Tumor Markers: When the Test Helps and When It Scares You for Nothing

Tumor markers are useful when the tumor is already known and the doctor is following how it responds to treatment. For finding cancer in a healthy person or for making a diagnosis, they are almost never suitable: they are wrong too often, in both directions. And the main rule I repeat to my patients: we look at the trend over several tests, not at a single number.

What a tumor marker is and why it isn’t a “cancer test”

A tumor marker is a substance produced by tumor cells, or by normal tissues in response to a tumor. Most often it’s a protein that can be measured in the blood.

The problem is that these same substances are also produced without cancer. The US National Cancer Institute (NCI) says it plainly: a high marker level doesn’t mean a person has cancer, and a normal level doesn’t rule it out. That’s why a tumor marker result is always interpreted together with a biopsy, CT, MRI and a physical examination.

Blood tumor markers and tumor biomarkers are not the same thing

Patients often confuse two different things. When people say “tumor markers”, they usually mean a blood test: CEA, CA 125 and so on. But the NCI also counts features of the tumor itself as tumor markers — mutations in its genes, receptors on its cells. These are tested in tissue obtained at biopsy or surgery, and sometimes in blood using a liquid biopsy.

The difference is fundamental. Blood markers help follow the trend. Tumor biomarkers help choose treatment: they determine whether hormone therapy, targeted therapy or immunotherapy is suitable. For example, the ER and PR receptors and the HER2 protein in breast cancer are markers too, but they have nothing to do with a CA 15-3 blood test. If your discharge summary includes molecular test results, that’s completely different information and needs to be discussed separately.

From here on, this article is only about blood markers.

Which markers are used and for which tumors

Below are the main markers I get asked about most often, and what they are actually used for.

Note the exceptions. PSA, AFP and hCG are among the few markers that genuinely play a role in diagnosis. In germ cell tumors (for example, of the testicle), AFP and hCG levels are even taken into account when determining stage and prognosis — more on staging in my article on the TNM system.

Under the Russian Ministry of Health clinical guidelines for colon cancer (2025), CEA is measured in all patients at initial staging — as a baseline to compare later results against.

Why tumor markers aren’t used to look for cancer

The NCI points out that circulating tumor markers perform poorly for screening (looking for cancer in people without symptoms) for two reasons:

• low sensitivity — in many people with a tumor the marker stays normal, especially at an early stage;
• low specificity — the marker goes up in benign conditions.

The Canadian Cancer Society adds one more point: marker levels often don’t rise until the disease has progressed. In other words, for early detection the marker lets you down exactly when you need it most.

PSA is a separate story: the NCI lists it among the markers that help in diagnosing prostate cancer. But who should be tested without symptoms, and when, is best decided together with a urologist rather than on advice from the internet.

What else raises tumor markers besides cancer

Patients often come to me with a raised marker they had tested “just in case”. According to the MSD Manual, these can cause a rise without a tumor:

• CEA — smoking (a history of 20 pack-years or more), liver cirrhosis, ulcerative colitis;
• CA 125 — any inflammation of the peritoneum (the membrane lining the inside of the abdomen);
• CA 19-9 — benign bile duct diseases and bile stasis;
• PSA — benign prostate enlargement, prostatitis, recent urological procedures;
• hCG — pregnancy;
• CA 15-3 — the manual notes that it can also be raised in other conditions.

A raised tumor marker in someone without a diagnosis is a reason to discuss the result with a doctor, not to diagnose yourself with cancer. Cancer is diagnosed only by biopsy.

Why the trend matters more than one value

When a tumor has been confirmed and the marker was raised before treatment, repeat measurements really do help. A falling level usually means the treatment is working. A steady rise is a signal to look more closely at the disease.

But a single data point means little. The Canadian Cancer Society specifically notes that a small rise may not be significant and that doctors look at the trend. And another important detail: chemotherapy can cause a temporary rise in the marker, as tumor cells break down and release it into the blood. If you panic at that moment and decide the treatment “isn’t working”, you could abandon a regimen that is actually doing its job.

What I usually advise patients:

• have the marker tested at the same lab each time — different test systems can give different numbers, and a “rise” sometimes turns out to be just a change of method;
• compare it not with the “normal range” on the form, but with your own previous values;
• come to your appointment with a table of all your results and the dates of your treatment cycles — that way the whole picture is visible.

If one “bad” test during treatment is worrying you, I’ll go through your results in a consultation together with your scan findings.

Why treatment isn’t changed based on one marker

The American Society of Clinical Oncology (ASCO), in its guideline on metastatic breast cancer, states that CEA, CA 15-3 and CA 27-29 can be used as an adjunct when making treatment decisions, but there isn’t enough evidence to monitor response to treatment by them alone. What’s more, ASCO considers it acceptable not to use these markers in this role at all.

In practice this means that the decision to switch regimens is made on the whole picture — CT, MRI or PET-CT findings, how you feel, examination, markers. I explained in detail what each imaging method shows in my article PET-CT, CT or MRI.

Markers after treatment has finished are a separate question. Whether they are part of your follow-up plan depends on the type of tumor: for some they are checked regularly, for others they aren’t tested routinely. I explain how follow-up after treatment works in my article on recurrence.

If your marker has gone up: what to do before seeing your doctor

Don’t change anything on your own — neither the treatment nor the timing of cycles. Then, step by step:

1. Find all your previous results for this marker and write them down with dates.
2. Note next to them the dates of your treatment cycles and most recent CT or MRI scans.
3. Check whether the latest test was done at the same lab.
4. Show the table to your doctor at your next scheduled visit.

How urgent this is depends on how you feel, not on the number. If you have new symptoms — pain, shortness of breath, jaundice, high fever — call your doctor straight away, whatever your test results.

Questions to ask your doctor

• Is a tumor marker needed for my particular tumor, and what are we testing it for?
• Was it raised before treatment started? If not, is there any point in monitoring it?
• How often should it be tested, and at which lab?
• What change in the level would be a reason for further tests?
• What will we do if the marker rises but the CT stays unchanged?

Sources

1. Tumor Markers — National Cancer Institute, 2023
2. Tumor Marker Tests in Common Use — National Cancer Institute
3. Tumor Immunodiagnosis — MSD Manual Professional Version
4. ASCO Clinical Practice Guideline: Use of Biomarkers to Guide Systemic Therapy for Women With Metastatic Breast Cancer — ASCO (The ASCO Post), 2015
5. Tumour markers — Canadian Cancer Society
6. Clinical guidelines “Colon and rectosigmoid junction cancer” — Russian Ministry of Health, 2025