Beta-2 microglobulin (B2M) is a protein found on the surface of most cells in the human body. This protein is released into the bloodstream as cells naturally turn over. The kidneys are responsible for filtering B2M from the blood, and a small amount is then excreted in the urine. Medical tests can measure its levels.
What is Beta-2 Microglobulin?
Beta-2 microglobulin is a small protein, approximately 11.8 kilodaltons (kDa) in size, which forms a component of major histocompatibility complex (MHC) class I molecules. These molecules are present on the surface of nearly all nucleated cells, playing a role in the immune system’s ability to recognize and respond to foreign substances or abnormal cells. B2M associates with the heavy chain of these MHC class I molecules, stabilizing their structure and enabling them to present antigens to T cells.
B2M is constantly shed from the surface of cells into the bloodstream. Once in the blood, this protein is primarily filtered by the glomeruli, the tiny filtering units within the kidneys. After filtration, most of the filtered B2M is reabsorbed and broken down by the proximal tubules of the kidneys, with only minimal amounts appearing in urine. This continuous production, filtration, and reabsorption maintain low and stable levels in healthy individuals.
Understanding the Beta-2 Microglobulin Test
The B2M test measures the amount of this protein in body fluids, most commonly blood serum or plasma, but also sometimes in urine or cerebrospinal fluid. B2M levels can serve as a marker for kidney function and indicators of increased cell turnover or immune system activation. The test is often used to help assess, monitor, or predict the course of certain medical conditions.
A blood sample for the B2M test is collected by a healthcare professional inserting a needle into a vein, usually in the arm. If a urine sample is needed, it may involve collecting all urine over a 24-hour period. A cerebrospinal fluid sample might be collected via a spinal tap if central nervous system involvement is suspected. The test results help clinicians gather information about a patient’s condition.
Normal Range and Interpretation
The normal range for beta-2 microglobulin in blood serum for adults is between 0.70 to 1.80 micrograms per milliliter (mcg/mL) or 1.5 to 3 milligrams per liter (mg/L). For urine, healthy individuals have concentrations at or below 300 micrograms per liter (mcg/L) or 0.04 to 0.22 mg/L. These ranges can vary slightly between different laboratories due to varying testing methods and equipment.
A result within the normal range suggests healthy kidney function and the absence of significant issues causing increased cell turnover or immune activation. Elevated levels of B2M in the blood can indicate impaired kidney function, an increased rate of cell turnover, or activation of the immune system. High levels in the urine, while blood levels may be normal, point to damage or dysfunction in the kidney tubules, which are responsible for reabsorbing B2M.
Significantly decreased B2M levels are less commonly observed and are not considered clinically meaningful for diagnostic purposes. An abnormal B2M result is not a standalone diagnosis for any specific disease but indicates that further investigation may be needed. Healthcare providers consider these results alongside other clinical findings, patient history, and additional diagnostic tests.
Conditions Associated with Abnormal Levels
Elevated beta-2 microglobulin levels in the blood are associated with conditions that impair its clearance by the kidneys or lead to increased production by cells. Kidney diseases, such as chronic kidney disease or end-stage renal disease, can cause B2M to accumulate in the blood due to inefficient filtration by damaged glomeruli. In individuals undergoing long-term hemodialysis, B2M can also aggregate and deposit in joints and tissues, leading to dialysis-related amyloidosis.
Certain cancers, particularly those affecting the blood and bone marrow, like multiple myeloma, lymphoma, and leukemia, can result in elevated B2M levels due to increased production from cancerous cells or a higher rate of cell turnover. In multiple myeloma, higher serum B2M levels correlate with a greater tumor burden and can indicate a more advanced stage of the disease, influencing prognosis. Solid cancers such as ovarian, gallbladder, prostate, breast, and renal cell carcinoma can also be associated with increased B2M levels.
Chronic inflammatory diseases, including rheumatoid arthritis, systemic lupus erythematosus, and Crohn’s disease, can lead to elevated B2M due to generalized immune system activation and increased cellular activity. Viral infections like Human Immunodeficiency Virus (HIV) and cytomegalovirus (CMV) increase B2M concentrations, reflecting the body’s immune response to these pathogens. Some medications, such as lithium or certain antibiotics like aminoglycosides, can cause B2M levels to rise by inducing kidney damage.
When elevated B2M is found in urine, even with normal blood levels, it points to damage or dysfunction of the renal tubules. These tubules are responsible for reabsorbing B2M. Conditions causing tubular injury, such as exposure to heavy metals like cadmium or mercury, or certain drug toxicities, can result in higher urinary B2M due to the tubules’ reduced ability to reclaim the protein. In individuals who have received a kidney transplant, an increase in urine B2M may signal early kidney rejection.