But a lot of people read all of this news and these statistics and may wonder if they could improve their colorectal cancer risk by testing for it even earlier than recommended. It’s an understandable reaction. So, what are the reasons a doctor might suggest you undergo screening for colorectal cancer even sooner than age 45?

What makes cancer screening tests so effective

A screening test identifies cancer in an individual who does not yet have signs or symptoms from the cancer – before you or your doctor even know it’s there. Over a 45-year period in the US from 1975 to 2020, it is estimated that over 1.3 million cancer deaths were averted due to screening programmes. These include mammograms reducing breast cancer deaths by 260,000 women; prostate-specific antigen (PSA) testing reducing prostate cancer deaths by 200,000 men; Pap smear testing reducing cervical cancer deaths by 160,000 women; and colonoscopies or other screening tests reducing colorectal cancer deaths by 740,000 people.

Ideal screening tests detect cancers in their early stages, when an intervention (such as removing the cancer with surgery) can prevent the cancer from spreading, and can treat it in its entirety.

Screening tests are best suited for identifying slow-growing cancers, and the recommended frequency of the screening test reflects how rapidly a given cancer typically grows: for breast cancer, a mammogram every year or two, but for colorectal cancer, a colonoscopy every 10 years or faecal immunochemistry test-DNA (FIT-DNA) every one to three years. (There are other screening tools available, but colonoscopy and FIT-DNA are the most commonly used.) A family history of cancer or personal risk factors may influence the optimal screening frequency in an individual.

I specialise in treating people with acute leukaemia, and my patients frequently ask me, “Why wasn’t my cancer caught earlier?”

We don’t have screening tests for cancers like leukaemia because they develop quickly, in weeks. By the time the leukaemia is diagnosed, it has already spread throughout the body – there is no early stage at which we can contain it. Even if there were, for a cancer that blossoms in weeks, there is no practical time when a screening test (an invasive bone marrow biopsy) could be performed to catch the cancer early – you would have to repeat the biopsy every month or two.

Good screening tests are also highly accurate. A colonoscopy to detect the adenomas (polyps, which are non-cancerous growths) that can become colon cancer has a sensitivity – the proportion of people a test correctly identifies as having cancer who actually have the cancer – that ranges from 67% to 98%. It has a specificity – the proportion of people who don’t have cancer that the test correctly identifies as not having cancer – ranging from 80% to 98%. The variability depends to some extent on the skill of the person performing the procedure. FIT-DNA appears to be just as accurate, but colonoscopy has an advantage: you not only can detect precancerous polyps, but can remove them at the same time.

Finally, a good screening test should present little risk to the person undergoing it. After all, the majority of people being screened fortunately will not have cancer. Screening tests should be as safe and non-invasive as possible, to avoid introducing any new health issues unnecessarily, and to increase the chances people will go get them done. Because after all, most people don’t look forward to undergoing a cancer screening test.

A large study conducted in Poland, Norway and Sweden invited over 28,000 people to undergo colonoscopy screening, with almost 12,000 agreeing. The investigators also found that you need to invite 455 people to undergo a colonoscopy to prevent one case of colorectal cancer.

When you might benefit from earlier colorectal cancer screening

The biggest risk factor for developing colorectal cancer is age. In the US between 2018 and 2022, there were 21.5 colorectal cancer cases per 100,000 people ages 40 to 44, but close to double that in people ages 45 to 49, and more than double that in people ages 60 to 64. Thus, the rationale to start colorectal cancer screening in your mid-40s.

Other conditions can raise the risk for developing colorectal cancer even more, and should prompt screening at an even younger age. These include inflammatory bowel diseases (IBD) such as Crohn’s or ulcerative colitis, a prior diagnosis of colorectal cancer or colonic adenomas, or known genetic disorders that predispose a person to a high lifetime risk of colorectal cancer.

For example, those with Lynch syndrome are born with a genetic mutation that prevents the cells in certain organs, such as the colon, from correcting mistakes made in their cells’ DNA. People with this inherited condition should start screening for colorectal cancer during their early 20s, or two to five years younger than the youngest relative diagnosed with colorectal cancer. People with familial adenomatous polyposis, another rare inherited condition, develop hundreds of polyps in their colon and rectum, any of which can blossom into cancer, and should start screening even earlier – at ages 10 to 15 – as their lifetime risk of developing colorectal cancer approaches 100%.

If you have multiple close family members, such as parents or siblings, or younger close family members who have been diagnosed with colon cancer or who have had precancerous polyps, especially before age 50, talk to your doctor about whether you should start colon cancer screening at an earlier age.

You might also go for a colonoscopy sooner if you’re having concerning symptoms and your healthcare provider wants to rule out colorectal cancer. In this case, it wouldn’t be considered screening, but rather a diagnostic test. Depending on the results, you may need to start going for screenings sooner or more frequently than you would have otherwise.

The most common symptoms associated with colon cancer are a change in bowel habits (like frequent diarrhoea), rectal bleeding and abdominal pain. If any of these persist for two months or longer, I advise seeing a doctor for further evaluation.

If you don’t have any symptoms, family history or a condition like IBD that increases your risk, it’s fine to wait until age 45. If you get the all-clear after your first colonoscopy, you may not need another one for 10 years.