CA-125 Explained: The Ovarian Cancer Marker

Health screening 09 October, 2026

Understanding the CA125 blood test

What is CA-125 and what does it measure?

CA-125 stands for cancer antigen 125. It’s named that way because it’s found on the surface of cancer cells, including ovarian cancer cells. Ovarian cancer is the sixth most common cancer in women, according to Cancer Research UK.

However, it is important to note that CA-125 also occurs in much smaller amounts on the surface of normal cells, such as healthy ovarian cells, womb lining cells and fallopian tube cells.

When cells with CA-125 on their surface are broken down, CA-125 is released into the bloodstream. A CA125 blood test measures CA-125 levels in the blood sample collected.

How and why is the CA125 blood test performed in the UK?

The CA-125 blood test is usually performed in the UK when a person with ovaries has symptoms that can be caused by ovarian cancer.

Having ovarian cancer symptoms does not definitively confirm that you have ovarian cancer, as these symptoms can also be caused by other health conditions.

The CA-125 blood test, therefore, provides a key input alongside clinical symptoms in determining who needs to go on to have further investigations. Your blood test results can then determine which follow up tests you need.

Most often, a CA125 blood test is recommended in a primary care setting, ie by a doctor at your GP surgery. Your GP will only recommend it after discussing your symptoms and, in some cases, examining your pelvic and abdominal area.

What is the normal range for CA-125 levels?

CA125 blood test results are measured in units per ml (U/ml). The vast majority of healthy women have blood levels of CA-125 that are below 35U/ml.

A very small minority of healthy women may naturally have levels higher than35 U/ml. However, in most cases, a CA-125 level of 35U/ml or higher is considered an abnormal test result. This means that there’s an underlying health problem.

A higher number does not mean ovarian cancer is more likely than a non-cancerous condition. This is because other conditions can cause levels that are just as high as those that occur in ovarian cancer.

The role of CA-125 in ovarian cancer diagnosis

Can CA-125 diagnose ovarian cancer or just indicate risk?

A CA125 blood test alone cannot diagnose cancer, whether it’s ovarian cancer or another type of cancer.

This is because you can have ovarian cancer but have normal CA-125 levels – this is called a false negative. According to Target Ovarian Cancer, false negatives occur in almost half of all cases of early stage ovarian cancer.

You can also have a high level of CA-125 but not have ovarian cancer – this is called a false positive. False positives can be caused by other medical conditions, such as endometriosis, pelvic inflammatory disease and liver disease.

A high level of CA-125 can also be caused by other types of cancer, including bowel cancer, breast cancer, cervical cancer, endometrial cancer and lung cancer.

Consequently, a CA125 blood test is not used as a screening tool in the UK for ovarian cancer.

Nonetheless, in around 8 out of 10 patients with late stage ovarian cancer, CA-125 levels are raised. The CA-125 blood test can, therefore, be used as part of the risk stratification process for ovarian cancer.

If you have ovarian cancer symptoms, they should always be investigated, particularly if they occur alongside elevated CA-125 levels. Your blood test results will help your doctor determine what type of further investigation is needed.

Recognising ovarian cancer symptoms alongside CA-125 results

High CA-125 levels and persistent symptoms of ovarian cancer should be investigated urgently to rule out ovarian cancer.

If ovarian cancer is confirmed through further tests, you can receive prompt treatment. Early detection and early diagnosis make a significant difference to the success of treatment, which may include surgery, chemotherapy and/or radiotherapy.

Ovarian cancer symptoms can include:

Persistent bloating of your tummy area
Changes in your bowel habits
Feeling full easily ie even if you have not eaten a lot
Loss of appetite
Pelvic pain and/or abdominal pain
Sudden urges to urinate or needing to urinate more often

Other symptoms of ovarian cancer include unintentional weight loss and fatigue.

Beyond cancer: other reasons for high CA-125 levels

High levels of CA-125 can be caused by several other conditions that are not cancerous. This includes conditions affecting a woman’s reproductive organs, such as:

Benign ovarian cysts – non-cancerous, fluid-filled growths in and on the ovaries
Endometriosis – when tissue similar to that lining the inside of the womb grows outside of the womb
Pelvic inflammatory disease – inflammation of the pelvic organs which may be caused by infection.
Uterine fibroids – non-cancerous growths within or on the womb wall

Other conditions that increase CA-125 that do not involve the reproductive organs include:

Autoimmune conditions eg lupus and rheumatoid arthritis
Gut conditions eg ulcerative colitis and diverticulitis
Heart conditions eg heart failure and pericarditis
Kidney disease
Liver conditions eg hepatitis and liver cirrhosis
Pancreatitis

CA-125 levels can also increase when you are having a period or ovulating, or if you’re pregnant. This is why it’s always important for patients to discuss CA125 blood test results in context and with a doctor.

Further tests after a high CA125 blood test result

If your CA125 blood test result shows that you have high CA-125 levels, your doctor will advise you on next steps. This will usually include tests at your local hospital or health centre often in conjunction with a Gynaecologist.

In most cases, your doctor will recommend an urgent ultrasound scan of your abdomen and pelvis. This is often performed as a transvaginal ultrasound scan where a slim, wand-like probe is inserted into your vagina. However, it may alternatively be performed by moving an ultrasound probe across your tummy.

The results of your ultrasound scan will determine which further tests you will need, such as a CT scan, laparoscopy and/or collection of a small sample of abnormal tissue (biopsy).

Find peace of mind with a CA125 blood test

Do you have a family history of ovarian cancer? Are you concerned about your ovarian cancer risk? Or perhaps you have persistent bloating and unexplained pelvic pain. A CA-125 blood test is a helpful tool on your journey towards an accurate diagnosis.

With Vista Health, you can benefit from a simple, at-home, pin-prick blood test for CA-125 when you book your Women’s Health Assessment.