CA 125 Blood Test: What It’s For & Reference Range

Updated in December 2022

The CA-125 test is a blood test that measure that measures the quantity of cancer antigen 125 in the blood. This a protein that becomes elevated in cases of ovarian cancer, and testing can be used to evaluate the risk for cancer or monitor response to cancer treatment. 

There are other health conditions that can cause elevated CA-125 levels, like endometriosis, pelvic inflammatory disease or cirrhosis. 

Although the CA-125 is a way to screen for ovarian cancer, is not a diagnostic tool. Other tests, like an MRI or biopsy, are required to confirm the presence of ovarian cancer.

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What it tests

The CA-125 test is ordered to help confirm suspicions of ovarian cancer in women who are at a higher risk for it. It can also be ordered to monitor the progression of an already-confirmed ovarian cancer, or to evaluate a patient’s response to cancer treatment. 

This test can also be used to identify the presence of endometrial cysts, peritoneal cancer, endometriosis, pancreatitis, pelvic inflammatory disease, cirrhosis and ovarian cysts. With these conditions, CA-125 levels can also increase.

Read more about the signs and symptoms of cancer that you should report to your doctor. 

How it’s tested 

The CA-125 blood test is done by collecting a small blood specimen that is sent to the lab for analysis. This exam can be also be done to other body fluid, like fluid extracted from the chest or abdominal cavity. This test does not require any fasting, and results are usually available within a day, depending on the lab. 

Reference range

The normal levels of CA-125 is up to 35 U/mL. Results above this level are considered to be abnormal, and are usually a sign of ovarian cancer or endometriosis. Nonetheless, more testing is require to confirm a final diagnosis. 

As mentioned, this test can also be ordered to evaluate cancer treatment responses. Levels will usually start to decrease if the patient responds well to treatment. On the other hand, if CA-125 levels progressively get higher, it means that treatment is not effective, and that another treatment approach may be necessary. Levels that get higher may also indicate that the cancer has spread and metastasized. Learn more about other blood tests for cancer that the doctor may order.