The CA 19-9 blood test measures a protein known as carbohydrate antigen 19-9. This protein is a tumor marker, a substance that may indicate the presence of cancer or certain noncancerous conditions. Cells in the pancreas, liver, and gallbladder naturally produce small amounts of CA 19-9. While healthy individuals have low levels of this protein, certain conditions can cause these levels to rise.
Primary Uses of the CA 19-9 Test
A physician may order a CA 19-9 test for several specific reasons, primarily related to managing a cancer diagnosis rather than for general population screening. One of its main applications is to monitor how well cancer treatment is working. For individuals undergoing treatments like chemotherapy for pancreatic cancer, a decrease in CA 19-9 levels can suggest that the therapy is effectively shrinking the tumor.
Another function of the test is to detect the recurrence of cancer after a patient has completed treatment. If CA 19-9 levels begin to rise after having previously fallen, it may be an early indication that the cancer has returned, prompting further investigation.
The test also assists in determining the prognosis for patients who have already been diagnosed with specific cancers. It is important to understand that the CA 19-9 test is not used by itself to diagnose cancer. Instead, it is used in combination with other diagnostic tools, such as imaging scans like CTs or MRIs and biopsies.
Interpreting CA 19-9 Levels
Interpreting CA 19-9 results requires considering the value in the context of an individual’s overall health. The normal range for CA 19-9 is less than 37 units per milliliter (U/mL), though this can vary slightly between different laboratories. Levels above this threshold are considered elevated and may warrant further investigation.
Elevated levels of CA 19-9 are most closely associated with pancreatic cancer, but they can also be linked to other malignancies. These include cancers of the bile duct, stomach, and colon. The degree of elevation is a significant factor in interpretation; for instance, a very high level is more suggestive of advanced pancreatic cancer than a mild or moderate increase.
When evaluating the results, a rising level may indicate that a tumor is growing or that a current treatment plan is not effective. Conversely, a decreasing level often signals that a tumor is shrinking in response to treatment. A stable level could mean the disease is not progressing.
Non-Malignant Conditions Causing Elevated Levels
An elevated CA 19-9 level does not automatically indicate the presence of cancer, as several non-malignant conditions can also cause this protein to increase. These conditions often involve inflammation or obstruction within the digestive system. Common benign causes include:
- Pancreatitis, which is an inflammation of the pancreas
- Liver diseases, such as cirrhosis and hepatitis
- Gallstones and conditions that cause a blockage of the bile duct
- Inflammatory bowel disease (IBD)
- Biliary infections, known as cholangitis
Test Limitations and Considerations
The CA 19-9 test has inherent limitations that are important to consider when interpreting its results. A small portion of the population, estimated to be between 5% and 22%, has a genetic trait that makes them “Lewis antigen-negative.” These individuals cannot produce the CA 19-9 antigen, meaning the test would show a normal or low level even if they had advanced cancer.
The test’s utility is also limited by its diagnostic sensitivity and specificity. It is not effective as a general screening tool for asymptomatic individuals because it can produce false-positive results, where levels are high due to benign conditions. This lack of specificity means it could cause unnecessary anxiety and lead to invasive follow-up procedures.
Furthermore, the test can also produce false negatives, where it fails to detect cancer that is present. This can happen in the Lewis-negative population or in the early stages of a malignancy when the tumor is not yet producing enough CA 19-9 to elevate blood levels significantly.