Lumbar puncture (LP), also known as a spinal tap, is a medical procedure in which a needle is inserted into the spinal canal, most commonly to collect cerebrospinal fluid (CSF) for diagnostic testing. The main reason for a lumbar puncture is to help diagnose diseases of the central nervous system, including the brain and spine. Examples of these conditions include meningitis and subarachnoid hemorrhage. It may also be used therapeutically in some conditions.

Why is a lumbar puncture done?

A lumbar puncture may be done to:

• Collect cerebrospinal fluid for laboratory analysis
• Measure the pressure of your cerebrospinal fluid
• Inject spinal anesthetics, chemotherapy drugs or other medications
• Inject dye (myelography) or radioactive substances (cisternography) into cerebrospinal fluid to make diagnostic images of the fluid's flow

How is the LP procedure performed?

For a lumbar puncture, you lie on your side with your knees drawn up toward your chest. This position helps widen the spaces between the bones of the lower spine so that the needle can be inserted more easily. A numbing medicine (local anesthetic) is put in the skin. Then a long, thin needle is put in the spinal canal to collect a sample of cerebrospinal fluid (CSF). Your doctor may need to move to another area of your spine if it is hard to get to the spinal fluid.

The color, blood cell counts, and amounts of protein, glucose, and other substances are measured in the CSF sample. Some of the sample may be added to a substance that promotes the growth of germs (such as bacteria or a fungus). This is called a culture. The pressure of the CSF also is measured during the procedure.