The Beta-2 Microglobulin (B2M) test measures the level of this specific protein in the blood, urine, or sometimes cerebrospinal fluid. This protein, part of the HLA (Human Leukocyte Antigen) system, is present on the surface of nearly all nucleated cells in the body and plays a role in immune system function. Elevated B2M levels in the blood may indicate activity of diseases associated with rapid cell proliferation (such as multiple myeloma, lymphoma, or leukemia) or kidney disorders (such as renal tubular damage). In patients with kidney failure, B2M accumulates in the blood due to impaired renal filtration, and its level is used as an indicator of kidney function. Increased B2M in urine may also indicate renal tubulointerstitial damage. This test is used to monitor response to treatment in blood cancers or to assess disease prognosis (such as staging multiple myeloma). The normal B2M level in blood is typically between 0.8 to 2.2 milligrams per liter (mg/L), but reference ranges may vary between laboratories. Interpretation of results should consider clinical conditions, kidney function, and other tests (such as creatinine or serum protein electrophoresis), as elevated B2M may also be seen in inflammatory conditions, autoimmune diseases, or with aging. In infectious cases like meningitis, measuring B2M in cerebrospinal fluid helps diagnose the cause.