The beta-2 microglobulin (B2M) test measures the amount of a small protein found on the surface of most cells in your body. Small amounts naturally shed into your blood and urine, but high levels can mean certain cells are turning over too quickly or your immune system is working overtime. Doctors usually order this test to help diagnose, monitor, or check the severity of blood cancers like multiple myeloma, lymphoma, and certain leukemias. It can also help evaluate kidney damage or inflammatory conditions, giving your healthcare team important insights into your overall health and treatment progress.