FIT (Fecal Immunochemical Test) & FOBT (Fecal Occult Blood Test) – We provide rapid colonoscopic assessment
These tests identify minute quantities of blood or blood break-down products in the stool that might indicate a slowly-bleeding polyp or cancer of the colon. The idea is that these tests will identify blood when there is so little of it that the patient does not see any blood with their eyes. This permits earlier identification of polyps and cancers and has been shown to reduce death from colorectal cancers in large population groups. These tests should NOT be used in patients with visible bleeding since visible bleeding is an entirely different matter and most often is not related to colon cancer.
The FOBT (Fecal Occult Blood Test, or Guaiac Test) was the original test. Patients put a sample of stool from three different days onto three cards, and mailed the cards into a laboratory. The laboratory would ‘develop’ the cards with hydrogen peroxide and those cards with blood on them would turn blue, indicating a ‘positive FOBT’ test in one, two or three cards. Unfortunately, the FOBT was wrong in up to 85% of cases (especially when only one card was positive), partly because it identifies ‘heme’, which is a break-down product from blood anywhere in the GI tract, even from the teeth. As well, patients had to restrict certain items in their diet when doing the FOBT test in order to not cause the test to be falsely positive, and often they did not do that properly. And if you colonoscoped a patient with a positive FOBT and found nothing, you then had to start thinking about putting a scope through their mouth and down into their stomach (gastroscopy) since the site of bleeding may have been way up there (ulcer, gastritis, stomach cancer, etc). So a better and easier test was needed, one that was more specific for microscopic blood originating in the colon or rectum exclusively.
The FIT test is a new test that identifies human haemoglobin in the stool and is taking over from the FOBT test. It is a bit more expensive for the government, but since it is more specific to colon blood, the hope is that there will be fewer expensive wild goose chases (additional tests) in patients who are positive but have negative colonoscopies. Because the test identifies human hemoglobin only, the source of that hemoglobin is more likely to be found in colon or rectum since hemoglobin from the stomach will have been broken down to heme by the time it gets to the colon. Since the FIT test does not identify heme, it is more specific as an identifier of minute amounts of blood from the colon or rectum. Unlike the FOBT which required the patient to take three days of stool samples, the FIT test only reuires a single sample, so it is easier for the patient to complete and hopefully that will translate into more patients doing it, and since it only identifies human hemoglobin, diet restriction should not be necessary (unless one is testing a group of cannibals). While we still expect there to be false positive FIT tests (the test is positive but the patient has no problem), they should be fewer, and when the colonosopy proves to be negative, the physicain does not have that same obligation to consider looking higher in the GI tract. In other words, it is easier to accept that a study is a false positive in the FIT-test-positive with a negative colonoscopy than it is in the FOBT-positive patient with a negative colonoscopy. This colon-specificity of the FIT tests therefore provides greater reassurance to the patient and physician should the colonoscopy be negative, as it will in many cases.
Unlike the FOBT test which was either ‘positive’ or ‘negative’, the FIT test comes with a ‘number’ or measure of amount of hemoglobin associated with the result. A normal FIT test in British Columbia is one that registers less than 50 nanograms per ml (ng/ml). Presumably there is always some hemoglobin in the stool as cells from the bowel wall shed normally, etc, so the screening experts, in this case the Cancer Agency, have selected a level of 50 ng/ml as the cut-off between normal and abnormal. So levels of 50ng/ml and higher are deemed to warrant further investigation, usually colonoscopy. At the Pezim Clinic we have colonoscoped many positive FIT patients and have seen numbers from 51 to over 20,000 ng/ml. It is not entirely clear how the value relates to risk of a polyp or cancer, but there does seem to be an increased risk with the higher numbers.
When a test, either the FOBT test or a FIT test come back positive, patients will often ask if they can just try it again and hope that it is negative the second time thinking that they can just forget about it if the second test comes back negative. Unfortunately, this is not the case. Tumors can bleed intermittently, so a positive test needs colonoscopy, even if the next ten prove to be negative. Once you have a positive test on file, you are ‘hooped’ and every MD who you see thereafter will be uncomfortable with it until a colonoscopy is done to prove whether the test is right or wrong.
At the Pezim Clinic we set a high priority on rapidly dealing with patients referred with a positive FIT or FOBT test. We know how anxiety-producing a positive FIT or FOBT test can be and we aim to complete our assessment within 3 weeks or less.
© Pezim Clinic, Vancouver, British Columbia, Canada