Definition

A procedure in which a thin, hollow needle is inserted into the lower part of the spine to collect a sample of cerebrospinal fluid (CSF). This is done by placing the needle between two bones in the spine and into the subarachnoid space (the fluid-filled space between the thin layers of tissue around the spinal cord). After the CSF sample is removed, it is checked under a microscope for signs of infection, inflammation, or diseases, such as cancer or certain nervous system conditions. A spinal tap may also be done to measure or reduce CSF pressure around the spinal cord and brain, to inject drugs, such as chemotherapy or anesthesia drugs, directly into the CSF, or to inject contrast dye for certain imaging tests. Also called LP and lumbar puncture.