CSF myelin basic protein is a test that measures the level of myelin basic protein in the cerebral spinal fluid (CSF). Myelin is a substance which covers the nerves and acts as an insulator. Without this insulation, information from nerves would be transmitted inefficiently which may result in weakness, sensory loss or other neurologic dysfunction. Myelin basic protein (MBP) refers to one of the components found in myelin and is a marker of its presence. MBP is measured in the CSF as part of an evaluation to determine if there has been inflammation and myelin breakdown in the central nervous system.