Spinal taps (lumbar punctures) help doctors diagnose many nervous system problems.

For this procedure, a needle is put into the lower back between the vertebrae to collect a sample of spinal fluid. The procedure usually takes no more than 15 minutes and does not require general anesthesia. Normal spinal fluid is clear and colorless. Abnormal fluid may appear cloudy, which can be a sign of an infection in the brain or spinal cord from meningitis, Lyme disease or some other inflammatory disease.

Other indicators in the fluid can signal a spinal cord tumor or a peripheral nerve disorder, such as a polyneuropathy or Guillain-Barré syndrome. Abnormal antibodies may suggest multiple sclerosis. Low glucose levels indicate an infection of the meninges (covering of the brain) or, in some cases, a cancer. Blood in the fluid may indicate a brain hemorrhage. A variety of diseases, including brain tumors and meningitis, can increase the fluid's pressure.