Why Does a FIT Result of 20 Micrograms Still Not Mean Bowel Cancer?

Geoffrey Chen

When Numbers Start Making Decisions · Season Five, “Things That Have Not Happened Yet” · Article 7

1. A positive letter changes the next step

Australia’s National Bowel Cancer Screening Program provides home screening to eligible people without symptoms. The kit detects haemoglobin in faeces even when blood is not visible. Under current Australian Government guidance, a result is positive when one or both samples contain at least 20 micrograms of haemoglobin per gram of faeces. Australian Government: Understanding bowel screening results

The word “positive” can sound like a diagnosis. Official guidance says otherwise. It means blood at or above the threshold was detected, not that bowel cancer has been found. Blood can have many other causes. The next step is to consult a GP and, commonly, receive a specialist referral for colonoscopy.

The result therefore resembles a fact in everyday language but operates as a triage boundary. It separates a large asymptomatic population into those who generally return to the screening cycle and those who need diagnostic investigation.

2. Twenty micrograms measures something narrow

The faecal immunochemical test, or FIT, uses antibodies to detect human haemoglobin. Bowel cancers and precancerous lesions can bleed, making blood a useful signal for further examination.

Blood is not specific to cancer. Haemorrhoids, inflammation and other conditions may cause bleeding. Some cancers do not bleed continuously or may not reach the threshold when the sample is taken. The Department advises that screening detects most—up to about 85%—but not all bowel cancers. A negative result is not proof that cancer is absent now and does not predict that it will never develop.

The precise measurement is haemoglobin concentration in faeces. It is not tumour size, cancer probability or disease severity. A message that says only “positive” suppresses the object of measurement and invites a proxy signal to be heard as a diagnosis.

Screening also applies to a defined population. People aged 45–49 can request a free kit, and people aged 50–74 are generally sent one every two years. Symptoms, personal history or a strong family risk may require a different clinical pathway. A home test cannot substitute for medical assessment.

3. Why draw the line at 20?

Lowering the threshold detects smaller amounts of blood and may increase sensitivity, but it also sends more people to follow-up colonoscopy. Raising it reduces follow-up demand and false positives while risking more missed lesions. Twenty micrograms is therefore both a measurement choice and a resource-allocation choice.

Colonoscopy is not unlimited or costless. It requires trained staff, facilities, bowel preparation and time, and carries small but real procedural risks. A national program must balance early detection, false-positive burden, participation and diagnostic capacity.

The threshold cannot be justified by laboratory performance alone. If a program persuades people to screen but cannot deliver timely diagnosis after a positive result, it creates waiting and anxiety without completing the intended health benefit. The legitimacy of the line depends on the whole service chain.

A common national threshold nevertheless supports quality control, comparison and resource planning. Allowing each laboratory to choose its own boundary would undermine consistency. Standardisation should organise initial triage without eliminating individual clinical judgement afterwards.

4. Positive changes an action, not the disease fact

Screening is well suited to opening a low-burden path towards diagnosis. The legitimate authority of a positive result is to prompt medical consultation, risk assessment and usually colonoscopy. It does not declare cancer, choose treatment or authorise conclusions about a person’s life.

A negative result also changes only the next step. An asymptomatic, average-risk person generally waits for the next routine screening round. Someone who develops rectal bleeding, persistent bowel changes or iron-deficiency anaemia should seek care rather than wait for another kit.

The direction is asymmetric. A screening number may reasonably open investigation, but it should not close a diagnostic pathway supported by symptoms or history. Where a missed diagnosis has serious consequences, clinical evidence must be allowed to override one negative screening result.

5. People near the threshold do not become two natural kinds

There is no disease cliff between 19.9 and 20.0 micrograms. Intermittent bleeding, sampling and laboratory variation can move a result. The national rule that either of two samples at or above 20 is positive already responds to some of this variability.

Individuals generally receive a category rather than the exact continuous result. That can protect them from over-interpreting tiny differences. It also makes a clear repeat-and-contact pathway essential when samples are damaged, inconclusive or mismatched.

An inconclusive result is not negative. It means valid knowledge has not been produced, so the process must be repeated rather than administratively returning the person to a lower-risk group.

Equity also appears after the threshold. Remote communities, low-income participants, people with language barriers and First Nations participants may receive the same positive result but unequal access to timely colonoscopy. A nationally uniform measurement can still enter an unequal decision chain.

6. How errors should be found and repaired

Program quality cannot be measured only by the number of kits mailed. It should track participation, positive rates, time to colonoscopy, cancers and precancerous lesions found, complications, group differences and interval cancers diagnosed after a negative screen but before the next round.

If positive participants wait too long for diagnosis, the answer should be capacity and referral improvement rather than raising the threshold merely to shorten the list. If one group shows unusual false-positive or missed-detection patterns, sampling, transport, test performance and population suitability need review.

Communication must be precise without offering false reassurance. A positive letter should name the haemoglobin threshold, state that the result is not a diagnosis and explain the next step and expected timing. A negative letter should retain the limitations of screening and the separate route for symptoms.

If a wrong or mismatched result delays diagnosis, correction requires more than rewriting a letter. The service must restore the clinical pathway, investigate harm and determine whether others in the same batch were affected.

7. Risk receives an action form through a chain

Bowel cancer does not begin to exist when FIT becomes positive, and haemoglobin does not refer itself for colonoscopy. Sample, antibody, threshold, registry, GP and endoscopy service collectively transform an invisible possibility into an actionable pathway.

This is a defensible use of forced structure. Without the 20-microgram line, a national program cannot triage consistently. The structure must remember that it measures blood, not cancer. When “positive” acquires authority without explanation, a person is linguistically sentenced before diagnosis.

Knowledge is distributed: the laboratory does not know a whole life; the GP does not possess national quality data; administrators do not know every patient. The chain allows knowledge to travel. When a link fails, the number produces anxiety without care.

Conclusion: keep the threshold and restrict the linguistic power of “positive”

A clear, common FIT threshold should remain and should be reviewed against current evidence and colonoscopy capacity. Twenty micrograms is suitable for initiating diagnosis and makes national quality and equity measurable.

But the result must be described in full as haemoglobin detected at or above the threshold, not shortened in messages or media to “positive for cancer”. A negative result cannot overrule symptoms or high individual risk. The program’s responsibility continues until timely diagnosis, rather than ending when a result letter is sent.

The most legitimate power of a screening number is to offer an earlier opportunity for confirmation before symptoms appear. It should not take the place of diagnosis before a disease has actually been established.

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