Cancer Antigen 15-3 (CA 15-3)
CA 15-3 is a protein that can appear in the blood in higher amounts when certain breast tissue conditions are present, and the test measures its level. It is used mainly to help monitor a condition that has already been diagnosed.
CA 15-3 is not used to screen for or diagnose disease on its own, because it is often normal early on and can be raised by unrelated conditions. Its main role is tracking trends over time in someone with a known diagnosis. A single value tells you very little without context.
A raised CA 15-3 is not a diagnosis and can stem from several benign causes, including liver disease, benign breast conditions, and some other inflammatory states. Levels can also fluctuate for reasons unrelated to any serious condition. Your clinician therefore focuses on the trend across repeated tests and your overall picture rather than one number.
A low or normal CA 15-3 is the common, expected result in most healthy people and is generally reassuring. In someone being monitored, a stable low value is typically viewed favorably.
There is no universal target, and reference ranges vary by laboratory, so the result should not be self-interpreted. A clinician should review CA 15-3 alongside your history and prior values to decide what, if anything, it means for you.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.