. [4],[5],[6],[7]  

The sensitivity to detect an advanced adenoma (polyp > 9mm, villous features or high grade dysplasia) is only 18%.  [4], [5],[6],[7]  When the guaiac FOBT is used annually, colorectal cancer mortality is reduced by approximately 30% and the incidence by 18% as found in a clinical trial setting.  [8],[9] 

Immunochemical-based Fecal Occult Blood Testing and the new Auto FIT example

The other type of FOBT, called Immunochemical Fecal Occult Blood Test (iFOBT) uses antibodies to detect human hemoglobin protein in stool. One example of the iFOBT test is the Auto FIT (FOBT-CHEK®oc) a new automated version of iFOBT recently made available in Canada that uses antibodies to detect human hemoglobin protein in stool. Much like the gFOBT, immunochemical based testing detects the presence of blood in the stool, but the main difference is that the iFOBT/AutoFIT uses a different technology to detect the presence of gastrointestinal bleeding. For this reason, it may be a more accurate way to screen for blood in the stools than the traditional fecal occult blood test. This test reacts to part of the human hemoglobin protein (heme), which is found in red blood cells and it is also less likely to react to bleeding from parts of the upper digestive tract, such as the stomach. As with the gFOBT, immunochemical based testing may not detect a tumor that is not bleeding. If blood is detected, the patient will require follow-up testing such as colonoscopy, to determine the reason for the presence of blood in the stools.






Several types of iFOBTs are in use worldwide and the test performance characteristics vary according to the test11. In some provinces the new automated fecal immunochemical test Auto FIT (FOBT-CHEK®oc) has been implemented as the screening method of choice. According to the manufacturer, Auto FIT (FOBT-CHEK®oc) is the first automated fecal immunochemical based test available in Canada. This advanced immunoassay specifically detects human hemoglobin. Therefore, there are no dietary or medicinal restrictions. In addition, only one sample is required. Individuals can collect the sample with ease with no interruption to their daily routine. Studies have shown that an easy to use collection device and no dietary restrictions lead to increased testing compliance. With increased compliance, increased sensitivity and specificity, more of the correct population of people tested is sent for further testing by colonoscopy.

Automated Fit 01 http://www.somagen.com/images/immunology/home.jpg
AutoFIT Personal Pack Collection Materials Automated Analyzer OC-Auto Micro 80 used for Auto FIT Testing



Automation allows for standardization and quality control in the laboratory as well as a closed system environment ensuring safety from biohazards for technologists running the test.

Supplies include a collection device/bottle, absorbent paper, biodegradable collection paper, and a sample mailing envelope. This personal pack/ kit will provide detailed instructions on how to collect the specimen and it will also provide an educational brochure on the benefits of screening. The manufacturer advises that AutoFIT can pick up as little as 0.3 ml of blood and that it significantly improves sensitivity and specificity over traditional guaiac based methods.14 Testing requires only one single sample collection with no dietary or medicinal restrictions resulting in increased patient compliance, a primary goal of colorectal cancer screening methods and programs. Both FOBT and iFOBT can help direct the “right” patients to colonoscopy thereby leading to the earlier detection of polyps and colorectal cancer.15

A summary of the differences between gFOBT (traditional guaiac) and AutoFit is shown below as well provided by Somagen (www.somagen.com).

 

Feature

Auto-FIT

Guaiac Test

Dietary Restrictions

None

Yes

Restriction on Medications

None

Yes

Number of Samples Required

1

3

Number of Days Required for Sample Collection

1

3

Specificity

99%

98%*

Sensitivity

100%

50%

Patient Compliance

91%**

23%

Methodology

Automated

Manual

Specimen Container

Completely Closed System

Open System, Risk of Exposure

Collection of Sample

Easy, one step

Subject to Patient Error

*Denotes strict adherence to sample collection regarding diet and medications
** European J of Cancer Prevention 2006; 15: 384-390

Disclosure:

The contents of this section were made possible through an unrestricted educational grant from Somagen Diagnostics Inc. The Colorectal Cancer Association of Canada is proud to partner with Somagen and Polymedco in their effort to promote widespread, population-based colorectal cancer screening across all the provincial jurisdictions of Canada. Their mandate is to reduce colorectal cancer mortality and incidence through the implementation of the most advanced immunoassay available to date. For more information on Somagen’s AutoFIT, please visit Somagen’s website at www.somagen.com or www.fobt-tests.com.

References:

  1. Leddin D, et al., Canadian Association of Gastroenterology and the Canadian Digestive Health Foundation:  Guidelines on colon cancer screening.  Can J Gastroenterol 2004; 18: 93-99
  2. Screening for colorectal cancer:  U.S. preventive Services Task Force recommendation statement. Ann Intern Med 2008; 149: 627-637
  3. Levin, B, et al., Screening and surveillance for the early detection of colorectal cancer and adenomatous polyps, 2008: a joint guideline from the American Cancer Society, the US Multi-Society Task Force On Colorectal Cancer, and the American College of Radiology.  CA Cancer J Clin 2008; 58: 130-160
  4. Collins, JF, et al., Accuracy fo screening for fecal occult flood on a single stool sample obtained by digital rectal examination:  a comparison with recommended sampling practice.  Ann Intern Med 2005; 142: 81-85
  5. Lieberman, DA, et al., One time screening for colorectal cancer with combined fecal occult-blood testing and examination of the distal colon.  N Engl J Med 2001; 345: 555-60
  6. Greenberg PD, et al., A prospective multicenter evaluation of new fecal occult blood tests in patients undergoing colonoscopy. Am J Gastroenterol 2000; 95:1331-8
  7. Imperiale, TF, et al., Fecal DNA versus fecal occult blood for colorectal cancer screening in an average risk population.  N Engl J Med 2004; 351:2704-14
  8. Mandel, JS, et al., Reducing mortality from colorectal cancer by screening for fecal occult blood.  Minnesota Colon Cancer Control Study. N Engl J Med 1993; 328: 1365-71
  9. Mandel, JS, et al., The effect of fecal occult blood screening on the incidence of colorectal cancer.  N Engl J Med 2000; 343: 1603-7
  10. Levin, Theodore, It’s time to make organized colorectal cancer screening convenient and easy for patients. Am J Gastroenterol 2009; 104: 939-941
  11. Hundt, S  et al., Comparative evaluation of immunochemical fecal occult blood tests for colorectal adenoma detection.  Ann Intern Med 2009; 150: 162-9
  12.  Levi, Z, et al., A quantitative immunochemical fecal occult blood test for colorectal neoplasia.  Ann Intern med 2007; 146: 244-55
  13. Hol, L., et al., Screening for colorectal cancer: random comparison of guaiac and immunochemical fecal occult blood testing at different cut-off levels. British J Cancer (2009); 100: 1103-1110
  14. Grazzini, G, et al., Immunochemical fecal occult blood test:  number of samples and positivity cutoff.  What is the best strategy for colorectal cancer screening? British J of Cancer 2009; 100:259-265
  15. Van Rossum,L, et al., Random Comparison of guaiac and immunochemical fecal occult blood tests for colorectal cancer in a screening population. Gastroenterology. July 2008; 135 (1): 82-90