Tumor Marker Tests Explained: Do They Really Detect Cancer?

A tumour marker test looks for certain substances or biological changes linked to cancer. These markers may be made by cancer cells or by normal cells in response to cancer. Most tumour marker tests use a blood sample, although some analyse urine, other body fluids or tumour tissue.

But does a tumour marker test actually detect cancer? Not on its own. A high tumour marker level does not always mean cancer, and a normal result does not always rule it out. Doctors use these tests along with symptoms, imaging, biopsy and other investigations to build a clearer picture.

What Is a Tumour Marker Test and How Does It Work?

Tumour markers are substances that can provide information about certain cancers. Many are proteins, while some are genetic changes or other features found in tumour cells. They are also called cancer markers or biomarkers.

A tumour marker blood test checks for these substances in your blood. Depending on the marker, doctors may also test urine, stool, bone marrow or tumour tissue.

A serum tumour marker test is a blood test that measures a particular marker in serum, the liquid part of blood.

The test itself is usually simple. For a blood test, a healthcare professional collects a small sample from a vein in your arm. The laboratory then measures the specific marker requested by your doctor.

Tumour marker tests may be used to:

• Support the diagnosis of certain cancers
• Help assess how advanced some cancers are
• Guide treatment decisions
• Monitor whether treatment is working
• Look for signs that cancer has returned
• Provide information about the likely course of some cancers

However, not every cancer has a useful tumour marker. This is one reason these tests cannot be used as a universal cancer test.

Common Tumour Markers and the Cancers They May Be Associated With

Different tumour markers are linked with different cancers. Some markers can also be raised in more than one type of cancer.

Tumour marker	Cancer it may be associated with	Common use
AFP	Liver cancer and certain germ cell tumours	Diagnosis and treatment monitoring
CA-125	Ovarian cancer	Diagnosis, treatment response and recurrence monitoring
CA 15-3 / CA 27.29	Breast cancer	Monitoring treatment and recurrence
CEA	Colorectal and some other cancers	Treatment monitoring and recurrence
CA 19-9	Pancreatic, gallbladder, bile duct and stomach cancers	Treatment monitoring
PSA	Prostate cancer	Screening discussions, diagnosis and monitoring
Beta-hCG	Certain germ cell tumours and choriocarcinoma	Diagnosis, staging and treatment monitoring

The exact use of each marker varies. Some are mainly used after cancer has already been diagnosed rather than to find cancer in someone without symptoms.

Can a Tumour Marker Test Confirm Cancer?

Usually, no. A tumour marker test cannot confirm cancer by itself.

This is because the same marker may be produced by cancer cells and non-cancerous cells. Some benign conditions can also increase certain tumour marker levels. At the same time, some people with cancer may have normal levels of a particular marker.

For example, if a tumour marker is higher than the usual range, it does not automatically mean that cancer is present. Your doctor may look at your symptoms, medical history and examination findings and may recommend imaging or a biopsy.

A biopsy is particularly important for many cancers because it allows doctors to examine cells or tissue directly.

So, think of a tumour marker test as one piece of the diagnostic picture rather than a simple positive-or-negative cancer test.

What Can Cause Elevated Tumour Marker Levels Besides Cancer?

A raised tumour marker level can sometimes occur without cancer. Depending on the marker, possible causes include:

• Infection
• Inflammation
• Non-cancerous growths or conditions
• Certain liver or kidney problems
• Smoking
• Some medications or medical procedures
• Other conditions that affect the particular organ being tested

The possible causes vary considerably from one marker to another. A high result should therefore be interpreted using the person's overall health and other test results.

This is also why it is not useful to compare your tumour marker number with someone else's. Each marker has its own reference range, and results can vary between people and laboratories.

How Are Tumour Marker Blood Tests Used in Cancer Diagnosis and Treatment?

Tumour marker blood tests can have several roles in cancer care. Their purpose depends on the type of cancer and the marker being measured.

Supporting cancer diagnosis

Some tumour markers can provide additional information when a doctor suspects a particular cancer. For example, AFP may be used when evaluating liver cancer or certain germ cell tumours. CA-125 can be used as part of the assessment of ovarian cancer.

However, the result is considered alongside other investigations rather than used alone.

Monitoring treatment

Doctors may measure a tumour marker before and during treatment. A fall in the level may suggest that treatment is working. However, marker levels can fluctuate, so doctors usually look at the trend along with scans, symptoms and other findings.

Checking for recurrence

Some tumour markers can help monitor people after treatment for signs that cancer may have returned. For example, CA 15-3 and CA 27.29 may be used in certain situations involving breast cancer, while CEA can be used to monitor some people with colorectal cancer.

Helping plan treatment

Not all cancer biomarkers are substances measured in a routine blood test. Some are genetic changes or proteins found in tumour tissue.

These biomarkers can help doctors determine whether a cancer may respond to a particular targeted treatment. For example, testing for certain molecular changes can help guide treatment in some lung cancers.

When Do Doctors Recommend Tumour Marker Tests?

Doctors may recommend a tumour marker test when there is a specific clinical reason to look for or monitor a particular marker.

It may be considered:

• When certain symptoms or test results raise suspicion of cancer
• After cancer has been diagnosed
• Before or during cancer treatment
• To assess treatment response
• During follow-up after treatment
• When monitoring for possible recurrence
• To help select certain targeted treatments when a relevant biomarker is present

Importantly, most tumour marker tests are not suitable as general cancer screening tests for everyone. A test is useful only when there is evidence that the particular marker can provide meaningful information in that situation.

Tumour Marker Test for Breast Cancer

People often search for a tumour marker test for breast cancer, particularly when they are worried about a lump or other breast changes.

CA 15-3 and CA 27.29 are blood-based markers associated with breast cancer. They may be used in certain situations to assess treatment response or monitor for recurrence.

However, a tumour marker blood test for breast cancer is not a stand-alone test for diagnosing breast cancer. A doctor may use breast examination, imaging and biopsy when appropriate.

Other biomarker tests performed on breast tumour tissue, such as oestrogen receptor, progesterone receptor and HER2 testing, can also provide important information for treatment decisions.

Tumour Marker Test for Lung Cancer

A tumour marker test for lung cancer can mean different things. Some blood-based markers may be used in specific situations, but they cannot independently confirm or rule out lung cancer.

Molecular testing is also important in some cases of non-small cell lung cancer. Doctors may look for changes such as EGFR, ALK, KRAS, ROS1, BRAF or other biomarkers because the results can help guide targeted treatment.

This type of testing may involve tumour tissue or, in selected situations, blood-based testing.

What Does a High or Normal Tumour Marker Result Mean?

A high tumour marker result does not automatically mean that you have cancer. Similarly, a normal result does not always mean that cancer is absent.

Doctors may be more interested in how a marker changes over time than in one isolated result. For example, a falling level during treatment may suggest a response, while a rising level may need further investigation. Even then, marker results need to be interpreted alongside other clinical information.

If your result is abnormal, your doctor may recommend repeat testing, imaging, a biopsy or another type of biomarker test, depending on the situation.

What Happens After a Tumour Marker Test?

Your next step depends on why the test was ordered and what the result shows.

If the result is within the expected range, your doctor may still recommend other tests if your symptoms or examination suggest a problem.

If the result is higher than expected, your doctor may look for other possible causes and decide whether further investigation is needed.

This may include:

• Repeat blood testing
• Imaging scans
• Biopsy
• Additional laboratory tests
• Molecular or genetic testing
• Continued monitoring

The result should always be interpreted in the context of your overall health rather than viewed as a diagnosis by itself.

The Bottom Line

A tumour marker test can provide valuable information during cancer diagnosis and treatment, but it is not a simple test that can tell you whether you have cancer. A high result may have causes other than cancer, while a normal result cannot always rule cancer out.

The test becomes more useful when its result is considered alongside symptoms, imaging, biopsy findings and other clinical information. If your doctor recommends a tumour marker test, ask what the particular marker is looking for and how the result will be used in your care.