This page was reviewed under our medical and editorial policy by
Bradford Tan, MD, Chair, Department of Pathology and Laboratory Medicine, City of Hope Atlanta, Chicago and Phoenix
This page was updated on June 7, 2023.
Antigens are substances that trigger the body’s immune system. High levels of a protein known as carbohydrate antigen or cancer antigen (CA) 19-9 may indicate the presence of certain malignant or benign health conditions.
CA 19-9 was discovered in the early 1980s by researchers working to identify tumor antigens in colorectal cancer. Since then, CA 19-9 has become a commonly used biomarker for pancreatic ductal adenocarcinoma (PDAC), a highly aggressive malignant cancer that accounts for more than 80 percent of pancreatic cancer cases, according to the National Cancer Institute. Although the CA 19-9 tumor marker is used to help detect pancreatic cancer, it’s also associated with cancers of the stomach, bile duct and colon.
This overview will cover the basic facts about CA 19-9, including:
A CA 19-9 test is one of a panel of tests used to detect early-stage cancer. As a tumor marker that’s produced by cancer and other cells in the body, CA 19-9 provides important clues about the presence and progression of cancer.
The CA 19-9 blood test is used to help support a patient’s cancer diagnosis and assess cancer treatment. Changes in CA 19-9 levels may tell the oncologist whether the tumor is stable, has grown or has become smaller in response to chemotherapy and other treatments. This information helps the cancer care team determine whether to adjust treatment or whether more tests are needed.
To perform the test, the doctor uses a small needle to collect a sample of blood from a vein in the patient’s arm. This procedure is usually quick, and the patient may feel a pinch when the needle is inserted. The sample is then evaluated under a microscope in a laboratory.
Because labs may measure CA 19-9 levels in different ways, it's important for patients to know which method was used to test their CA 19-9 levels. If possible, patients should get any follow-up CA 19-9 tests measured in the same way by the same lab so the doctor can compare results over time.
It’s possible for someone to have small amounts of the CA 19-9 protein in his or her blood. The normal CA 19-9 range in the average person is between 0 and 37 units per milliliter (U/mL).
CA 19-9 levels above 37 U/mL are generally considered elevated and warrant further testing. Cancers that may cause elevated CA 19-9 levels include:
CA 19-9 levels don’t necessarily correlate to a specific type of cancer. These levels vary depending on the patient’s age, gender and stage of disease.
Several benign conditions may also cause an increase in CA 19-9 levels, including:
CA 19-9 levels above 37 U/mL may indicate cancer. It’s also possible to have a benign condition that causes CA 19-9 levels to rise. And sometimes a person will fall in the normal range for CA 19-9 even if he or she has a type of cancer that usually produces it.
A CA 19-9 test is one component of a comprehensive cancer screening. Cancer experts evaluate tumor marker tests in conjunction with other blood tests, physical examinations and diagnostic imaging tests for a more accurate assessment of the patient’s condition.
If the patient has been diagnosed with cancer, he or she may have subsequent CA 19-9 tests during the course of treatment. Changes in CA 19-9 levels provide clues about the progression of cancer. If CA 19-9 levels remain unchanged, the cancer may be stable. Increasing levels may suggest the tumor has grown.
Declining levels of the CA 19-9 antigen may indicate treatment is helping and the tumor is getting smaller. A decline followed by an increase in CA 19-9 levels may indicate the tumor has returned or become larger. CA 19-9 fluctuations are possible and should be evaluated further.