Why Screen for Bowel Cancer?
Bowel cancer is the 4th most common type of cancer. Sometimes patients present with symptoms and this leads to investigation. Bowel screening aims to prevent cancer or identify it an early stage when it is most amenable to treatment.
Advancing age, family history and life style factors are risk factors for bowel cancer.
As part of the NHS bowel cancer screening programme people are currently tested every 2 years between the ages of 60 and 74 years. There are currently plans however to extend the NHS screening programme to younger age groups.
Bowel screening programmes have most commonly relied upon stool tests investigating for traces of blood within the faeces. Stool testing offers a simple-to-perform, non-invasive approach to cancer testing. Historically, Faecal Occult Blood Tests (FOBTs) were used for population screening but these have been superceded by Faecal Immunohistochemical testing (FIT).
What is FIT?
The Faecal Immunohistochemical Test (FIT) is an investigation for bowel cancer. It is easily performed from a stool sample. It yields a measurement that describes a direct likelihood for the presence of bowel cancer. FIT uses antibodies to detect haemoglobin (a component of blood). It has been evaluated in many clinical studies and is now an established diagnostic test used:
i. by the general public who do not have bowel symptoms as part of national bowel cancer screening programmes in England, Holland, Italy and many other countries.
ii. by patients who have bowel symptoms to help determine whether they should go on to have further bowel investigation (colonoscopy or CT virtual colonoscopy).
The ‘science bit’ behind FIT and other screening tests
The reliability of FIT
The accuracy of FIT is superior to other older faecal tests such as guaic Faecal Occult Blood Tests (gFOBT). FIT is quantitative - it gives a score. The higher the score the greater the likelihood of serious bowel disease (such as cancer). FIT’s accuracy therefore depends to some extent on the level that is used to be considered ‘positive’. The lower the threshold is set for regarding the test as ‘positive’, the more people will be advised to undergo further investigation. A study published by Imperiale and colleagues in the Annals of Internal Medicine (2019) investigated the sensitivity of FIT against colonoscopy findings in thirty one clinical studies. They observed that ‘a threshold of 10 µg/g resulted in:
a test sensitivity of 0.91 (95% CI, 0.84 to 0.95) and a negative likelihood ratio of 0.10 (CI, 0.06 to 0.19) for colorectal cancer, whereas a threshold of greater than 20 µg/g resulted in specificity of 0.95 (CI, 0.94 to 0.96) and a positive likelihood ratio of 15.49 (CI, 9.82 to 22.39).
for pre-cancerous lesions (adenomas), sensitivity was 0.40 (CI, 0.33 to 0.47) and the negative likelihood ratio was 0.67 (CI, 0.57 to 0.78) at 10 µg/g, and specificity was 0.95 (CI, 0.94 to 0.96) and the positive likelihood ratio was 5.86 (CI, 3.77 to 8.97) at greater than 20 µg/g.
What does this all mean? - this test will pick up at least 9 out of 10 bowel cancers. It also suggests that the likelihood of a false positive test i.e. people who test positive but turn out to have no cancer, is low (about 1 in 20 cases). In addition, a reasonable proportion of pre-cancerous lesions will be picked up by FIT.
The advantage of FIT as a screening tool is that it is simple to undertake and relatively inexpensive. This leads to high rates of compliance. Furthermore, invasive diagnostic tests such as a colonoscopy can cause anxiety in some patients leading to them avoiding testing. When colonoscopy has been compared to FIT as a screening tool in a population of 50-69 year olds without symptoms more patients took up FIT than colonoscopy. In addition, similar numbers of cancers were found in both FIT and colonoscopy groups. Colonoscopy was however superior for finding advanced adenomas (potentially precancerous polyps) (Quinetero and colleagues, N Engl J Med 2012).
Enhanced FIT testing - there is recent evidence that carrying out two qFIT stool tests two weeks apart may improve its diagnostic accuracy. The double testing was carried out in patients with high risk bowel symptoms. In this study the double testing reduced missed cancers by a half when compared to single testing (Gerrard AD et al, Br J Surg 2023).
Risk associated with screening tests
IT IS IMPORTANT TO NOTE THAT FIT IS NOT 100% RELIABLE. A positive test can lead to some patients requiring further investigations that are negative. Conversely, sometimes a negative FIT test may occur in a patient who has a cancer (see above). FIT may also miss some advanced polyps that can later develop into cancer. It is however a useful simple test that can be used to screen patients who do not currently have symptoms to help determine who should go on to receive further investigation. Similarly, it can be used in patients who have symptoms to triage those who need additional bowel testing (such as colonoscopy or CT scanning). Colonoscopy, and to a lesser extent CT Virtual colonoscopy, are invasive procedures. Extremely rarely colonoscopy can result in serious bowel injury requiring surgery.
The Benefits of FIT testing
The benefits of FIT for screening in people who don’t have symptoms
Early cancers are usually amenable to curative treatment. Bowel cancer risk increases with age. The majority of bowel cancers occur after the age of 50 years. Screening programmes aim to investigate people without symptoms to identify the presence of early cancers. In many countries FIT is replacing the older guaic Faecal Occult Blood Test (gFOBT) as there is considerable evidence to suggest that it is more accurate for the detection of colorectal cancers. Moreover, there are fewer dietary interactions than were associated with FOBT. Importantly, FIT relies upon a single stool test and clinical trials suggest that it has improved uptake when compared to older stool tests that required multiple samples.
The Benefits of FIT for patients with bowel symptoms
Symptoms are often unreliable when used for diagnosing bowel cancer. Symptoms unfortunately mostly develop when tumours are advanced and in some such cases treatment is less effective. In addition, many non-cancerous bowel conditions share symptoms with bowel cancers leading to diagnostic confusion. The ‘gold standard’ for diagnosis amongst symptomatic patients is colonoscopy. Some patients may not however wish to have an invasive test such as colonoscopy in the first instance, or, are not suitable candidates for one. They may prefer a CT scan instead (this is also known as ‘virtual colonoscopy’ and involves CT imaging following inflation of the large bowel via a short tube inserted into the anus). These procedures are indicated when patients have clinical features suspicious of bowel cancer. Many patients do not however meet the threshold for invasive investigation. The National Institute for Health and Care Excellence (NICE) in the United Kingdom has recommended the use of FIT by general practitioners for patients who have bowel symptoms but do not meet the criteria for bowel investigation through the ‘suspected cancer pathway’. Specifically, patients whose FIT is greater than 10 micrograms of haemoglobin per gram of faeces should be referred for specialist bowel evaluation and investigation.