A lumbar puncture also known as a spinal tap, is done to obtain samples of the clear, colorless fluid that surrounds the brain and spinal cord (cerebrospinal fluid, or CSF) to diagnose any disease or injury that affects the brain, spinal cord, or their protective membranes such as meningitis, hemorrhage, cancer, inflammation etc. This test is usually done to evaluate the causes of numbness or paralysis, stiff neck with a high fever, delirium, or sudden onset of severe headache. This test is also done to inject anesthetics or medications in to the cerebrospinal fluid, to decrease the pressure in the brain caused by an excess of CSF and to inject a dye that is used to produce X-ray images of the spinal cord.

Care before the procedure
Before you have a lumbar puncture, inform your doctor if you are taking any medications especially blood thinning medications, allergic to any medications or anesthetics, have had bleeding problems and if you are or might be pregnant. A detailed neurological exam should be done before performing the test in people who have bleeding disorders and are suspected of any illness of the brain.

How the test is done
The procedure involves inserting a needle into the spinal cord to obtain samples of the cerebrospinal fluid. The person is made to bend over while sitting in a chair or lie down on one side with knees flexed toward the chest. These positions help to separate the vertebrae of the lower back. The area where the needle is to be inserted is cleaned and medication is injected to provide local anesthesia. Then the spinal needle is inserted and the fluid is collected or drugs injected. The pressure of the fluid is also measured during the test. The procedure usually lasts 15 to 20 minutes.

Samples are analyzed for colour, blood cell counts, protein, glucose and other substances. Samples are also placed under conditions that promote the growth of infectious organisms, such as bacteria or fungi, to check for infection of the CSF.

After the test, a dressing is applied on the puncture site and the person may be instructed to lie flat up to 12 hours to avoid the post-procedure spinal headache. The person is also advised to drink plenty of fluids to help the body replenish the cerebral spinal fluid. He has to report any numbness and tingling to the legs, bleeding or oozing from the puncture site, or headaches that fail to improve to the doctor.

Risks
The main risk from a lumbar puncture is a spinal headache which usually last for few hours. These headaches can be avoided by lying flat for several hours and drinking plenty of fluids. Spinal headaches occur due to a change in cerebral spinal fluid pressure as a result of removing a sample for testing. Severe cases may require pain medication or another special procedure to repair the spinal tap site.

The test can be dangerous causing severe injury and even possible death, for people who have increased pressure within the brain, bleeding disorders or infection in the brain. A detailed neurological exam should be done before performing the test in such cases.