Introducing the Faecal Immunochemical Test (FIT)
19th April 2019
When diagnosed at stage 1, 90% of people with bowel cancer will survive for at least five years. This is compared to a survival rate of less than 10% when diagnosed at stage 4. Bowel cancer screening reduces bowel cancer mortality by detecting the disease in early (often pre-symptomatic) stages. A guaiac-based faecal occult blood test (gFOBT) has always been the type of bowel cancer screening used in the UK. However, the gFOBT is now being replaced by a Faecal Immunochemical Test (FIT).
What is FIT?
FIT is a form of faecal occult blood test used to identify and quantify the amount of human blood in a stool sample. The test specifically detects human haemoglobin, using antibodies. An abnormal result suggests there may be bleeding in the gastrointestinal tract requiring further investigation, usually via a colonoscopy.
Asymptomatic FIT Testing
The FIT bowel screening programme is aimed at people without any signs or symptoms of bowel cancer, who meet the eligible criteria. The kit is sent directly to the individual in the post and is completed at home before being returned to the screening hub.
The threshold for determining an abnormal result is relatively high and GPs are informed of all results electronically. The GP will be notified whether the result is normal or abnormal but will not receive a numerical value.
Individuals with an abnormal result are invited for a colonoscopy for further investigation. Individuals with a normal result will be eligible for future screening every two years.
If the screening FIT result is normal but colorectal symptoms develop, GPs should consider the FIT symptomatic pathway. This is because the threshold for detection of blood in symptomatic patients is much lower than the asymptomatic screening FIT test.
FIT vs gFOBT
- FIT specifically detects human haemoglobin, and therefore only identifies human blood in stool samples, unlike gFOBT
A FIT result is not affected by blood ingested via diet, which is not true for gFOBT.
- FIT is associated with higher uptake
Pilot studies investigating the use of FIT have reported a higher uptake in those invited, compared to gFOBT. This is true across demographics, including in men and lower socioeconomic groups. For example, in two of the five screening hubs used in a pilot in England, 66.4% of individuals invited for a FIT test attended, compared to 59.3% for gFOBT.
There are numerous potential factors contributing to the greater uptake of FIT. For example, FIT requires only one stool sample, compared to the three samples required for gFOBT. Also, there is no need to store the stool samples for several days for FIT testing, unlike gFOBT. The whole process for FIT is much more user-friendly and convenient in comparison to gFOBT.
- A machine analyser generates an objective numerical result for FIT, while gFOBT requires subjective human judgements of a colour change
FIT simply measures micrograms of human haemoglobin per gram of faeces. The threshold for a normal or abnormal result can be changed by altering the numerical value. As a general rule, the lower the threshold, the more sensitive the test will be and the more cases of cancer that will be detected and ultimately more lives will be saved. However, it is also true that with a lower threshold, more colonoscopies will be carried out (with the associated risks) which may show no signs of cancer (false positives).
Symptomatic FIT Testing
Where available, a symptomatic FIT test should be offered to individuals who have certain colorectal symptoms (NICE DG30 criteria) that do not meet the threshold for an urgent cancer referral (2-week wait [2WW]). The kit is given to the patient via the GP or can be sent directly to the patient by the lab following a GP’s request. The completed kit can be returned to the GP or to the screening hub via the post.
The threshold for determining an abnormal result is relatively low and GPs are informed of the result (normal or abnormal) electronically which usually includes a numeric value.
Individuals with an abnormal result are not automatically referred; GPs need to send them on a 2WW. Individuals with a normal result may still require routine referral or further investigation. These individuals may still have cancer – healthcare professionals need to be vigilant for ongoing, changing or worsening symptoms.
Up until now, clinicians have usually referred patients with lower gastrointestinal symptoms for further investigations via colonoscopy, resulting in an increasing demand for these services. Colonoscopies are invasive, expensive and carry risk. Furthermore, there is currently a low conversion rate to cancer. In July 2017, NICE published guidance recommending FIT to guide referral for colorectal cancer in primary care. A threshold of 10 micrograms of haemoglobin per gram of faeces is advised for ruling out bowel cancer. The use of this simple, inexpensive diagnostic test is hoped to provide further assistance in determining the most appropriate pathway for further investigation.
It is recognised that the human blood detected by FIT could be associated with several other conditions beyond cancer. Although, it was found that up to 28.9% of people with a false-positive FIT result for bowel cancer, did have a form of serious bowel pathology, such as IBD. It was therefore concluded that it is possible that the number of false-positive results from FIT could be partially offset by detecting other treatable bowel diseases.
The Role of Healthcare Professionals
Although it is expected that the introduction of FIT will increase bowel screening uptake, healthcare professionals also have a vital role in supporting this uptake.
It is important healthcare professionals assist patients to make an informed decision about whether to take part in the screening, being aware of both potential harms and benefits. Some harms patients ought to be aware of include false positives and negatives, over-reassurance following a normal result and the risks associate with a follow-up colonoscopy.
Even when an individual has a normal FIT screening result, it is vital that GPs continue to be alert to the possibility of bowel cancer as some cancers may be missed. Healthcare professionals ought to continue to remind patients of the signs and symptoms of bowel cancer and to seek medical advice if they notice anything new or unusual, even if they have recently taken a FIT test. Also, it is important that healthcare professionals and patients are aware that bowel screening is still indicated for eligible patients, regardless of any previous use of symptomatic FIT.
Faecal Immunochemical Tests (FIT) can help rule out colorectal cancer in patients presenting in primary care with lower abdominal symptoms: a systematic review conducted to inform new NICE DG30 diagnostic guidance. Westwood et al. (2017), BMC Medicine