Lumbar puncture (spinal tap)

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What does a spinal tap tell my doctor?

Mayo Clinic neurologist Jerry Swanson, M.D., and colleagues answer select questions from readers.

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Lumbar puncture (spinal tap)

A lumbar puncture, also called a spinal tap, removes a small amount of cerebrospinal fluid (CSF) — the fluid that protects your brain and spinal cord from injury — for laboratory analysis. The test also measures the pressure in CSF fluid. Results of a lumbar puncture can help diagnose:

• Serious infections, such as meningitis, polio and encephalitis
• Bleeding around the brain (subarachnoid hemorrhage)
• Certain cancers involving the brain and spinal cord
• Certain inflammatory conditions of the nervous system, such as multiple sclerosis and Guillain-Barre syndrome

During a lumbar puncture procedure, you typically lie on your side with your knees drawn up to your chest. A local anesthetic is injected in an area over your lower spine to reduce discomfort from the procedure. Then your doctor inserts a needle into your spinal canal, measures the pressure and collects fluid for analysis. The entire procedure usually takes about 45 minutes. Once the procedure is complete, you'll need to lie flat for 20 minutes to one hour.

Lab technicians check for a number of things when examining spinal fluid, including:

• General appearance. Spinal fluid is normally clear and colorless. If it is cloudy, it may indicate infection.
• Protein — total protein and the presence of certain proteins. Elevated levels of protein may indicate infection or other condition.
• White blood cells. Increased numbers of white cells in spinal fluid may indicate infection.
• Sugar (glucose). A low glucose level in spinal fluid may indicate infection or other condition.
• Bacteria and fungi.
• Polymerase chain reaction — which helps identify certain viruses such as meningitis.