What Is the Cancer Antigen 15-3 Blood Test?

A tumor marker test used to monitor metastatic breast cancer

Various tools are used to monitor cancer progression and treatment response. For some cancers, tumor marker blood tests, such as the CA 15-3 blood test, can be indicative of tumor changes.

The cancer antigen 15-3 (CA 15-3) biomarker test is used to monitor breast cancer and its response to treatment. The CA 15-3 antigen is a protein released into the bloodstream by breast cancer, various other cancers, and even some benign (noncancerous) conditions.

CA 15-3 is one of several substances classified as tumor markers, which may increase as a malignancy progresses and decrease as a tumor gets smaller in response to cancer therapy. It is one of several tumor markers used to monitor people with stage 4 breast cancer (also known as metastatic breast cancer). While some oncologists (specialists who treat cancer) will use the test as a way to detect breast cancer recurrence, it is not recommended for this purpose.

Read on to learn more about the CA 15-3 blood test, what it does and what it means, and what you can expect when this test is ordered for you.

Purpose

CA 15-3 is an antigen that's normally found in breast tissue. Antigens are proteins that identify a cell, acting as its "signature."

While the CA 15-3 antigen does not cause cancer, the level can increase as cancer cells multiply. The number of CA 15-3 antigens will increase with a tumor's growth.

That said, not all breast cancers produce CA 15-3 antigens. This is especially true with early-stage breast cancer. Fewer than half of patients with early-stage breast cancer will show elevations in CA 15-3 values and up to 80% of people with metastatic breast cancer (breast cancer in late stages that has spread) will have increased CA 15-3 levels.

These dynamics make the CA 15-3 useful for monitoring stage 4 breast cancer or secondary tumors in other parts of the body.

Your oncologist may order a CA 15-3 test for one of these two reasons:

  • By regularly tracking your CA 15-3 values, your oncologist can assess how effective your cancer treatment is.
  • If CA 15-3 antigens rise over a certain level, it may indicate worsening of your disease and may guide a search for metastasis in other parts of the body (most often the bones or liver).
Conditions Linked to CA 15-3 (Besides Breast Cancer)

Verywell / Theresa Chiechi

Limitations: Sensitivity and Specificity

Although the CA 15-3 test is valuable for monitoring breast cancer, it is less reliable when used for cancer screening. This is partly because the CA 15-3 is not a highly sensitive test and it's not specific to breast cancer.

Other benign and malignant conditions can also cause increases, such as:

And pregnancy can increase CA 15-3 levels.

The CA 15-3 test also has low sensitivity because breast cancer doesn't always cause it to increase. In fact, according to a 2015 study from Germany, the CA 15-3 test has a sensitivity of only 55.6% when used to diagnose metastatic breast cancer.

Due to its potential for false-positive results (results that read as positive for cancer when there is no cancer), the American Society of Clinical Oncologists (ASCO) advises against the use of tumor marker tests to screen for new or recurring breast cancer.

Routine mammograms are considered the frontline tool for breast cancer screening in most countries.

Risks and Contraindications

There are few risks associated with the CA 15-3 test. The test requires a blood draw, which may cause mild pain, redness, or bruising. Light-headedness and fainting may also occur.

Less commonly, the blood draw may cause a hematoma, which is an accumulation of blood under the skin. Most small hematomas go away on their own; larger ones may require treatment.

Infection is rare following a blood draw if standard health precautions are taken.

Before the Test

Measuring CA 15-3 requires a blood test typically performed along with a complete blood count (CBC), liver functions tests, and kidney function tests. There are no preparations needed for any of these tests.

Timing

The blood draw takes only a few minutes. Depending on the registration and the waiting time, your appointment can take 30–90 minutes.

Location

The CA 15-3 test can often be performed at the oncologist's office during your visit. Otherwise, you may be referred to a nearby lab.

What to Wear

You should choose a top that has either short sleeves or sleeves that can be easily rolled up. If you plan to return to work after the test, you may want to wear long sleeves to cover the bandage or puncture mark on your arm.

Food and Drink

There are no food or drink restrictions for the CA 15-3 test.

If you have small veins, it often helps to drink a couple of glasses of water before the test. Doing so won't affect the results, but may plump your veins and make the blood draw easier.

Medications That Alter Results

Certain medications and supplements can interfere with the CA 15-3 test. Among them is biotin (also known as vitamin B7, vitamin B8, vitamin H, or coenzyme R). The CA 15-3 test relies on biotin to bind to the CA 15-3 antigen and may be affected if too much biotin is consumed.

While the recommended daily intake of biotin is unlikely to cause any harm, high-dose biotin supplements may trigger false-negative readings. Stop taking any supplement containing more than 0.03 milligrams of biotin 72 hours before the test.

In addition, the targeted cancer drug Afinitor (everolimus) may cause paradoxical effects. The drug may cause CA 15-3 levels to rise when treatment is working (resulting in a false-positive result) or drop when treatment is failing (resulting in a false-negative result that appears negative when it is actually positive).

You should tell the phlebotomist if you are on Afinitor so a notation can be made for the reviewing pathologist.

Cost and Health Insurance

The cost of a CA 15-3 test can vary by location and generally runs between $50–$80; sometimes more. If you have to pay out of pocket or have high copayment costs, it helps to shop around for the best price. Even if you have insurance, check that the lab is an in-network provider; otherwise, your claim may be denied.

The test doesn't usually require preauthorization (the insurance carrier having to approve the test before it is performed), but you may be limited as to how many tests you can take each year. Check your policy or call your health insurer for details.

If you are uninsured or struggling with treatment costs, ask your oncologist if there are any financial assistance programs you qualify for. You should also contact the Cancer Financial Assistance Coalition (CFAC), a group of national organizations that provide financial help to people with cancer.

What to Bring

You will need to bring a form of ID, your health insurance card, and either a credit card or check to cover any out-of-pocket expenses. Find out which form of payment the office accepts before you go to your appointment.

During the Test

When you arrive at the lab, you will be asked to sign in and complete a registration form. Many labs will ask you to pay your coinsurance or co-pay costs in advance; others will bill you later. A consent form may also be provided.

The blood draw, also known venipuncture, is performed by a phlebotomist.

Throughout the Test

The phlebotomist will be wearing disposable gloves and will begin by gently pressing their fingers against your skin to locate the best vein.

Once the vein is selected, they will place a tourniquet around your upper arm and ask that you make a fist. You may be asked to pump your fist several times if your veins are small. The site is then swabbed with an alcohol pad.

You will feel a small prick as the needle is inserted. For the CA 15-3 test, the phlebotomist will need to obtain a minimum of 0.3 milliliters of blood. The vacuum-sealed test tube will usually have a red top or a gel barrier.

Once the needle is withdrawn, a cotton ball or gauze is placed on your arm until the bleeding stops. The phlebotomist then applies an adhesive bandage over the puncture wound.

Post-Test

You should be able to leave shortly after your test as long as you are not bleeding or feeling woozy. You can then continue your day as usual.

After the Test

You may feel sore or develop redness or bruising at the puncture site. These issues will usually resolve on their own without treatment. If needed, you can take Tylenol (acetaminophen) to help alleviate the pain.

If you develop a small lump, you can apply a cold compress on your arm for 20 minutes several times a day to relieve the swelling. Do not apply ice directly to the skin or leave it on for longer.

Call your healthcare provider if you develop any signs of infection after a blood draw, including a high fever and persistent or worsening pain, swelling, or tenderness at the puncture site.

Interpreting Results

The results of your test should be sent to your oncologist within three to five working days. The report will include a reference range outlining the normal and abnormal CA 15-3 levels. This is based on the expected values within a population and can vary slightly from one lab to the next.

Generally speaking, a CA 15-3 value of 30 units per millimeter (U/mL) or less is considered normal. With that being said, a single value is generally not as helpful in diagnosis as serial values, in which your results are routinely measured during the course of treatment.

Your CA 15-3 level must be evaluated in conjunction with a physical examination, diagnostic imaging, and other blood tests.

Higher CA 15-3 levels typically correspond with more advanced stages of breast cancer. The highest levels tend to be seen in metastatic breast cancer, particularly when the liver or bone is involved. However, CA 15-3 levels can be low or normal even in advanced disease since not all breast cancers produce CA 15-3.

Sometimes CA 15-3 levels can be abnormally elevated during the first four to six weeks of new cancer therapy. Any treatment that disrupts a tumor can cause a temporary rise in tumor markers. To avoid misinterpretation, the CA 15-3 test should be performed at least two to three months after starting a new treatment.

It is important to note that when CA 15-3 levels are elevated with benign conditions, the levels tend to remain stable. It is only when levels rise that further investigation may be warranted.

Follow-Up

If your CA 15-3 results are elevated, your oncologist will likely order additional tests. If the elevation is minimal, the healthcare provider may take a watch-and-wait approach and order a repeat test several weeks later.

If elevation persists or increases, your healthcare provider may order additional testing.

This may include:

Summary

While a CA 15-3 blood test might be helpful to your doctor as part of the bigger picture, it is not used alone to monitor your response to treatment. Because of all of the factors that can cause fluctuating antigen levels, it's best to see this with other monitoring tests.

It is most effective in monitoring advanced breast cancer and its response to treatment, but even with metastatic breast cancer, this is not the only evaluation tool available.

A Word From Verywell

You might experience stress or anxiety when you're having blood tests to monitor metastatic breast cancer, especially if you have a sudden rise in a tumor marker. However, it is important to remember that the CA 15-3 provides only a glimpse of what is going on. Results should be examined in the context of your health and other diagnostic evaluations.

Having a temporary increase in CA 15-3 levels does not necessarily mean that your cancer is spreading. Along those same lines, a temporary decrease does not necessarily mean that the cancer is disappearing.

Frequently Asked Questions

  • What does a high cancer antigen 15-3 mean?

    A CA 15-3 level often is associated with breast cancer. It may indicate that cancer that you were treated for has returned. Non-cancer-related explanations can include endometriosis, pelvic inflammatory disease, chronic hepatitis, and more. Your doctor will do more tests to explore the underlying cause of the elevated marker.

  • What is the normal range of the CA 15-3 test?

    A CA 15-3 measure that's less than 30 units per milliliter (U/mL) is considered normal.

  • What is the difference between the CA 27.29 and CA 15-3?

    Both of these test for cancer antigens. However, CA 27.29 and CA 15-3 measure different tumor markers. While their results are often related, they are different and should not be used interchangeably.

  • What does it mean when my tumor markers go down?

    While it may mean your cancer is responding to treatment, advanced breast cancer can have low tumor markers, specifically, low CA 15-3 levels. Doctors can use them as one piece of information among many. Your doctor will be able to provide you with more information based on other tests they do.

Originally written by
Pam Stephan
Pam Stephan is a breast cancer survivor.
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