CA 15-3
Definition
Measures the CA 15-3 protein in the blood. This protein is the product of the MUC1 gene and can be released into the blood by breast cancer cells.
Purpose
The use of CA 15-3 is narrow and clearly defined: the test is used to assess whether treatment is working or whether the cancer has recurred, particularly for monitoring treatment in advanced-stage breast cancer, and for monitoring circulating MUC1, CEA and possible tumor cells in patients with metastatic disease.
In other words, CA 15-3 is intended for monitoring in diagnosed and usually advanced-stage/metastatic breast cancer.
Results
- A level that falls during treatment → treatment may be working
- A level that rises after treatment → there may be a recurrence
Values are compared over time; a single measurement is not meaningful on its own.
Normal Values
Laboratory-specific.
Risks and Limitations
Three important limitations:
1. It cannot be used for screening or early diagnosis. The general rule for tumor markers applies here as well: tumor marker tests used for cancer screening cannot diagnose cancer on their own; a biopsy is required for a definitive diagnosis.
2. It usually does not rise in early stages. All sources limit its use to advanced-stage/metastatic disease. A normal result is expected in early-stage breast cancer; a normal CA 15-3 does not rule out breast cancer.
3. The general evidence problem with tumor markers:
For a tumor biomarker test to be used to guide clinical care in standard practice, it must have high analytical validity and clinical utility. Very few tumor biomarkers meet these high standards.
Non-cancer elevations: As with other tumor markers, benign breast diseases and some liver conditions can also raise CA 15-3, so the result is interpreted together with the clinical picture and imaging.