CPD: Key questions on the use of FIT in primary care

Gastroenterologists Dr Michael Colwill and Dr Andrew Poullis explain the principles and practice of using the faecal immunohistochemical test (FIT) to guide referrals for suspected lower gastrointestinal (GI) cancer

Module summary

There have been a number of recent updates to national guidelines on use of the FIT to guide referrals for suspected lower GI cancer. In light of the most recent publication of NICE diagnostic guidance on use of FIT and its incorporation into NICE guidance on suspected cancer referrals, gastroenterologists Dr Michael Colwill and Dr Andrew Poullis explain the evidence-based principles behind FIT and its current recommended use in general practice.

Learning objectives
This module will enhance GPs’ understanding of the FIT including:

• The rationale for the use of FIT to guide cancer referral decisions in general practice.
• The appropriate diagnostic thresholds and why they are used.
• Which patients should be asked to perform a FIT and how the result should be interpreted.
• The purpose of the FIT in patients with overt rectal bleeding.
• When and what further investigations are required in patients with a negative FIT.
• How to manage a FIT-positive patient with negative colonoscopy.

1. How does the faecal immunohistochemical test (FIT) used in primary care differ from the test used in bowel cancer screening?

Since 2019, the UK national bowel cancer screening programme (BCSP) has used FIT rather than faecal occult blood (FOB) to screen asymptomatic individuals for colorectal cancer (CRC). The difference between this test and the FIT used in primary care is the threshold for further investigation and referral to secondary care. The current BCSP guidelines mean that anyone with a FIT of more than 120 mg Hb/g in England, Wales and Northern Ireland or a FIT of more than 80 mg Hb/g in Scotland will be offered a colonoscopy.

The FIT used in primary care to test symptomatic individuals usually has the cut-off of 10 mg Hb/g for further investigation. The test itself, based on an antigen-antibody reaction to a breakdown product of human haemoglobin, is more sensitive and more specific than the previously used FOB.

Historically, alongside a FOB stool test, everyone aged 55 and over was offered a flexible sigmoidoscopy as part of the BSCP. However, this has been stopped to allow the BSCP to focus on eventually offering anyone aged 50-74 a FIT through the programme, as part of the NHS long term plan.

2. Which patients should have a FIT?

NICE diagnostic guidance, DG56, published in August 2023,¹ and incorporated into the NICE guidelines on referral for suspected cancer,² states which patients should have a FIT performed to guide referral for suspected CRC. These are adult patients with:

• Abdominal mass, or
• A change in bowel habit, or
• Iron-deficiency anaemia, or
• Aged 40 and over with unexplained weight loss and abdominal pain, or
• Aged under 50 with rectal bleeding and any of the following unexplained symptoms:
  • Abdominal pain
  • Weight loss, or
• Aged 50 and over with any of the following unexplained symptoms:
  • Rectal bleeding
  • Abdominal pain
  • Weight loss, or
• Aged 60 and over with anaemia even in the absence of iron deficiency.

Patients fitting the above criteria who return a positive FIT (≥10 mg Hb/g) should be referred on the urgent two-week-wait cancer pathway.

It should be noted that with new symptoms, patients should be offered a FIT even if they have had a negative result through the UK BCSP.

Note also that patients with a rectal mass, unexplained anal mass or ulceration do not need a FIT and should be referred directly on an urgent suspected cancer pathway.

This module has received funding from Bowel Cancer UK with no involvement in the content.

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Dr Michael Colwill is luminal gastroenterology research fellow and Dr Andy Poullis is gastroenterology consultant, both at St George’s University Hospitals NHS Foundation Trust in London