Beta-2 microglobulin, or B2M, is a protein usually measured in a blood sample. It can be used as a tumor marker to help assess certain cancers of the blood or bone marrow and follow treatment. A high B2M result alone does not diagnose cancer.
The kidneys remove B2M from the blood. Levels can rise when the kidneys do not filter it well. Inflammation and some infections can also increase B2M. A high value therefore cannot be read simply as a measure of how much cancer is present.
In someone with a diagnosed cancer for which B2M is useful, changes over time can help assess the response to treatment. The clinician considers kidney function and other findings alongside the result. The test name alone does not tell you why it was ordered in your case.

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