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Cologuard® vs. Colonoscopy: Which test is better?

Doctor about to perform colonoscopy

In the US, colorectal cancer is currently the second leading cause of cancer deaths in men and women combined. Virtually all health professionals agree that screening average-risk people starting at age 45 for colorectal cancer is the best way to prevent this disease. There are a few highly effective tools available for colorectal cancer screening: This article will focus on the two most popular tests, colonoscopy and the Cologuard test.

Most people are familiar with colonoscopy, but in case you’re not…colonoscopy is a safe 15-20 minute outpatient procedure that is done using sedation (so it’s painless) that entails using a flexible scope inserted in the rectum (the end part of the colon) and carefully advanced to the cecum (the other end of the colon) to inspect the entire colon lining for the presence of polyps, tumors, and other abnormalities. A polyp is a “precancerous” growth that is common and if not removed can slowly grow and eventually may turn into colon cancer. During a colonoscopy, the doctor can both find polyps and remove these polyps at the same time. Therefore colonoscopy can both diagnose cancer and polyps, but more importantly colonoscopy can prevent colorectal cancer from occurring in the first place by removing precancerous polyps years before they would have otherwise become cancer. A normal colonoscopy typically does not need to be repeated for 10 years. If polyps are found and removed, a colonoscopy will be recommended sooner, to make sure any new polyps that grow will be found and removed before having a chance to turn into colon cancer. Colonoscopy is considered the gold-standard colon cancer prevention test and is the preferred test by most medical societies.

Image of colonoscopy with polyp removal.

Cologuard® is a non-invasive, commercially-available screening test for colorectal cancer. The test is ordered by a doctor (typically a primary care provider) and mailed to the patient’s house. When the urge to have a bowel movement strikes, the patient places the Cologuard collection device over the toilet and makes a deposit. There are a few simple preparation steps such as adding in a small amount of liquid that the company provides, and then the entire container of poop is mailed back to the company in the provided packaging. The Cologuard test looks for blood and certain DNA mutations in the stool to determine if a test is positive or negative. A week or so later the doctor gets a report indicating the result. If the test is negative, it only “protects” the patient for 3 years.

So which test is better, colonoscopy or Cologuard?

Well, it really depends on what the goals are…

As a general philosophy, it’s much more effective to prevent a disease from occurring rather than waiting for the disease to occur, then reacting to it. When the disease in question is colon cancer, preventing it starts with healthy diet and lifestyle as well as screening the population at large for polyps, the precancerous growths that cause colon cancer. To have a colonoscopy and remove a significant polyp is akin to stopping a future cancer from occurring in the first place. When effective preventive tests like colonoscopy exist, to wait until a patient has developed cancer and then treating the cancer is like waiting until you have been in a car accident to then put on your seatbelt…it’s too late. Although there are excellent treatments available for colorectal cancer nowadays, including surgery and chemotherapy, treating cancer is not the goal of screening. The goal of screening is to not develop cancer of the colon in the first place!

Typically, when an effective prevention technique exists (like removing precancerous polyps during colonoscopy) the earlier we can screen for colon cancer the better. This is why colonoscopy is the preferred test for younger healthier people starting at age 45. Save a 45-50 year old from colon cancer and you will potentially give that person 30-40+ years of life having not developed colon cancer. Saving a 79 year old from colon cancer is still a commendable goal, however the average 79 year old typically won’t have as many quality years left “in the tank” compared to the average 45 year old.

So why does this philosophical stuff matter when it comes to picking a colorectal screening test? Well, understanding what these tests do helps you understand how to apply the proper test to your individual goal.

If this article is already too long and you just want the bottom line, here it is: Colonoscopy is the superior test for most people, especially “younger” people (age 45-mid 70s). It can both detect and (more importantly) prevent colorectal cancer. It is semi-invasive and less convenient when compared to Cologuard testing. Cologuard is an easier test but plagued by false negatives and false positives. Cologuard does not necessarily prevent cancer, it only detects cancer after cancer has occurred, or at best detects large polyps that are close to becoming cancer. Cologuard should be considered for older patients (age 75+), for patients that may not have the best overall health, or for patients who have specific reasons why they cannot have a colonoscopy.

OK, you asked for it! Here are more details, starting with the Cologuard test:

The study that determined the characteristics of the Cologuard test basically performed the test on almost 10,000 patients at average-risk of colon cancer, and then had the patients undergo colonoscopy as the gold-standard test. The results of the Cologuard test were not available to the patients or the endoscopist at the time of the colonoscopy. The major results of this study showed that the Cologuard test had a sensitivity (the amount of times it picked up colorectal cancer when cancer was indeed present) of 92%. It was far less sensitive for picking up advanced precancerous polyps, at only 42%. It turns out that sensitivity is the main thing we care about in a screening test: we want the test to miss none of the patients who have the disease. A perfect screening test would have a sensitivity of 100%, meaning that if 100 people have colon cancer and have the test, all 100 people will get a “positive” test result, meaning no false negative tests.

Sensitivity isn’t everything however…we also want a test that gives a negative result when someone does not have the disease in question. That is, if you take a group of 100 people that do not have colorectal cancer, a perfectly specific test will have 100 “negative” results, meaning no false positive tests.

What does a positive Cologuard test mean?

First and foremost, a positive result on the Cologuard test means that you need to have a colonoscopy. Not a virtual colonoscopy, or another stool test, or another scan of some sort…you need a real optical colonoscopy. Luckily, only about 4% of people with a positive Cologuard test will have cancer found on colonoscopy. 51% will have a precancerous polyp. The rest (45%) will have nothing found on colonoscopy. So to simplify even further, just a little more than half of people with positive results will have something abnormal (cancer or a polyp) found on colonoscopy.

What does a negative Cologuard test mean?

A negative test means that there is a less than one-percent chance of having cancer found on colonoscopy. However, about 34% of people with negative tests still have precancerous polyps found on colonoscopy, with the remainder (66%) of people with negative Cologuard results having truly negative colonoscopies.

What is immediately apparent from these numbers is that Cologuard rarely misses cancer. However, if we count polyps as a significant finding, there are plenty of false-positive results (45%) and plenty of false-negatives too (34%).

A word on how health insurance companies view Cologuard

While not important to the medical reasoning behind choosing colonoscopy or Cologuard, for some people it is important to note the finances of each test. Either colonoscopy or Cologuard can be considered a screening test, and is typically covered by health insurance plans without an out-of-pocket cost. However, if a Cologuard test is positive (remember that 45% false positive rate discussed above), the insurance company now views the necessary colonoscopy as a diagnostic colonoscopy, not a screening colonoscopy. Diagnostic tests often have an out-of-pocket responsibility for the patient and in the case of a colonoscopy this can be in the thousands of dollars range. This is something rarely discussed when ordering a Cologuard test in the primary care setting, but that we often need to educate patients about when it’s time to book their colonoscopy to follow up a positive Cologuard test.

What about colonoscopy? Are there any downsides?

In good hands, colonoscopy is an excellent test—it’s the best test we have in the fight against colon cancer. However, no test is perfect and colonoscopy is no exception. Even though colonoscopy is the gold-standard test, here are some of the negative things to know about colonoscopy.

Colonoscopy requires a bowel preparation, meaning you have to take either a liquid prep or pill prep to clean out the colon the day before. It’s not painful, but prepping for a colonoscopy is far from a good time. Colonoscopy has small but real risks, such as bleeding, infection, perforation of the bowel, and anesthesia problems. However these risks are very rare, and in with a skilled team the risk of a serious complication is far less than 1 in 1,000 procedures. Colonoscopy also has a miss rate for polyps and even cancer. It is very hard to define an actual number of missed lesions because it’s difficult to perform a study on the gold-standard test (colonoscopy) as there is no better test to compare it to. That being said, colonoscopy can miss small polyps around 20% of the time, and can even miss cancer a few percent of the time. The devil is in the details however: Missing a significant lesion during an outpatient screening colonoscopy in a properly prepped patient (meaning the patient did the bowel cleanse effectively) with a doctor that performs high-quality colonoscopy (meaning the doctor spends adequate time and uses excellent technique to find and remove polyps) is quite a rare event and is something that is difficult to study given variations in quality practice between doctors even in the same community or hospital system.

Here is a quick pros and cons table to help clarify all of the above

Measure Colonoscopy Cologuard®
Prevention of colon cancer? Yes Not really
Repeat a normal test every 10 years 3 years
Overall convenience Bowel prep and 1 day off work No prep but have to handle stool
Overall invasiveness Moderately invasive Not invasive
Accuracy Very accurate Not very accurate
Biggest upside of the test Better cancer prevention and accuracy Easy and can do it at home
Biggest downside of the test Bowel prep and less than 0.1% chance of complications Lots of false positives that will require a colonoscopy anyway

How do I choose between colonoscopy and Cologuard in my practice?

I typically reserve Cologuard testing for patients that just need to know if they have cancer right now, and are not in good condition to undergo colonoscopy due to other major health issues. A patient that has not been screened recently who is approaching 80 years old, and who has one or more major cardiovascular or pulmonary issues is a good candidate for Cologuard testing. For pretty much everyone else, colonoscopy is by far the better test.


Imperiale TF, Ransohoff DF, Itzkowitz SH, et al. Multitarget stool DNA testing for colorectal-cancer screening. N Engl J Med 2014;370:1287-97.

Cologuard website:

Cologuard is a registered trademark of Exact Sciences Corporation.

What Causes Colon Polyps?


When it comes to colon cancer prevention, the polyp is the key player to know. Colon polyps, called adenomas, are precancerous growths originating from the inner lining of the colon wall. There are other types of polyps in the colon which are not considered precancerous, but for our purposes in this article we will consider the terms colon polyp and adenoma to be one and the same.

Polyps are important to know about because they are the precursors to colon cancer. That is, virtually all cases of colon cancer began many years earlier as a small polyp growing in the colon. For the average person, it takes many years for these small polyps to appear on the scene, which is why we don’t usually recommend colonoscopy until a person is 50 years old. However, it’s not totally uncommon to find significant-sized polyps in younger people in their 30s or 40s (and even in their 20s!) From the time a polyp starts growing, it is thought to take more than 10 years for the polyp to grow into cancer…and not all adenomatous polyps grow larger or turn into cancer at all. However, many polyps will continue to have cellular changes such as mutations that will promote growth of the polyp and eventually transformation into colon cancer.

By understanding what causes colon polyps, we can understand what causes colon cancer. Here are some risks factors for getting colon cancer/polyps that an individual cannot change:

  • Age: The risk tends to go up as one gets older.
  • Sex: Men seem to have a slightly higher chance of having polyps when compared to women.
  • Family history: A close relative with polyps or colon cancer makes your risk go higher.
  • Race: Black men and women have the highest risk of developing colon cancer.

However, roughly half of all cases of colon cancer (and by extension, colon polyps) are a result of modifiable risk factors. These are the things that you can control. If we know what these risk factors are, maybe we can make better choices for ourselves and our families. The following is a list of the known risk factors for developing colon cancer/polyps:

    • Alcohol: Alcohol use is closely tied to colon cancer risk. Unfortunately, even for “social” drinkers, the risk goes up by about 10% if you only drink less than 1 drink per day. If you have 2-4 drinks per day, the risk increases by 23%!


    • Red meat: Beef, veal, lamb and pork (despite the advertisements) are considered red meats. Regular consumption of about 100 grams of red meat per day (about the amount found in 2 regular-sized McDonald’s hamburgers) can increase the risk of colorectal cancer by 17%. I like the idea of “everything in moderation” so let’s aim to only eat red meat about once per week.
    • Processed meat: This seems to be a real bad one…processed meat is often also red meat and is defined as meat that has been salted, cured, smoked, fermented, or treated with other processes to improve flavor or preserve the meat. Processed meat is bacon, sausages, hot dogs, cured ham, etc. The risk of colorectal cancer goes up by 18% for those who eat just 50 grams per day of processed meat (this is half the amount of red meat needed for the same risk).

processed meat

    • Lack of dietary fiber: Eating fiber is good for your colon for many reasons, and transmits a decreased risk of colon cancer as well. An extra 10 grams of fiber in the diet can drop colon cancer risk by 10%, but don’t stop there: Men should get 38 grams of fiber per day, and women are recommended to eat 25 grams per day. An apple has about 4 grams of fiber in it, so that’s a lot of apples to eat every day! Alternatively, you should eat a variety of plant-based foods and take in fiber with each meal and snack.


    • Smoking: This seems obvious. Smoking causes all kinds of cancer. It also increases the risk of colon polyps and colorectal cancer. Smoking is not a great way to stay healthy.
    • Obesity: Being overweight or obese increases the risk of colon cancer. This is independent of physical activity. That is, the excess weight itself seems to be tied to cancer risk, likely due to changes in inflammatory and growth signaling molecules, among other factors.
    • Lack of exercise: Interestingly, staying physically active can reduce the risk of colon cancer by a whopping 25%! The minimum amount of exercise recommended for this purpose is about 2.5 hours per week of moderate exercise, or 75 minutes per week of intense exercise, but more is better. Again, the reasons for this are somewhat unclear but likely rooted in positive changes in insulin resistance, growth factors, inflammatory factors, and who knows what other things that are modified by exercise. We weren’t born to sit around, going from couch to car to chair and back again…we were born to move, so do something active on a regular basis!


  • Coffee: Well this should really read “lack of coffee” because coffee seems to be somewhat protective against developing polyps and colon cancer! There are not many high-quality studies on this topic, but what is out there suggests a 25% decrease in colorectal cancer with coffee consumption, possibly due to the antioxidants found in coffee.
  • Calcium: This is controversial. On one hand it seems that high consumption of dairy products like milk is associated with a lower risk of colorectal cancer. At best, supplementing calcium seems to have no effect on cancer risk for better or worse. However a very recent study suggests an increased risk of adenomatous polyps from taking both calcium and vitamin D supplements in combination, so more research will be needed to figure out the exact role of calcium, vitamin D, and the combination of both for colon cancer prevention.

And let’s not forget the final factor that leads to colon polyps and cancer in many individuals:

  • Bad luck: Unfortunately some people just have bad luck. We doctors don’t understand everything! This is why it’s still important to get screened for colorectal cancer at the appropriate age even if you don’t have any family history or symptoms, and are a thin, non-smoking, vegan, fitness-guru teetotaler!

March is Colorectal Cancer Awareness Month

colon cancer prevention

It’s March, which means it’s Colorectal Cancer Awareness Month!

Preventing colon cancer is one of the most important things that we gastroenterologists get to do. Even if you’re not particularly interested in colon cancer, there will still be some interesting stuff for you to read this here as I will cover a little bit about nutrition, exercise, and healthy lifestyle choices as they relate to colon cancer prevention. I also look forward to sharing a somewhat humorous and true story about bowel prep from the perspective of a patient, and that patient is me!

First, I’d like to review some basic facts about colorectal cancer for any new readers out there:

What is colorectal cancer?

Cancer, in general, is when your own cells develop DNA mutations and eventually stop obeying the normal control signals from the body that function to tell the cells when to stop dividing and where not to grow (obviously this is a gross oversimplification). These rebellious cancer cells multiply and form a tumor, which is just a mass of cancerous cells. Tumors can grow into other organs and cause damage, blockages, bleeding, and other types of badness. The cells inside the tumor can also spread through the bloodstream or lymphatic system and land in other locations in the body, a process known as metastasis.

Colorectal cancer is when this process happens in the colon (or the rectum, which is the end portion of the colon). The cells that transform into the typical type of colon cancer originate from the inner lining of the colon and turn into a type of cancer called adenocarcinoma.

How does colorectal cancer happen?

Colorectal cancer occurs when a precancerous polyp (known as an adenoma) forms inside the colon and slowly accumulates additional genetic mutations, causing the polyp to grow larger and act more aggressively, eventually invading into the muscle layer of the colon wall and becoming full-blown cancer. We think this process takes about 10-20 years to occur, which is a very important fact when it comes to colorectal cancer prevention. This long sequence, from adenoma to cancer, is the reason why screening can prevent colon cancer—we can intervene during the long precancerous stage and change the natural history of the disease. Stated more simply, we can remove the precancerous polyp before it actually turns into colon cancer, therefore preventing colon cancer from developing at all!

How do we prevent colon cancer?

All professional gastrointestinal societies recommend starting to screen most people for colorectal cancer starting at fifty years old. However, true prevention really starts many years before most people have to worry about getting a colonoscopy! Diet, exercise, and many lifestyle choices can increase or decrease the risk of developing colorectal cancer. We will cover this important topic in more detail later this month.

As far as screening goes, there are various tests available to look for both colorectal cancer and adenomatous polyps. I have covered these topics in more detail in past articles, so I will invite new readers to peruse the links below for more info:

However, if you are in a hurry and don’t want to read those older articles, we can boil down the testing recommendations for most individuals as follows: To prevent colorectal cancer, the average-risk man or woman should have a colonoscopy starting at age 50, and then every 10 years thereafter, until about 75-85 years of age. If polyps are found, they are removed during the colonoscopy, and the next exam is scheduled sooner than 10 years later.

colon cancer awareness

Can young people get colon cancer?

Most cases of colon cancer are diagnosed in people in their late sixties and early seventies, with rectal cancer being diagnosed somewhat earlier (early sixties). The good news is that colorectal cancer rates in the over-fifty population are on the decline! This may be due to several factors such as the increase in effective colorectal cancer screening programs and a decline in the popularity of smoking. However, new data is showing that the rate of colorectal cancer in young people is actually on the rise! Although it is still relatively rare, the rate of colorectal cancer is increasing in the 30- and 40-year-old age group.

We are not quite sure why colorectal cancer is increasingly developing in the younger population. Various theories exist, including the influence of obesity, inactivity, food additives, poor diet, and even antibiotic exposure. All we can conclude at this point in time is that symptoms that could be consistent with colorectal cancer should not be ignored just because a patient is relatively young.

Well, that about wraps up the basics on colorectal cancer. I am going to hit this topic from all angles this month, so be sure to keep reading!

For a quick reference on colorectal cancer, see the American Cancer Society’s publication Colorectal cancer facts and figures 2017-2019.

Think Twice Before Large Colonic Polyp Surgery

polyp removal

Large colonic polyps used to be managed by a surgeon, similar to how colon cancer is still managed…cut it out and hope for the best. However, it is clear that when possible, these large non-cancerous polyps are more safely removed using advanced endoscopic techniques, such as endoscopic mucosal resection (EMR). Despite excellent safety data on endoscopic removal of large polyps, there is still a reluctance by many physicians to send their patients for EMR. Why wouldn’t a fellow endoscopist sent their patients for a procedure that is as effective, safer, with no significant recovery time, and far less expensive when compared to surgery? I’m not entirely sure…maybe there is unfamiliarity with the technique among many physicians, or perhaps there is ignorance about the actual risks of surgery. Or maybe (just maybe) sending a patient for surgery is a gastroenterologists way of protecting his or her delicate ego…as in, “If I can’t remove it than it can’t be done by anyone with a scope!”

The truth is, it’s all about the positioning of the polyp. Is the polyp in a place that is able to be approached by the scope in the right position to facilitate removal? Sometimes the biggest polyps are easy to remove because they’re in an ideal location. Other times, a relatively small lesion proves impossible to remove because it is just at a bad angle and can’t be made better no matter how the endoscopist tries to reposition things.

Here is a recent case of a large polyp resected using standard EMR technique. As always, the patient should be followed up closely to make sure the resection was complete.

A-C: Large tubular adenoma in rectum. D: After resection, large vessel visible at base. E: After coagulation of vessel using hot forceps. F: After clip placement.


When compared to surgery, which carries a 14% risk of a major postoperative event (e.g.: need for colostomy, major infection, anastomotic leak, need for reoperation, cardiovascular event, blood clot, etc.) and a 1-in-140 risk of death in 30 days (which rises to 3% in the over-80 patient age group), endoscopic polypectomy is much safer. The most common risks of endoscopic removal of large polyps includes bleeding (6.5%), perforation (1.5%), need for emergency surgery (1%), and a 1-in-1250 risk of death.