Cancer Antigen 15-3 (CA 15-3) is a protein found on the surface of breast cancer cells, which can be measured in the blood to provide information about the disease. This marker is utilized primarily in the management of breast cancer. No single measured level can definitively confirm a cancer diagnosis, and the marker is not specific enough for initial screening of the general population. The test serves as one tool in a larger set of diagnostic and monitoring procedures.
Understanding the CA 15-3 Tumor Marker
CA 15-3 is a mucin-like glycoprotein, a large protein with attached sugar molecules, which is encoded by the MUC1 gene. This protein is a fragment of the MUC1 molecule, which is normally found on the surface of various epithelial cells, including those in the breast. In many breast cancers, the MUC1 protein is overexpressed and shed into the bloodstream, where the CA 15-3 test measures it.
The biological role of the MUC1 protein is thought to involve cell adhesion, and its overexpression in cancer may facilitate tumor cell invasion and metastasis. The test is ordered to track the progression of specific cancers, most often advanced or metastatic breast cancer. This marker is not used for cancer screening or initial diagnosis because only a minority of early-stage breast cancer cases show elevated levels. Furthermore, many people who are otherwise healthy or who have benign conditions can also have elevated CA 15-3 levels.
Interpreting Specific CA 15-3 Values
The concentration of CA 15-3 in the blood is measured in units per milliliter (U/mL). The generally accepted “normal” range is typically considered to be below 30 U/mL or 31 U/mL, though the exact cutoff can vary slightly between different laboratories and testing kits. A result within this range usually suggests no significant cancer activity or stable disease in a patient being monitored.
Values that rise above this threshold suggest the possibility of malignancy, especially in a patient with a known breast cancer diagnosis. Higher CA 15-3 levels are commonly correlated with a greater burden of cancer. The highest concentrations are often seen when the cancer has metastasized, particularly to the bones or liver. However, a high CA 15-3 number does not guarantee a cancer diagnosis; approximately 20% to 30% of patients with advanced breast cancer may have tumors that do not shed the marker, resulting in a normal value despite the presence of disease.
Reasons for Elevated Levels Beyond Cancer
The lack of specificity means that an elevated CA 15-3 level is not automatically an indicator of breast cancer. The marker can be elevated in a variety of non-cancerous conditions. Liver diseases, such as hepatitis and liver cirrhosis, can cause increased levels because the liver is responsible for clearing the marker from the circulation.
Benign conditions of the breast, ovaries, and lungs can also lead to mildly or moderately elevated CA 15-3 results. Other malignancies, including cancers of the lung, colon, ovary, and pancreas, can also cause the levels to rise. When interpreting a result, clinicians must consider the patient’s entire medical history and other diagnostic test results to determine the true cause of the elevation. These non-cancerous elevations tend to remain relatively stable over time, unlike the increasing trend often seen with active cancer progression.
Monitoring Treatment and Recurrence
The CA 15-3 test is primarily used to monitor the effectiveness of treatment and to watch for recurrence in patients with metastatic breast cancer. Doctors rely on the trend of the numbers over a series of tests, rather than focusing on a single snapshot value. A consistently decreasing level of CA 15-3 often suggests that the current therapy is working and the tumor burden is shrinking.
Conversely, a sustained increase in the marker’s concentration may indicate that the tumor is not responding to treatment or that the cancer is progressing or recurring. For example, a rising trend can sometimes predict the recurrence of the disease several months before it becomes visible on imaging scans or causes symptoms. However, CA 15-3 levels can temporarily rise during the initial four to six weeks of a new treatment due to the death of cancer cells, a transient increase that does not necessarily signify disease progression.
The results of the CA 15-3 test are always used in conjunction with other clinical evaluations and diagnostic imaging. A change in the CA 15-3 level alone is insufficient to change a patient’s treatment plan. Any suspicious rise in the marker must be confirmed by imaging studies, such as CT or PET scans, and a thorough physical examination to ensure the most accurate assessment of the patient’s condition.