The spinal cord is a bundle of nerves that run from the base of the brain all the way down the back. They are responsible for controlling muscles and movement as well as the sense of touch. It is protected by the bones of the spine, and a layer of fluid which surrounds it, called the cerebrospinal fluid.

A spinal tap is a procedure done to look for infection, cancers, and bleeding in this area by taking a sample of the cerebrospinal fluid. During this procedure, the patient will be asked to lie on her side, with the knees drawn up to the chest. Sometimes the patient will be asked to bend over while sitting.

Once the doctor locates the appropriate area in the lumbar spine, the region is cleansed and numbed with a local anesthetic. Then a long needle is inserted between two of the vertebrae until it reaches the space just outside the membrane that surrounds the cerebrospinal fluid, the dura. It then pierces the dura and reaches the cerebrospinal fluid. Then, a tap is inserted to draw the fluid. Fluid is collected into vials, which are sent to a laboratory for analysis. The procedure is finished by placing a bandage at the injection site.

The patient is asked not to bathe for 24 hours.

The lab will analyze the fluid and look for several different things. Presence of red blood cells may indicate a hemorrhage around the brain or spinal cord. White blood cells and or certain proteins may be an indication of a tumor or an infection, such as meningitis.