THEantigen tumor 125 (or CA 125, Cancer Antigen 125) is a protein produced by a multitude of cells, and in particular by ovarian cancer cells.

It is found in tiny quantities in the pleura (tissue that surrounds the lungs), pericardium (tissue surrounding the heart), or the peritoneum (tissue surrounding the organs of the abdomen) of healthy adults.

The CA 125 assay is primarily used to monitor the patient’s response to treatment for ovarian cancer : it gives indications as to the effectiveness of the treatment, or the occurrence of a recurrence.

After treatment with surgery and chemotherapy, patients should therefore be dosed with CA 125 every 2 to 4 months for 2 years, every 6 months during the third year, and then every year. 

The test is not used to screen for ovarian cancer for the following reasons:

• the level of CA 125 is not higher than average in women with early ovarian cancer

• the presence of CA 125 is not specific to ovarian cancer (it may be high in the presence of mild disease or cancers other than ovarian cancer). It is even consistently elevated in about 1% of healthy women.

The examination consists of a sample venous blood sample, usually performed at the crease of the elbow. The assay is based on the use of an anti-CA 125 antibody capable of recognizing the protein present in the patient’s serum. 

The concentration of CA 125 is considered normal when it is less than 35 U / ml (units per milliliter). Note that this threshold may vary slightly depending on the laboratories performing the analyzes.

Following a ovarian cancer, the fall in the level of CA 125 means that the treatment worked well. On the contrary, if the level of CA 125 increases, it means that the cancer is still progressing or that it has come back (this is called a recurrence).

Note that a slight increase in the level of CA 125 is not necessarily a bad sign. Rather, the physician observes the increasing trend over time.

A rise in CA 125 levels may indicate the presence of non-cancerous conditions:

• an ovarian cyst

• inflammation of the pancreas

• liver disease (cirrhosis, hepatitis)

• inflammation of the pleura

• a fibroid (benign tumor growing on the lining of the uterus)

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

• or simply pregnancy or menstruation

It can also be a sign of cancers:

• ovarian cancer

• but also cancer of the uterus, breast, lung, stomach, pancreas, liver or colorectal cancer