CA125 blood test

What is a CA125 blood test?

CA125 is a protein that both women and men have in their blood. A high level of CA125 in your blood is a possible indicator of ovarian cancer, as CA125 can sometimes be produced by ovarian cancer cells. A CA125 blood test is used to check the level of the CA125 protein in the blood to support the diagnosis of a number of things including ovarian cancer. 

When might a CA125 blood test be necessary?

If your GP thinks your symptoms could be a sign of ovarian cancer, they'll recommend you have a blood test to check the level of a protein called CA125 in your blood. Your GP will undertake an examination of your tummy and/or internal vaginal examination before they make this referral. The CA125 test can either be carried out at your local doctor’s surgery, or you will be referred to your local hospital the same way you would for any other blood test. 

CA125 blood test results

The normal range for CA125

For most women, a CA125 level below 35 units per milliliter (U/mL) is considered normal. However, it’s important to note that some women naturally have levels above this range without any health concerns. (Source: NHS, CA125 Testing Guidelines)

Elevated levels of CA125

A raised CA125 level means your CA125 reading is higher than the normal range. A high level alone does not confirm ovarian cancer, as various benign conditions can elevate CA125, such as fibroids or endometriosis.

Low CA125 levels

Having a low CA125 reading is generally not a concern and does not indicate health problems. Most people with low CA125 levels are healthy and do not have ovarian cancer.

What can cause elevated CA125 levels?

Ovarian cancer

Elevated CA125 levels may sometimes be associated with ovarian cancer, as some cancerous ovarian cells release CA125. However, other tests including an ultrasound and a biopsy are required for a full diagnosis.

Chest infections, pregnancy, endometriosis, fibroids

Other health issues can also increase CA125 levels. Conditions such as chest infections, pregnancy, and benign ovarian cysts may raise CA125 without any link to cancer. Elevated levels of CA125 may also be a sign of:

• Benign ovarian cysts
• Chest infection
• Endometriosis
• Fibroids
• Liver cirrhosis
• Menstruation
• Pancreatitis
• Pelvic inflammatory disease
• Pregnancy
• Renal failure.

Having a raised level of CA125 is not a diagnosis of ovarian cancer, but it should lead to further investigations. These will usually be in the form of a referral for an ultrasound scan.

What if my CA125 blood test is normal?

It is important to continue to monitor your symptoms as sometimes your CA125 levels can be normal in the early stages of ovarian cancer. If you've had a normal test result but your symptoms don't improve, go back to your GP as you may need to be re-tested.

Things other than ovarian cancer that could present with symptoms such as bloating, stomach pain, difficulty eating and weeing more frequently include:

• Gallstones
• Gastro infections
• Indigestion
• Irritable bowel syndrome (IBS)
• Pulled muscles
• Pregnancy
• The onset of the menopause
• Urinary tract infections.

Your GP should continue to investigate to see if your symptoms are linked to any of the above, or something else.

Can a CA125 blood test be used as a screening tool for ovarian cancer?

We’re often asked whether the CA125 blood test can be used as a way of screening women for ovarian cancer and have to share the disappointing news that it can’t. Here we dig into the reasons why, and what research is taking place in the search for a screening tool.

Why can't the CA125 blood test be used as a screening tool for ovarian cancer?

It is not just ovarian cancer that can cause an elevated CA125 reading, many other things can cause this to be the case (a chest infection, pregnancy, endometriosis, or fibroids for example), along with the fact that some women will have a naturally elevated level with no cause for concern.

Not all women with a raised CA125 level will have ovarian cancer, and it is also important to consider some women may have ovarian cancer but have a normal CA125 reading.

Annually screening all women with a CA125 blood test could lead to many being referred for unnecessary further investigations, causing needless worry and the possibility of having to undergo inappropriate surgery for no reason.

In 2021 the results of a 20-year study looked at the effectiveness of CA125 blood tests and ultrasound scans as screening tests for ovarian cancer. The study concluded that using the CA125 blood test or ultrasound scans to test for ovarian cancer did not help people to live longer and is therefore not recommended to be used as screening tests for ovarian cancer.

What other research is happening in screening for ovarian cancer? 

Ovarian Cancer Action funds the work of Professor Ahmed at the University of Oxford and so far, the team has made several exciting discoveries that have taken them closer to answering that question. They have found that the number of cells that have a protein called SOX2 is markedly increased in the fallopian tubes of women with or at high risk of ovarian cancer. Having a better understanding of how the disease develops is key to developing a screening tool.

Although they have made some exciting discoveries, there is still a lot more research to be done. Identifying the SOX2 protein is an important step forward but it’s very difficult to get to, meaning a screening tool centred around it would be quite invasive. They are now looking for other changes that take place in the body simultaneously to the SOX2 protein production and, by harnessing different markers, they hope to find another marker that is easier to test for.

They hope to complete their next stage of investigations within the next five years and then the next step would be to translate their findings into clinical research.

Reviewed: September 2022

Next review: September 2023