Faecal occult blood testing

The test

Detects occult blood in stool as a surrogate for colorectal neoplasia
*iFOBT (faecal immunochemical test, FIT) has replaced the older guaiac test (gFOBT)*

	iFOBT / FIT	gFOBT (obsolete)
Detects	Human globin antibody-based	Haem peroxidase activity
Specificity	High - human blood only	Low
Dietary/drug restriction	None needed	Red meat, peroxidase-rich vegetables, vitamin C
Upper GI blood	Not detected (globin digested) - a feature, not a flaw	Detected
Quantitative	Yes - threshold adjustable	No

National Bowel Cancer Screening Program: free biennial iFOBT kit mailed to Australians aged 45-74 (automatically invited from 50; 45-49 can opt in, since July 2024)
Participation only ~40-45% - the programme's biggest limitation is uptake, not test performance
Sensitivity for colorectal cancer ~65-80% for a single round (higher over repeated rounds); for advanced adenoma ~25-40%
Specificity ~95%
Of those with a positive iFOBT: ~1 in 32 have colorectal cancer and ~1 in 7 have an adenoma
- So ~3% cancer yield - most positives are not cancer, but most warrant colonoscopy

Colorectal neoplasms bleed intermittently from friable surface vessels
- -> a negative test does not exclude cancer; repeated rounds are what generate the mortality benefit
Right-sided and small lesions bleed less -> lower sensitivity proximally
Antibody binds human globin
- -> upper GI bleeding is not detected, because globin is digested proximally
- -> a positive iFOBT localises bleeding to the colon or distal small bowel
Causes of a positive test other than neoplasia: haemorrhoids, diverticular disease, IBD, angiodysplasia, NSAID injury, infection, anticoagulation, menstrual contamination

Asymptomatic, average-risk, age 45-74: biennial iFOBT
*Not for people at increased risk - family history, prior adenoma, IBD, hereditary syndromes -> colonoscopy surveillance instead*
Not recommended over age 75 (harms outweigh benefit), or with limited life expectancy

*iFOBT must NEVER be used to "rule out" cancer in a symptomatic patient*
- Rectal bleeding, iron deficiency anaemia, change in bowel habit, weight loss, abdominal mass -> colonoscopy, regardless of iFOBT result*
- A negative iFOBT in a symptomatic patient is the classic cause of delayed diagnosis
(In some UK-style systems quantitative FIT is used to triage symptomatic referrals; in Australian practice, symptoms mandate colonoscopy)

Colonoscopy - target within 30 days
Do not repeat the iFOBT to "confirm" it
If colonoscopy is normal, consider gastroscopy only if iron deficiency or upper GI symptoms coexist

Positive iFOBT -> colonoscopy, ideally within 30 days
- Delay beyond 120 days is associated with more advanced stage at diagnosis
Negative iFOBT -> repeat in 2 years; a single negative test is not reassurance
Ensure the result is communicated and acted upon - loss to follow-up after a positive screen is a recognised patient safety failure
Counsel on the meaning of the result: positive does not mean cancer (~3% cancer yield), negative does not mean no cancer
Address the modifiable determinants of participation: kit return, GP endorsement (the strongest single predictor of participation), language and cultural barriers, Aboriginal and Torres Strait Islander access

National Bowel Cancer Screening Program; Cancer Council Australia colorectal cancer guidelines
Higher risk groups requiring colonoscopy instead: family history (2 FDRs, or 1 FDR <50), personal history of adenoma or CRC, IBD, Lynch syndrome, FAP, MUTYH-associated polyposis, acromegaly
Iron deficiency anaemia - a colonoscopy indication in its own right
Anticoagulants and NSAIDs - increase positivity without increasing cancer yield (but do not stop them or delay the colonoscopy on that basis)
Haemorrhoids and diverticular disease - the commonest benign explanations, and the commonest reason a positive result is wrongly dismissed

Biennial FOBT screening reduces colorectal cancer mortality by ~15-25% in randomised trials, and more in modelled programme terms
Benefit accrues by detecting cancers at an earlier stage and by removing adenomas (incidence reduction)
The adenoma-carcinoma sequence takes ~10 years, which is what makes a 2-year interval adequate
Interval cancers occur (~20-30% of screen-era cancers) - typically right-sided, serrated pathway, non-bleeding
Harms: false positives leading to colonoscopy (perforation ~1 in 1,000, bleeding ~1 in 300), over-investigation, anxiety
The NBCSP is projected to prevent tens of thousands of deaths over its first decades - contingent almost entirely on participation rates

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