CA 15-3

What is being analysed?
Cancer antigen 15-3 (CA 15-3) is a normal product of breast cells. Its levels often rise when there is cancer in the breast, because the antigen is encoded by a gene that is overexpressed in breast tumours. CA 15-3 does not cause cancer: rather, it is a protein released by tumour cells that becomes a useful marker for following the course of the disease.

CA 15-3 is raised in about 30% of women with localised breast cancer and in 75% of women with metastases.
CA 15-3 levels can also be raised in some healthy people and in people with other types of cancer, such as bowel or lung cancer, or conditions such as cirrhosis, hepatitis and benign breast disease.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
CA 15-3 is not sensitive or specific enough to be used for breast cancer screening. It is a tumour marker used to monitor the patient’s response to treatment and to detect recurrence.

CA 15-3 is sometimes used to give additional information about where the cancer has spread (bone or liver) and to give an idea of its size.

When is it requested?
CA 15-3 may be requested together with other tests, such as oestrogen and progesterone receptors, Her2/neu and BRCA-1 and BRCA-2 mutation testing, when advanced breast cancer is diagnosed, to define its characteristics and choose treatment.

If the baseline (pre-treatment) CA 15-3 concentration is high, the marker can be used to monitor treatment and, if measured at regular intervals, to detect recurrence of the disease. CA 15-3 is not usually measured in breast cancer diagnosed early, before it has spread, because it is not raised in most early cancers.

What does the test result mean?
In general, the higher the CA 15-3 concentration, the more advanced the cancer and the larger the tumour. The concentration tends to rise as the cancer grows. In breast cancer with metastases, the highest CA 15-3 levels are seen when the cancer involves the bones and/or liver.

Mild and moderate increases in CA 15-3 are also seen in a variety of conditions, such as liver and pancreatic cancer, cirrhosis and benign breast disease, and even in some apparently healthy people. In non-cancerous conditions, raised CA 15-3 concentrations tend to remain stable over time.

Low CA 15-3 concentrations do not rule out cancer. The tumour may be at too early a stage to produce high concentrations of the marker. In addition, 25-30% of people with advanced breast cancer have tumours that do not produce CA 15-3.

Is there anything else I should know?
CA 15-3 levels are not usually measured immediately after treatment starts, because temporary rises and falls in antigen levels unrelated to the course of the disease can be seen. After treatment starts, a few weeks usually pass before CA 15-3 measurements begin.

1. Can CA 27.29 be measured in addition to CA 15-3?
2. If there is a strong family history of breast cancer, should I be screened with CA 15-3?
3. What can I do to lower my CA 15-3 level?

1. Can CA 27.29 be measured in addition to CA 15-3?
CA 27.29 and CA 15-3 are proteins encoded by the same gene (MUC1). They are measured for the same purpose. The CA 27.29 test was introduced more recently than CA 15-3 and many doctors consider them equivalent. CA 27.29 is therefore measured instead of, not in addition to, CA 15-3.

2. If there is a strong family history of breast cancer, should I be screened with CA 15-3?
CA 15-3 is not recommended as a test for early detection of breast cancer.
Raised CA 15-3 concentrations can have other causes, and negative results do not rule out cancer. CA 15-3 should be measured only after breast cancer has been diagnosed

3. What can I do to lower my CA 15-3 level?
There is nothing you can do to lower your CA 15-3 level.