In a screening programme there are several possible tests and examinations that can be performed. These possibilities are listed below:
Fecal Occult Blood Testing (FOBT)
Here a small stool sample is taken and examined for the appearance of blood.
The rationale for this examination is based on the observation that cancerous mucosa bleeds more than normal mucosa. As polyps also bleed, particularly if they are large, and are more common than cancer, FOBT will also detect polyps. The advantages of a non-invasive test is combined with the disadvantages of either low sensitivity (people with early cancers will be missed) or low specificity (many people without bowl cancer, adenomas or polyps will undergo full bowel examinations). Screening intervals for average risk individuals are suggested to be every 1 to 2 years.
Screening can be divided into so called mass screening and opportunistic screening. Mass screening is the presumptive identification of unrecognised disease by the use of tests to identify apparently well people who probably have a disease from those who probably do not. Mass screening is usually organised by the government and offered to the whole population of a country. Opportunistic screening is considered to be a form of screening in which the main object is to detect disease by bringing individual people to treatment. Opportunistic screening is also called “case finding” performed with the help of general practitioners.
Flexible Sigmoidoscopy (FS)
The effectiveness of screening sigmoidoscopy is supported by direct evidence from three case control studies suggesting that this screening method can reduce incidence and mortality (number of deaths from bowel cancer) of the distal colon and rectum. Although sigmoidoscopy has no effect in reducing incidence/mortality of cancer in the proximal colon the rationale of using sigmoidoscopy for screening is that the majority of all colon cancers develop in the distal part of the colon within the reach of a sigmoidoscope.
Fecal Occult Blood Testing & Flexible Sigmoidoscopy
The combination of both screening methods (FOBT and FS) may correct some of the limitations of each method used alone. Evidence that these two modalities together are more effective than either one alone comes from a non-randomised controlled trial of FOBT in patients who had had a screening sigmoidoscopy. Indirect evidence is that FOBT alone is a weak strategy for detecting polyps (anywhere in the bowel) and that sigmoidoscopy cannot find adenomas or cancers in the proximal colon.
Double-contrast barium enema (DCBE)
There are no studies evaluating whether screening DCBE alone reduces the incidence or mortality from colorectal cancer in people at average risk of the disease. The use of this method is based on the evidence that screening DCBEs can image the entire colon and detect cancers and large polyps almost as well as colonoscopy and better than FOBT or sigmoidoscopy. The procedure is probably safer than sigmoidoscopy or colonoscopy (apart from the x-ray exposure). This method can, however, miss small polyps and does not permit removal of polyps and biopsy of cancers. The identification of artifacts is not easy so abnormal barium enema may need a subsequent colonoscopy.
Colonoscopy
There are also no studies evaluating the use of colonoscopy alone, however, colonoscopy was an integral part of randomised and non randomised controlled trials of FOBT that showed a reduced mortality in screened patients. Also, colonoscopy is analogous or even superior in performance and effectiveness to sigmoidoscopy, which has proven to reduce colorectal cancer mortality. Because colonoscopy permits visualisation of the entire colon directly, detection and removal of polyps and biopsies of cancers throughout the colon it can be very well considered for screening average-risk individuals. Colonoscopy involves a greater risk and inconvenience to the patient than sigmoidoscopy, and not all examinations can visualise the entire colon. Recommendations suggest a screening interval of 10 years because there is strong direct evidence that only few clinically important lesions are missed by this examination.
Virtual colonoscopy is in a way similar to DCBA; instead of using standard x-ray imaging techniques, pictures from computed tomography (CT) are used in combination with 3D rendering software to show the inside of the bowel.
Main disadvantages of this method are high costs and reduced sensitivity for small polyps. On the other hand the acceptance of people to undergo VC compared to colonoscopy seems to be higher, strictures do not hinder the inspection of the proximal colon and it is possible to detect extracolonic findings.
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