Cancer Marker Tests and False Positives Explained

A cancer marker test measures specific substances, often proteins or other biological molecules, that may be produced by cancer cells or by the body in response to cancer. These substances are commonly called tumour markers or cancer biomarkers [1].

Depending on the marker, testing may involve a blood sample, urine sample, or occasionally another type of body fluid or tissue. Examples include prostate-specific antigen (PSA), CA-125, CA 19-9, carcinoembryonic antigen (CEA), and alpha-fetoprotein (AFP).

However, tumour markers are not automatically cancer-specific. A raised level may occur because of a non-cancerous condition, inflammation, infection, or other factors.

A cancer marker test can provide useful information, but an elevated result does not by itself confirm cancer because non-cancerous conditions can also cause abnormal marker levels.

This distinction is important because an abnormal result can understandably cause anxiety. Understanding what these tests can and cannot tell you can help put results into perspective.

Why Can Cancer Marker Tests Produce False Positives?

A false-positive result occurs when a test suggests that a condition may be present when it is not. With tumour markers, this can happen because the marker may also increase in benign conditions.

For example, CA-125 can be elevated in certain non-cancerous gynaecological conditions, while PSA can increase because of benign prostate enlargement or inflammation [2]. Similarly, some markers can be affected by liver disease, infection, or other medical conditions.

This means a raised tumour marker test should not automatically be interpreted as evidence of cancer.

Healthcare professionals consider the result in context, including the person's age, symptoms, medical history, risk factors, previous results, and the specific marker being measured.

Common Tumour Markers and Their Uses

Different markers are associated with different cancers and clinical situations. Some are more useful for monitoring an already diagnosed cancer than for detecting cancer in someone without symptoms [3].

The usefulness of a cancer marker test depends on the clinical situation. A doctor may recommend one or more markers based on symptoms, an existing diagnosis, treatment monitoring, or other findings.

Are Tumour Markers Used to Diagnose Cancer?

A tumour marker test result is rarely sufficient on its own to diagnose cancer.

If a marker is elevated, additional investigations may be needed to determine why. Depending on the situation, these could include repeat blood testing, imaging such as ultrasound, CT, MRI, or other examinations.

In some circumstances, a biopsy may ultimately be required to determine whether cancer cells are present. The appropriate next step depends on the suspected condition and the individual patient's circumstances.

This is why an abnormal cancer marker test should be viewed as one piece of information rather than a definitive diagnosis.

Why Can a Marker Be Elevated Without Cancer?

There are several possible explanations for an elevated tumour marker.

Benign Medical Conditions

Some non-cancerous conditions can increase particular markers. For example, inflammation, infections, benign growths, or organ-related conditions may affect results.

Individual Biological Differences

Marker levels can naturally vary between people. Age, sex, hormonal changes, and other biological factors can influence certain results.

Lifestyle and Other Factors

Some tumour markers can also be affected by lifestyle factors. For example, smoking can influence CEA levels [4].

Laboratory Variation

Laboratory methods and reference ranges can differ. Small changes between results may not necessarily indicate a meaningful change in health.

For these reasons, doctors generally look at trends and the overall clinical picture rather than relying on a single number.

What Happens After an Abnormal Result?

If your cancer marker screening result is higher than the laboratory's reference range, try not to assume the worst. An abnormal result does not necessarily mean that cancer is present.

Your doctor may first review your medical history and symptoms. Depending on the marker and the degree of elevation, they may recommend repeating the test after an appropriate interval.

Further testing may be recommended if the result remains elevated, increases over time, or is accompanied by other concerning findings.

The appropriate approach varies considerably between different tumour marker tests. A mildly elevated result may require different follow up plan from a substantially abnormal result combined with symptoms or abnormal imaging.

Can Tumour Markers Be Used for Cancer Screening?

The role of tumour markers in screening varies by cancer type. Some markers are not accurate enough to be used alone for screening people who have no symptoms.

For example, PSA testing may be discussed as part of prostate cancer risk assessment, but it has limitations and requires shared decision-making [5]. CA-125 is not generally considered a stand-alone screening test for ovarian cancer in average-risk women without symptoms [6].

Cancer screening should therefore be based on established screening recommendations and individual risk factors rather than assuming that a broad panel of tumour markers can detect every cancer.

When Might Tumour Marker Testing Be Useful?

Tumour markers can have an important role in certain clinical situations. Doctors may use them to support the evaluation of a suspected condition, establish a baseline before treatment, monitor response to treatment, or watch for possible recurrence after treatment.

Their value is often greater when interpreted alongside other information. For example, a consistent change in a marker over time may provide more useful information than a single isolated result.

However, the meaning of a change depends on the particular marker and the person's clinical circumstances.

Should You Be Concerned About a False Positive?

It is understandable to feel concerned after receiving an abnormal result. However, a false positive is one of several possible explanations.

Do not attempt to interpret a tumour marker result without considering the laboratory's reference range and your personal medical context. Even results that appear significantly elevated may have causes other than cancer.

Speak with your healthcare professional about what the result means, whether additional testing is needed, and whether any temporary factors could have influenced it.

Conclusion

A cancer marker test can provide valuable clinical information, but it has limitations. An elevated tumour marker does not automatically mean that cancer is present, and false-positive results can occur because of benign medical conditions, inflammation, lifestyle factors, biological variation, and other causes.

The most appropriate response to an abnormal result is not to assume that cancer has been diagnosed. Instead, discuss the result with a healthcare professional who can consider your symptoms, medical history, risk factors, previous results, and other investigations.

Ultimately, tumour markers are most useful when interpreted as part of a broader medical assessment. Understanding this can help reduce unnecessary anxiety while ensuring that genuinely concerning findings receive appropriate follow-up.