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Also known as Cancer Antigen 15-3 test.

What is a CA 15-3 Test?

A CA 15-3 test detects the amount of cancer antigen 15-3 (CA 15-3) in your blood. CA 15-3 is a substance that boosts your body's immune system. Some cancer cells release the CA 15-3 antigen into the blood. Therefore, this test usually helps monitor certain types of cancer. However, breast cancer is most likely to release CA 15-3, especially in cases where breast cancer comes back after treatment. 

Antigens like CA 15-3 that give cancer information are called tumor markers.

What is the test used for?

As mentioned above, CA 15-3 is a tumor marker so, it is used if you are already diagnosed with breast cancer. Your doctor suspects that it has spread to other parts of the body or metastasized, or you may have this test along with other tests such as hormone receptor testing and HER2 status testing to determine how breast cancer treatment is working and look for cancer that has recurred, after treatment.

However, this test is not used to detect early-stage breast cancer because the levels of CA 15-3 are seldom higher than usual at this stage.

When and why do you need a CA15-3 Test?

You may need a CA 15-3 test if you are currently undergoing treatment or have previously had therapy for invasive breast cancer. Your doctor may order this test to comprehend how well the medicine performs or if he wants to check for breast cancer recurrence. A CA 15-3 is also helpful to determine whether a tumor is spreading or reducing in response to treatment.

Your doctors may also advise a CA 15-3 test if you are diagnosed with advanced breast cancer.

CA, 15-3 levels are increased in about 80% of people with breast cancer spread to other body areas. Again the test might not be as helpful for people with early-stage or localized breast cancer, as less than 50% of these cases will involve elevated CA 15-3 levels.

What kind of sample is required for the test?

The CA 15-3 test is a blood test and usually takes place at a doctor's office or a hospital.

  • A phlebotomist will wear disposable gloves and start by pressing his fingers on your skin to locate the vein during the test.
  • After the vein is selected, he will tie a band around your upper arm and ask that you make a fist. He might also ask you to pump your fist several times if your veins are small. The puncture site will be cleaned with an antiseptic.
  • You will feel a slight prick or pain when the needle is injected. For the CA 15-3 test, the technician needs to collect a minimum of 0.3 milliliters of blood. 
  • After the blood is collected, a cotton ball is put on your arm until the bleeding stops.

Do you need to prepare for the test?

Usually, you don't need to prepare for a CA 15-3 test, but you should tell your doctor if you take any medications, supplements, or herbal remedies before the test. Your doctor may suggest withholding some substances a few days before lab tests.

For instance, tell your doctor if you take biotin supplements to grow hair and nails because biotin may interfere with various lab test results.

Are there any risks to the test?

A CA 15-3 test poses minimal risks. As we know, the test needs a blood draw to cause mild bruising or redness at the puncture site. You may also need lightheadedness and faintness. However, infection is very unlikely after blood draw if standard health precautions are taken.

Sometimes, the procedure may cause the leakage of blood under the skin, which leads to hematoma formation. Small hematomas are usually not a cause of concern and go away on their own; larger ones may need treatment.

What do the test results mean?

A typical test result for CA 15-3 levels is usually 30 units per milliliter (U/ml) or less. According to experts, CA, 15-3 increases gradually with the advancement of breast cancer. Therefore, increased CA 15-3 may indicate that cancer has metastasized.

CA 15-3 levels that persist in increasing may mean that the:

  • Treatment is effective
  • Cancer is persisting to spread
  • Cancer is recurring

Similarly, a decreasing pattern of CA 15-3, or levels coming back to normal ranges, may indicate that you are responding to the treatment effectively. However, in some cases, people with normal levels of CA 15-3 might have breast cancer. In the initial stages of breast cancer, CA 15-3 quantities may not be enough to detect a tumor. 

Research also suggests that 20-25% of people with advanced breast cancer do not release CA 15-3 antigen. You may require additional tests to confirm supporting the CA 15-3 test findings and determine how cancer affects the body.

How accurate is the test?

Some laboratories use different methods to test CA 15-3, so the outcomes may vary. If your doctor thinks you may need several CA 15-3 tests over time, they may ask you to get the tests done from the same laboratory using the same method every time. It helps doctors diagnose and compare the results correctly. 

Another important thing we like to mention is that not every doctor uses CA 15-3 test to assess the disease, as other tumor markers are more sensitive. In addition, the CA 15-3 test is not always a trustworthy indicator of breast cancer or recurring breast cancer. Generally, increasing CA 15-3 shows that breast cancer is growing or recurring while decreasing levels suggest that treatment works efficiently. However, other factors can also play a role and affect CA 15-3, such as ethnicity and other medical conditions.

Low Levels of CA 15-3

Women without breast cancer can have comparatively lower levels of CA 15-3 if:

  • They are perimenopause
  • Have had uterine fibroids
  • Have taken birth control pills for an extended period

High Levels of CA 15-3

The increased CA 15-3 levels can be found in Hispanic and Black females. 

Your healthcare provider will order other tests to confirm any CA 15-3 test findings to evaluate how the treatment works and make a diagnosis.

Related tests: Cancer Antigen-125, Cancer Antigen 19-9, BRCA TM COMP

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