How reliable are CA blood tests for breast cancer monitoring?

Tumor marker blood tests like CA 27-29 and CA 15-3 can be part of breast cancer monitoring, but they're generally considered supplementary tools rather than definitive indicators on their own. These markers can fluctuate for various reasons, including non-cancer related factors, which is why oncologists typically look at trends over time along with imaging and physical exams rather than isolated values. Many patients find it helpful to discuss what these specific results mean in the context of their unique treatment history and current health status with their medical team, as they can provide the most personalized interpretation of these monitoring tools.