THEantigen tumor 15-3 (or CA 15-3, Cancer Antigen 15-3) is a protein produced by a multitude of cells, and in particular by breast cancer cells.

The CA 15-3 assay is not used to diagnose breast cancer, nor to detect it early. On the other hand, it is effective for checking the patient’s reaction following treatment for breast cancer: the dosage makes it possible to give indications as to the effectiveness of the treatment, or the occurrence of a recurrence.

The examination consists of taking a sample of venous blood, usually taken from the fold of the elbow. The assay is based on the use of an anti-CA 15-3 antibody capable of recognizing the protein present in the patient’s serum.

The concentration of CA 15-3 is considered normal when it is less than 30 U / ml (Units per milliliter). Note that this threshold may vary slightly depending on the laboratories performing the analyzes.

The drop in CA 15-3 levels following breast cancer means that the treatment has worked well. On the contrary, if the level of CA 15-3 increases, it means that the breast cancer is still progressing or that it has come back (we speak of recurrence).

Note that a slight increase in the level of CA 15-3 is not necessarily a bad sign. Rather, the physician observes the increasing trend over time.

An increase in CA 15-3 levels may also mean the presence of non-cancerous conditions:

• mild breast or ovarian disease

• pelvic inflammatory disease

• liver disease (cirrhosis, hepatitis)

• endometriosis (formation of tissue made up of endometrial cells – the lining that lines the uterus – outside the uterus)

• or simply pregnancy (the level of CA 15-3 may increase in the last trimester) or breastfeeding in progress

It can also be a sign of cancers:

• breast cancer: CA 15-3 levels are high after advanced breast cancer, but this is rarely the case in the early stage

• but also cancer of the ovary, lung, stomach, pancreas, liver