Everything you need to know about the FIT test for colorectal cancer

1 in 20 people will suffer from bowel cancer. Because of this the NHS national bowel cancer screening programme invites everyone between the ages of 60 and 74 years of age to take a home screening test. This looks for the presence of blood in the stool. This test is about to change, and in this blog I will explain what the new test is, how it works and why it is important. 

What is a FIT test?

FIT stands for Faecal Immunochemical Test. It is a test that analyses the presence of blood in faeces to the nearest microgram. This test that can be done at home and sent to the laboratory for analysis.

Why do we need a FIT test?

40,000 cases of colon cancer are diagnosed in the UK each year and it’s the third commonest cause of cancer deaths. The earlier the cancer is detected, the better the chance of survival (figure 1).

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Figure 1. Survival from colon cancer in patients with stage IV disease is considerably worse at one year when compared to Stage I disease. This data is from cancer research uk, and more information can be found here

The problem is that early bowel cancers are commonly asymptomatic and very difficult to detect. But, silent bowel cancers do pass small amounts of blood into the stool. Typically, this is in quantities that are not visible to the human eye. However, the FIT test can detect this very accurately, and this simple test is now being used to detect bowel cancer. The NHS is due to role out the FIT test as part of its national bowel cancer screening programme in 2018.

What are the benefits of a FIT test over other tests?

The previous test used to screen for colon cancer was called the Faecal Occult Blood Test (FOBT). Recent studies have shown that FIT is more accurate for the diagnosis of CRC than FOBT. This is because it is more specific for bleeding from the colon as opposed to the stomach or the small bowel, and its accuracy is not affected by diet or medications. FIT also offers some significant advantages over this test. For example, it only requires one stool sample and the analysis can be automated. The test is also more objective and reproducible.

How does it work?

FIT measures the levels of blood in the stool known as faecal haemoglobin (FHb). FIT detects the globin protein component of haemoglobin in the blood using antibodies. A test called an ‘immunoassay’ then measures the development of antibody-globin complexes.  

While some degree of gastrointestinal bleeding is physiological, FHb concentration is related to the severity of any disease that affects the colon, although it is highest in patients with colorectal cancer. Therefore the FIT test is way of detecting the presence of blood so that we can identify patients at risk of having bowel cancer.

FIT provides a quantitative analysis. FHb cut-off levels can therefore be calibrated depending on the patient population being studied. The challenge is to get this callibration right so that as many cancers as possible can be detected while minimising the need for unecessary endoscopy. 

How do I provide a sample?

Your doctor or the screening programme will provide you with a FIT testing kit with a clear set of instructions. There are a number of different companies offering this test, and the exact sampling process may vary slightly between them. However, a video on how to provide the sample can be found below for one such test. 

Are there any problems with FIT?

There aren’t many, but no test is perfect. FHb levels vary by age, sex, ethnicity and deprivation. It also varies by collection and storage techniques, the number of samples tested, or the analyser and process employed.  Finally, FHb may also be undetectable in cases of high risk adenoma or inflammatory bowel disease (IBD), and levels of blood found in stool vary in CRC. 

If you are very worried about your risk of colon cancer, then it is best to ask your GP to make an appointment to see a specialist.

What will happen if I have a positive FIT test?

A positive test does not necessarily mean you have bowel cancer. But, in the event of a positive FIT test, you would typically need a more formal examination of the colon to rule out the presence of a cancer. This would either mean a colonoscopy or a CT scan known as a virtual colonoscopy.

If your test is positive, we will be in contact to advise you to see a specialist and to arrange further tests.

Colonoscopy remains the gold standard test for detection of CRC and is the first line test in 68% of patients referred with suspected colorectal cancer.  If a suspected cancer is found, it will be biopsied for histological confirmation, and tattooed to enable intra-operative localisation.  Smaller polyps may also be removed and other bowel conditions diagnosed. In 2014-15, over 250,000 patients were referred with suspected colorectal cancer. Only 3-4% of these patients were found to have bowel cancer.

What does a negative FIT mean?

Patients with undetectable or very low FHb levels can be reassured that no further investigation is needed for bowel cancer with a high degree of certainty.  

Links and other information

NHS NICE FIT study

NHS bowel cancer screening programme