Dr John P Wright
Updated: 22 September 2008
Many routine medical check ups include an occult blood test to detect early colonic cancer. Is this pious hope or is there a serious down side? The motivation for this is the belief that polyps and colonic cancers bleed and this can be detected by the testing the faeces. Unfortunately not all polyps and cancers do bleed and there are other sources of faecal blood that may confuse. For instance anal fissures and haemorrhoids are common sources of rectal bleeding. A meaty meal, a slowly oozing gastritis due to the ingestion of non steroidal drugs may add to the normal amount of blood found in faeces. In theory of course blood in the upper gastrointestinal tract should be absorbed as any food would be. To avoid these false positives, occult blood tests are generally designed not to pick up these normal low levels of blood in faeces. To avoid food related false positives, antibody tests for human protein have been developed. Another area of false positives is that due to medications such as vitamin C
The downside of occult blood testing is the cost of the annual check, the anxiety while waiting for the results and then the non-compliance with the annual check. The most important downside is the false sense of security that a negative test gives.
When performing occult blood testing a 2 day high fiber diet is recommended prior to the testing of two patches of stool on 3 different stool specimens. If a test is positive the chance of a colon cancer is about 2%. This means that for every cancer found 50 people have “unnecessary” colonoscopies. These of course are the lucky ones as at least they now know that they have neither polyp nor cancer. A frequent practice is to re-hydrate dried occult blood test cards. This practice increases the sensitivity of the test but as indicated above also increases the false positive rate and therefore the colonoscopy rate.
Several large studies have shown that mortality from colon cancer is reduced by about 30% if a prospective program of faecal occult blood testing s instituted. Conversely almost 40% of people have a colonoscopy during the program and overall mortality is not really influenced. It is suggested that the apparent success of the programs is related to the numerous colonoscopies carried out.
If occult blood testing has too many false positives and almost half the people will come to colonoscopy anyway is it not better to go straight to colonoscopy? It is normally recommended that a screening colonoscopy be performed when a person is 50 years old and in view of a false negative rate to repeat the procedure again at 55 or 60. Flexible sigmoidoscopy has been shown to reduce mortality due to colon cancer by about 33%. If we view colonoscopy as simply a longer sigmoidoscopy we could expect even better results. The evidence that such a program will detect more cancers is however not there. The problem is complicated by the perception that colonoscopy is expensive, requires an unpleasant colonic preparation, is painful and has risks. Expense is of course a relative term and has to be related to the value obtained IE is there or is there not colonic neoplasm. The expense of colonoscopy can be significantly lowered by for instance performing the procedure it in an endoscopy suite rather than a full theatre. The cost has also to be compared to the hazzle of 2 days dieting and collecting 3 specimens every year. Not to forget the 40% who will come to colonoscopy anyway. The unpleasant colonic lavage is difficult to avoid. The use of small volume preparations containing sodium phosphate are dangerous and should not be used. The pain and complication rate is operator dependent. Not all colonoscopists are equally trained or proficient. For about 80% of people colonoscopy is no more painful than a mild colonic cramp. This is shown be the increasing number of people who are selecting not to have conscious sedation so that they are able to drive home afterwards.
In spite of this reality many people are considering virtual colonoscopy. Unfortunately this procedure still requires a bowel clear out and some insufflation. It is therefore not the painless easy procedure many had expected and abnormalities require colonoscopic verification. Studies have show that the vast majority of people prefer a competent video colonoscopy to a virtual examination. But in people in whom a colonoscopy can not be completed a virtual examination is mandatory.
In conclusion where competent colonscopists are available a routine colonoscopy at 50 years is recommended with a final check 10 years later. If the person is in a high risk group or has a positive family history than colonoscopy is performed between 45 and 50 years of age. This should be at 10 years before the index case had the diagnosis made and then repeated 5 yearly.
Radiological virtual colonoscopy has a place when ideal conditions can not be met but the use of annual testing for faecal occult blood should probably be replaced with the more definitive imaging now available.