Ask the Expert: Which Colon Cancer Screening Method is Best?

Colon cancer affects five percent of Americans, or one in 20 individuals. During 2017, it is estimated that 95,520 new cases of colon cancer and 39,910 new cases of rectal cancer will be diagnosed. The best way to prevent colon cancer is through regular screening, but which test is best? There are several options from which to choose when it comes to colon cancer screening, and people may disagree about which screening is preferable.

Sanjay Reddy, M.D., is a Board Certified gastroenterologist with Palm Endoscopy Center in Altamonte Springs, Florida. Dr. Reddy discusses the various colon cancer screening methods and which screening offers the best prevention against colon cancer.

Would you please give a short description of the colon cancer screenings that are available for patients?
Colon cancer screenings can be divided into two categories. The first category is tests that detect colon cancer and polyps.

  • A colonoscopy allows your gastroenterologist to view the entire inner lining of your large intestine for polyps, tumors, ulcers, or inflammation by using a thin, flexible tube called a colonoscope. If polyps are discovered during the procedure, your doctor can remove them during the procedure, and then test them to determine risk for colon cancer. Because it is active prevention, allowing both diagnosis and treatment, colonoscopy is widely accepted as the gold standard for colon cancer screening.
  • Flexible sigmoidoscopy is an abbreviated version of a colonoscopy. Your gastroenterologist uses a scope to inspect the lining of the lower colon and rectum, but the upper colon is not examined.
  • Virtual colonoscopy or CT colonography is performed by a radiologist who uses multiple computerized tomography (CT) images to create a detailed picture of the inside of your colon. This procedure can show larger polyps and cancers, but does not include any treatment or removal.
  • A double contrast barium enema uses a series of x-rays to evaluate your colon and rectum. Barium, a contrast dye, is placed into your rectum through an enema, and then air is added. The barium and air help to outline the colon and rectum on the x-rays so your doctor can examine the films for abnormalities in the colon.

The second category is tests that mainly detect cancer.

  • Guaiac-based fecal occult blood test (gFOBT) and fecal immunochemical test (FIT) detect blood in a stool sample through a chemical reaction.
  • Stool DNA test (sDNA) is a suite of DNA testing which looks for certain abnormal sections of DNA (genetic material) from cancers or cancerous polyps.

Cologuard has received a great deal of attention lately. How does it compare to the other stool tests as a screening method for colon cancer?

The stool DNA test, commonly known as Cologuard, has been on the market since 2014. All three stool tests are non-invasive and easy to complete, looking for evidence of cancer growing in the colon. To give a little history, the guaiac-based fecal occult blood test was the original stool test (and is still available today). It tests for the presence of blood in the feces, but medications and food can affect its accuracy. The fecal immunochemical (FIT) was the next stool test that was developed. FIT tends to be more accurate than its predecessor, and typically, medicines or food do not interfere with the test.

Cologuard can be described as a suite of tests, which includes the FIT. It is the first stool-based colon screening test that detects the presence of altered DNA which may indicate certain kinds of abnormal growths related to colon cancer. Therefore, it is the most comprehensive among the stool tests. It is set apart from the other two screenings because it detects the presence of DNA in the stool. If a colon growth is present, pieces of DNA will slough off in the stool and this can be detected by analyzing the stool sample. Because it is covered by Medicare and most private insurance companies, this test is available to most adults between the ages of 50 and 75 years of age.

One of the drawbacks of Cologuard is that a positive stool DNA test simply means that you will have to schedule further testing to determine the source of the DNA. Your doctor will recommend that you have a colonoscopy for a full examination of the colon lining and the possible removal of polyps.

For negative stool DNA tests, we see further limitations. There is a lack of long term follow-up studies, so there are no real guidelines by which to determine when the next test should take place, or how to best follow-up a negative result. Some physicians recommend DNA tests yearly, while others recommend it every 10 years (based on high quality colonoscopy studies).

Finally, it has really only been validated for cancer in later stages of growth, which may be more difficult to treat in some case.

Why is it important that doctors fully inform their patients regarding the various types of colon screenings?

Patients need to be informed about the types of colon screenings so they can understand the benefits and limitations of each choice. At first glance, a non-invasive test like the Cologuard or FIT may appear to be a better option because it requires less of a time commitment.

A patient may think that all screenings are created equal, so he or she may choose a stool test because it is simple. However, detecting blood in the stool does not indicate the source, or even that a cancer is present. Additional testing is then required for clarification of this ‘easy’ test.

A negative stool DNA test, in the case of a small polyp that has not yet turned into a cancer (but is highly likely to) may provide reassurance, but not the protection for up to 10 year, indicated by large high-quality colonoscopy studies. Lack of guidance on when to repeat a stool test may further confuse the screening efforts of a well-intentioned primary care provider.

On the other hand, a colonoscopy may initially seem time-intensive and overly invasive. The bowel preparation and clear liquid diet takes effort and time, so it is understandable why some patients hesitate to schedule a colonoscopy. However, when we compare screenings that detect blood or DNA versus screenings that detect cancer and can remove the risk from polyps, we see some very divergent characteristics.

A colonoscopy inspects the entire colon, and precancerous polyps are removed before they can develop malignancies. After this explanation by the doctor, the bowel preparation and fasting have a distinct function: to create an optimal environment for a gastroenterologist to find and remove precancerous polyps. When doctors explain why a colonoscopy is the most effective and efficient in the areas of diagnosis, treatment and prevention, the effort becomes secondary to the preventative benefits.

If a colonoscopy is better than a stool test, can a virtual colonoscopy be just as effective without the invasiveness?


The term virtual colonoscopy is simply advertising and like any other commercial, there is fact and then there is hype. I prefer the actual name of the procedure, CT colonography. This procedure, plainly stated, is a really nice CAT scan of an organ. It is performed by a radiologist, not a gastroenterologist. It may be helpful; it may find a suspicious lesion- but it may also be expensive, and it could lead you to having a colonoscopy to confirm a questionable result. Since it requires a laxative solution, as colonoscopy does, and you aren’t comfortably sleeping while your colon is pumped full of air, I’m not really sure why you would pick this test.

In fact, as far as I know, most insurances will not cover this procedure routinely. It is a very high- quality CT scan with beautiful images, but I think it is important for patients to know that this procedure is heavily marketed as an alternative to a colonoscopy, but it is not a colonoscopy. Another technology is being used to “virtualize” the gold standard in colon cancer screening, and it is being branded as the next best alternative. Colonoscopy is universally recognized as the most effective screening method for detecting and preventing colon cancer, and the CT colonography acknowledges that- by calling itself a virtual colonoscopy. There is no substitute for the real thing.

When presenting screening options to your patients, do you encourage everyone to get a colonoscopy?

We as physicians are not doing right by our patients if we don’t tell them that a colonoscopy is the most effective colon screening. The colonoscopy is the most comprehensive test available, and we have specific guidelines from the U.S. Preventative Services Task Force as to how often to repeat the exam, so we can confidently assure our patients that it is best. All positive test results from alternative screening options require patients to schedule a diagnostic colonoscopy, so why not just schedule the screening colonoscopy in the first place? As advocates for our patients, we must tell them that a colonoscopy is the only method that, when repeated in regular intervals, actually prevents colon cancer.

Now, there are certain instances where I would not suggest a colonoscopy. For patients who have heart disease or lung disease, perhaps a sedated test isn’t the best choice for routine screening in that person. It may be safer for them to choose a non-invasive stool test as a first step to screening. The stool DNA test was developed to give patients choices. Research shows that when patients are given choices, they are most likely to say “yes” to colon cancer screening.

For the patients who cannot or choose not to have a colonoscopy, I want some other options to offer them. Cologuard is a good test, but it can never offer the preventative benefits of a colonoscopy. Some patients choose to pair colonoscopy with a stool test. The U.S. Preventative Services Task force recommends that screening colonoscopies are repeated every ten years, but many of my patients include the FIT as part of their colon cancer screening regimen between colonoscopies. The bottom line is to pick the best test for that patient. I do this by having a discussion about the merits of the screening, and not just picking a test because of cute marketing.

What is your main goal in working with patients in your practice?

What I really want to see is our overall screening rates increase and colon cancer incidence decrease. One out of every three Americans between the ages of 50 and 70 are not getting screened for colon cancer. We are seeing the effects of this fact because currently, colon cancer the third-leading cause of cancer death in the United States among men and women.

The National Colorectal Cancer Roundtable began an initiative a few years ago called 80% by 2018. Their goal is to achieve a nationwide screening rate of 80% by next year, which means that we have a lot of work to do. The first step is to spread awareness. We must inform our patients of the prevalence of colon cancer and give them the options for colon cancer screening. Some doctors say, “The best colon cancer screening is the screening that gets done,” but I would say, “The best colon cancer screening is the quality screening that works.” I think that, when it comes to colon screenings, there are good, better and best categories. Suggesting colonoscopy as the best and most effective colon cancer screening, as well as presenting other options, will, in time, boost screening rates and reduce colon cancer mortality.


Dr. Reddy received his Medical Doctorate from the University of Miami in 1998 and completed his Residency in Internal Medicine and his Fellowship in Gastroenterology and Liver Disease at Rush University Medical Center in Chicago. Dr. Reddy is Board Certified in Gastroenterology and serves on the Faculty of the University Of Central Florida College Of Medicine.

Repeatedly recognized by his patients and colleagues as one of Orlando Magazine’s “Top Doctors”, Dr. Reddy has received the American Medical Association’s “Distinguished Physician Award”, the “Patient’s Choice Award”, and has been named to the “Most Compassionate Doctors” by vital.com. To learn more about Dr. Reddy and his practice, please click here.

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