All about colonoscopy screening for colon and rectal cancer including understanding the FIT test

All About Screening for Colorectal Cancer
Screening for colorectal cancer is the process of looking for cancer in patients who do not yet have symptoms.  If colorectal cancer is found before it produces symptoms, it is highly curable (90%).  By the time symptoms occur, it is less curable (50%).  Screening also means looking for adenoma polyps, the precursors of the majority of colorectal cancers.  While it took a long time for authorities and payers to accept the benefits of colorectal cancer screening, we have now evolved well beyond that.  The current debate no longer centers on whether screening is worthwhile but on which tests to use, at what ages, and how often.  There are a number of ways of doing colorectal cancer screening:

Digital examination of the rectum by a physician
Limited to the detection of problems within a couple of inches or so. Notoriously unreliable.

Checking for microscopic blood in the feces (stool): FIT Test (Fecal Immunochemical Test) or FOBT (Fecal occult blood test)
The most common way to screen for colorectal cancer, fecal blood tests detect invisibly small amounts of blood break-down products in stool.  While not particularly accurate, they have been shown to reduce death from colorectal cancer in large populations, and so represent a significant advance in public health.

The original fecal blood test was the FOBT, which required a patient to take samples from three successive stools and place the sample on a series of three panels on a card that was then mailed in to the laboratory.  A technician in the laboratory would then add hydrogen peroxide to the three panels and if there was blood in the sample it would turn blue.  A blue panel meant the stool was determined to be ‘positive’ for blood.  Well, not really blood, the test actually detected ‘heme’, which is a breakdown product of blood (from the hemoglobin in blood breaking down into heme and globin).  The problem is that the heme could have come from blood anywhere in the gastrointestinal tract, even from the mouth or stomach.  So a positive FOBT meant that there was some blood from somewhere.  It was not specific to a colon lesion.  Moreover, the heme could have come from meat eaten by the patient and there were also a number of dietary items that could turn the panels blue.  Thus, the FOBT had problems with ‘false positive’ results, i.e. the test was positive but the colon, and indeed the entire patient could be normal.  In general, if a patient had one of three panels positive, the FOBT was wrong 85% of the time.  Sounds pretty bad.  But think about it, if it was right 15% of the time and it was truly identifying serious disease, that’s pretty significant.  So the test did reduce death from  colorectal cancer.  When two of the panels were positive the likelihood of a problem in the colon was greater, and even greater when all three panels were positive.  Another one of the problems with the FOBT was that the patient had to collect three samples on different days, thus complicating the test and increasing the likelihood that people would not do it.  It was just too much work.

The FIT test is the newer fecal blood test and has just been approved (covered by the Ministry) for general use in British Columbia.  The FIT test uses immune testing (antibodies matching with protein) to identify human hemoglobin, the protein in human blood.  This represents several important advances over the FOBT.  Firstly, it looks for human hemoglobin so it will not be positive from a rare steak.  Secondly, hemoglobin from the stomach and above (mouth, esophagus) will be broken down by the time it reaches the colon, and so the finding of hemoglobin in the stool localizes the bleeding site to the colon.  These two features should reduce the number of false positive FIT tests.  Thirdly, the FIT test is a single stool test, and it is hoped that this will make patients more inclined to do it and this in itself is an advance since no test can reduce colorectal cancer death rates if the patients will not do it.  Finally, the colonoscopist can feel more confident that a positive FIT test is a false positive test if the colonoscopy is normal than he or she could with the FOBT.  With the FOBT test, in the event of a positive FOBT test and a negative colonoscopy, the examiner then had to consider scoping the esophagus and stomach and essentially looking everywhere for the source of the heme identified by the FOBT.  With the FIT test however, a negative colonoscopy in a FIT positive patient is more likely a true false positive for colon disease and the doctor does not need to consider looking at other areas higher in the gastrointestinal tract.

Three other points about fecal blood tests need to be made.  Firstly, they should not be done on patients who are having frank (visible) bleeding.  Visible bleeding is a whole different issue from microscopic bleeding.  If a person says they are seeing blood, we don’t need to do a fecal blood test to find out if they are telling the truth, we just believe them and they should be investigated accordingly.  The vast majority of patients with visible blood will have a benign problem, and if they contaminate a fecal stool test for microscopic blood with real amounts of visible blood, the doctor will assume there is something mores serious in the colon.  So if there is visible blood, a fecal test such as the FIT or FOBT should NOT be done.  Secondly, in patients who need regular colonoscopy because of family history of colon cancer or other risk factors, FIT  or FOBT tests should not be done, since they will increase the number of unnecessary colonoscopies that patient will receive because of false-positive tests.  So a patient who is scheduled to have a colonoscopy every five years or so, should avoid FIT and FOBT tests and just stick to their much more accurate colonoscopy testing.  Finally, a downside of fecal microscopic testing is when the test is falsely negative.  This means that the patient actually has colon pathology but the test is negative.  The problem with this result is that it may provide false reassurance to the patient and physician and so subsequent symptoms may be ignored for a longer time in the setting of a negative FIT and the diagnosis of colon disease may ultimately be delayed.  In a recent study of 187 patients diagnosed with colorectal cancer at the Pezim Clinic, of the patients that underwent FOBT testing prior to referral to the Clinic, 14% had a falsely negative fecal occult blood test.    

Flexible sigmoidoscopy
An examination using a flexible scope.  This scope is suitable for biopsies and polypectomies.  The aim is to go about 35 to 60 cm (the colon is 160 cm).  It is as accurate as colonoscopy within the range that it is used.  In fact, may specialists will use a colonoscope to do a flexible sigmoidoscopy since a colonoscope is exactly the same as a flexible sigmoidoscope, but longer.  Flexible sigmoidoscopy has a simpler bowel preparation than colonoscopy, and no sedation is used.  The simpler bowel prep (usually just a couple of small enemas) does preclude doing polypectomies during flexible sigmoidoscopy.

Combined with a CT Colon, flexible sigmoidoscopy can be a fairly good way of screening the colon if colonoscopy is unavailable or not possible.  In a recent review of 187 patients diagnosed with colorectal cancer at the Pezim Clinic, flexible sigmoidoscopy would have detected 79% of them.  The risk of perforation is thought to be less than for colonoscopy, 1 in 6,000.  Surprisingly, even though flexible sigmoidoscopy is shorter, because it is done without sedation, and often by less experienced examiners, it can be more painful than colonoscopy.

A 160 cm flexible scope permits visualization of the entire colon and rectum and allows the examiner to do biopsies and polypectomies.  Colonoscopy is the gold standard for colorectal examination with a 97% accuracy rating.  A full oral bowel
preparation is required (see bowel preparation for colonoscopy).  Intravenous sedation is used to minimize discomfort.  The risk of damage to the colon is low, but it is not zero.  The rate of perforation of the colon during colonoscopy is 1 in 3,000, or up to 1 in 500 if a polyp is removed.  Pezim Clinic staff has performed over 11,500 colonoscopies and flexible sigmoidoscopies with no perforations.  In approximately 1 in 40 cases, the scope will not go in as far as required so the procedure will be incomplete.

Barium Enema
This is an x-ray of the colon.  In larger centers it has been superseded by CT Colon (colonography/virtual colonoscopy), which is more accurate.  In a barium enema, the radiologist instills a fluid (barium) into the colon through a small tube placed into the anus before the procedure in order to make the colon visible on the x-rays.  The procedure requires a full oral bowel prep.  The accuracy is about 85%, and no sedation is given.  Most patients remember a barium enema as a very unpleasant experience.  Barium enema has a low (1 in 10,000) perforation rate, but complications from perforation may be severe and long lasting.  If a polyp or other abnormality is seen on a barium enema, the radiologist cannot do anything about it other than recommending a colonoscopy as follow-up – this necessitates some further delay and another bowel prep.  This test is expensive for out-of-hospital x-ray centers to do (they used to do a lot of them) and MSP does not pay them enough to keep up with costs, so many such centers will no longer do them.  A barium enema combined with flexible sigmoidoscopy makes for a high quality screening program in areas in which colonoscopy or CT Colon are not available.

CT Colonography/CT Colon
A computerized x-ray of the colon.  It is more accurate than a barium enema but less accurate than a colonoscopy.  CT Colon requires a full oral bowel prep, and no sedation is given.  The lack of sedation may or may not be a plus, depending upon the patient and the nature of the colon problem being investigated.  At the beginning the CT Colon, the radiologist distends the colon by pumping gas into it via a small tube placed through the anus.  The perforation rate for CT Colon should be much less than for colonoscopy, but it will not be zero due to this requirement to distend the colon.  It is not accurate for polyps less than o.6 cm in diameter or in areas that do not distend well, such as the sigmoid colon if there are many diverticulae present.  As well, CT Colon is not good at looking at the anal canal or very low rectum. As with barium enema, CT Colon may require follow up colonoscopy for clarification or removal of polyp(s).  An interesting feature of CT Colon is that the radiologist can also scan other organs at the same time so the patient can get information about other body parts, although the resolution for these other organs is not as good as a standard CT scan.

Which test is best? It all depends on what is most important to you.
If you are looking for safety above everything else, it is the FIT test.  It’s not the most accurate test in the world, but the only real risk would be falling into the toilet while attempting to retrieve the stool.

If the highest level of accuracy and reassurance is what you want, colonoscopy is the best test.  With it comes the highest risk of complications, but that risk is fairly low in the hands of an experienced examiner and a reasonably healthy patient.
Colonoscopy also has the advantage of permitting the doctor to remove polyps or to biopsy abnormalities.  Colonoscopy and polypectomy is the most powerful cancer prevention technique currently available in modern medicine.  It does have the downside that not all patients can be colonoscoped fully.  In about 3 – 5% of patients it can be too difficult to insert the colonoscope all the way to the end of the colon.  If this happens, another test will be needed to complete the assessment, usually a barium enema or CT colonography.

CT Colon is somewhere in between a FIT test and a colonoscopy, more towards the accuracy of colonoscopy.  CT Colon has the drawback of less accuracy than colonoscopy and the inability of the examiner to do something if an abnormality is found.  But it is quite safe.  CT Colon does not use sedation (although some patients wish they would), so that is a plus for some.  Follow-up colonoscopy may be recommended in up to 15% of cases.  The radiation required for CT Colon is equivalent to about 350 chest x-rays.  This may sound like a lot but one gets a similar dose on a transatlantic flight.

So what’s the bottom line?
FIT and FOBT testing is safe and have made a positive impact when attempting to identify disease in a large community.  FIT testing seems to have been a significant advance over FOBT testing.  However, neither are reliable enough to use for testing an individual who really wants to know whether he or she has a colon problem.  Colonoscopy is the gold standard for examination of the colon and rectum.  With a 97% accuracy, 95% completion rate, the ability to take biopsies and remove polyps, and a low perforation rate in experienced hands, it is the way to go for patients that are in a reasonable state of health.  In addition, a negative colonoscopy is much more reassuring than a negative barium enema or a negative CT Colonography.  A CT Colon or barium enema may be recommended for those patients who are less fit (have more medical problems) and who may not tolerate intravenous sedation or in whom colonoscopy is incomplete or not available.  As we learn more about CT Colon and it becomes more widely available, it will assume a larger role in colorectal cancer screening.  For now, however, CT Colon is not recommended for screening in patients with a positive FIT test.

© Pezim Clinic, Vancouver, British Columbia, Canada