Fecal Occult Blood Testing:
This test is based on the premise that a polyp or tumor will bleed intermittently and testing the stool for trace amounts of blood which may not be visible to the naked eye may enable us to detect the presence of these polyps or early cancers. Generally, there are two types of FOBT - one that tests for a reducing reaction and the other that is specific to human haemoglobin. The haemoglobin specific test is more accurate as it removes the false positive results from other chemicals in the stool that may cause a reducing reaction. A positive result should be further evaluated with COLONOSCOPY as this provides us the opportunity to not only diagnose the source of bleeding, but also remove it.
Unfortunately, FOBT only has a sensitivity of about 40-50% - which means that more than half of patients who test negative may still have a polyp or tumor. False positive tests can also occur if one is taking certain drugs or food products that may cross react with the reagent. The main advantage of FOBT is the low cost which makes it affordable and it is widely employed throughout the world for population screening of AVERAGE RISK individuals.
Double Contrast Barium Enema:
Double Contrast Barium Enema (DCBE) is a study where a tube is inserted into the rectum and x-ray contrast and air are injected into the colon. By turning the patient and applying pressure to the abdomen, it is possible to outline the length of the colon using x-rays. The procedure requires cleansing of the colon with laxatives and is uncomfortable for patients.
One of the inherent difficulties with DCBE is that visualization of the rectum is often poor and lesions can be missed. Therefore, the recommended practice is to always perform a sigmoidoscopic examination of the rectum when DCBE is used to evaluate the colon. The other limitation is that patients who fail to clear the colon of feces cannot be accurately assessed, and there is no ability to remove lesions if they are not seen on the study. An abnormality seen on a DCBE will need further colonoscopy.
CT Colonography aka Virtual Colonoscopy
This technique utilizes helical CT images that are reconstructed by the computer to create a 3 dimensional image of the colon. The computer generated image can be rotated and one can view the inside of the colon just as if one were performing a colonoscopy. Air is insufflated through a rectal tube instead of x-ray contrast to distend the colon and delineate the colonic wall.
The main limitations of this study are that mucosal abnormalities and small lesions can be missed or difficult to interpret. There is also no ability to obtain tissue from the colon for histological testing which is required to confirm the presence of cancer. In order for the study to be accurate, the colon must be cleared of fecal material, and the patient is required to take laxatives as in a normal colonoscopy.
Colonoscopy is currently still the GOLD STANDARD for evaluation of the colon. The procedure involves inserting a fibreoptic flexible instrument with a camera system through which the endoscopist is able to not only visualize any lesions within the colon, but also remove them or obtain tissue specimens for further examination. Bowel preparation with laxatives is necessary to optimize the visualization but even when there is some fecal material left in the colon, it is possible to remove it by means of water irrigation of the area via the scope.
Patients often experience pain or discomfort during the procedure, especially when the scope is being inserted, and so it is customary to sedate the patient during the procedure. If a polyp is detected, the endoscopist will attempt to remove it for histological examination. If the lesion is too large to remove safely with the colonoscope, the endoscopist is able to obtain tissue for diagnosis and sometimes “tattoo” that part of the colon so that it can be easily identified by the surgeon if resection of the lesion is found to be necessary. This is especially helpful with small tumors when laparoscopic (keyhole) surgery is performed so that the surgeon is able to correctly localize the site of the tumor.
The most serious complication of colonoscopy is perforation of the colon - especially if a polyp was removed. Perforation can also occur if there is a sharp angulation of the colon, making insertion of the colonoscope difficult. Excessive force during insertion - especially if the endoscopist is inexperienced - can cause a tear of the colonic wall.