Introduction
In New Zealand working as a GP on the front line, i am aware of 3 main types of bowel cancer ‘screening’ modalities. Colonoscopy, stool based tests including the outdated poor performance FOB Tests, and the newer ones such as Faecal immunichemical tests (True Proactive FIT test) and cross sectional imaging for example CT and MRI there may be more obscure ones but these are the main validated ones i come accross in my daily practice and also as a preventative health doctor.
Colonoscopy
Colonoscopy is considered the gold standard investigation for assessing and screening for bowel cancer also known as colorectal cancer. The procedure usually takes a few hours in total and is performed by a highly trained clinician called an endoscopist, often a doctor.
During the procedure, the patient is usually given sedation. A flexible camera is passed gently through the anus and along the bowel until it reaches the furthest point a standard colonoscopy can examine, known as the caecum.
One of the major advantages of colonoscopy is that it allows direct visualisation of the bowel lining. This means large cancers can be identified, and smaller growths called polyps can also be detected and removed proactively before they have the chance to develop into cancer.
However, colonoscopy does have some downsides. It is an invasive test and requires full bowel preparation beforehand, which many people find unpleasant. While generally very safe, colonoscopy is not completely risk free. A perforation or tear of the bowel wall can occur in approximately 1 in 1000 procedures.
Stool based tests
Stool based screening tests are widely used around the world to screen for bowel cancer and colorectal cancer. These tests look for signs of cancer or advanced polyps by analysing a small stool sample that can be collected at home and sent to a laboratory for testing.
There are several different types of technology used in stool based boewl cancer screening tests, including FOB tests, Faecal immunochemical (FIT) Tests and also DNA analysis tests, i will discuss these in more detail beow
The main advantages of stool based screening tests are that they are non invasive, do not require bowel preparation, and can be completed at home without sedation or time off work. This makes them far more acceptable for many people and helps increase participation in screening programmes. FIT testing in particular has been shown to reliably detect many bowel cancers at an early stage.
However, stool based tests are screening tools rather than definitive diagnostic tests. They cannot see the bowel directly and cannot remove polyps. A negative result does not completely exclude bowel cancer, and false positive and false negative results can occur. If a stool based test is positive, further investigation with colonoscopy is usually recommended to identify the cause.
Imaging / Scans
Imaging based tests can also be used to investigate the bowel and screen for bowel cancer in selected situations. These include CT scans, MRI scans, and CT colonography sometimes referred to as a virtual colonoscopy. These tests use advanced imaging technology to create detailed pictures of the bowel and surrounding organs without inserting a camera through the bowel.
The main advantage of imaging based screening is that it is less invasive than a traditional colonoscopy and does not usually require sedation. CT colonography in particular can provide a good overview of the colon and is useful for detecting larger polyps and cancers, however that tends to be more invasive than the other tests as it requires full bowel clearance and a tube to inflate the bowel with air while it is scanned.
However, imaging tests have limitations. They are less sensitive than colonoscopy for small or flat polyps and cannot remove polyps if they are found. If an abnormality is detected, a follow up colonoscopy is still required for biopsy or treatment. CT based tests also involve exposure to radiation, while MRI scans are more time consuming and less widely available. Imaging therefore tends to play a complementary role rather than replacing colonoscopy or stool testing in bowel cancer screening.
Colonoscopy vs FOB vs FIT vs DNA vs CT colon MR, a table for direct comparison.
| Test | Invasiveness | Cancer detection | Polyp removal | Preparation | Risks |
|---|---|---|---|---|---|
|
Colonoscopy Direct camera exam to caecum, usually with sedation |
Invasive | Excellent | Yes | Full bowel prep | Small risk, perforation about 1 in 1000 |
|
FOB test Older stool test for hidden blood |
Non invasive | Lower | No | None | No procedural risk |
|
FIT test Modern stool test for human blood |
Non invasive | Good | No | None | No procedural risk |
|
Stool DNA test Stool markers for cancer and advanced polyps |
Non invasive | Good | No | None | No procedural risk |
|
CT colonography Virtual colonoscopy using CT imaging |
Minimally invasive | Moderate | No | Mild prep | Radiation exposure |
|
CT or MRI scan Cross sectional imaging, not a primary screening test |
Non invasive | Poor for early disease | No | None | Radiation with CT |
So what screening tests are best for most people?
This sounds like a simple question on the surface, but it quickly becomes more complex once you dig into it. The “right” approach to screening depends on a whole range of factors, including personal preference, past medical history, existing health conditions, individual risk factors, and access to medical care, including the ability to afford private screening tests.
When i am in clinic i have to take into account a lot of individual factors before recommending a particular course of action. A frail 80 year old on a pensioner with COPD and taking anticoagulants which can both increase the risks of colonoscopy and with no access to private healthcare may reasonably choose a less intensive screening option, or maybe no screening at all . Compare that with a healthy 40 year old executive with a strong family history of bowel cancer and the financial means to access comprehensive private screening, who may opt for a much more proactive approach.
This is why bowel cancer screening is never truly one size fits all. As discussed previously the New Zealand bowel cancer bowel screening programme takes a kind of half way house position, it offers screening with the accurate FIT Tests every 2 years from 58-74. It would be impossible, unaffordable, unsafe and unethical for everyone in the country to have a colonoscopy or FIT test every year so they have adopted a less agressive screening policy.
The Colonprev trial
The COLONPREV trial was a large Spanish randomised study involving more than 57,000 average risk adults aged 50 to 69, comparing invitation to a one time screening colonoscopy with invitation to biennial FIT testing. After 10 years of follow up, colorectal cancer mortality was very similar in both groups, with no statistically significant difference between colonoscopy and regular FIT screening. Although colonoscopy detected slightly fewer cancers overall, the key message was an organised programme of repeated FIT testing with appropriate colonoscopy follow up performed just as well as a one off colonoscopy strategy in preventing death from bowel cancer over the first decade. The study highlights that participation and regular testing may matter more than simply choosing the most invasive test.
Summary
Ultimately, the best screening strategy depends on individual risk, medical history, personal preference, and access to care.
But if I am being completely honest, most of the patients I see are not actively screening for colorectal cancer at all. They are simply waiting for symptoms to appear. By the time symptoms develop, we are no longer talking about prevention. We are talking about diagnosis.
Screening is about acting before there is a problem.
For many people, starting with a simple annual FIT test is a very sensible first step. It is safe, non invasive, easy to complete at home, and gives you a regular check in each year. If it is positive, you move on to colonoscopy. If it is negative, you gain reassurance and continue monitoring.
The most important step is not choosing the most invasive test. It is choosing to start.
