Brain and spinal cord are bathed in fluid. A spinal tap, also called a lumbar puncture, is a procedure doctors use to remove and test some of this liquid, called cerebrospinal fluid (CSF). It helps them diagnose disorders of the brain and spinal cord, including multiple sclerosis.The results of the procedure can help doctors see whether your body’s immune system is attacking itself, which is what happens in multiple sclerosis. If you have the condition, your CSF (spinal fluid) will have higher amounts of certain proteins. If someone’s CSF doesn’t have these proteins, though, they might still have multiple sclerosis -- 5% to 10% of people with the condition never show signs in their spinal fluid. Also, these signs can show up in a number of other diseases, too. So a spinal tap by itself can't confirm or rule out a diagnosis of multiple sclerosis. It must be part of the total picture of testing for the disease.To start, you’ll lie on your side with your knees drawn as close to your chest as possible. Or you'll sit with your arms and head resting on a table. After the skin around your lower back is cleansed and covered, you’ll get medicine to numb that part of your body. Your doctor will put a long, thin hollow needle in your low back between two bones in your lower spine and into the space filled with CSF. He’ll take 1-2 tablespoons of fluid and remove the needle. The procedure doesn’t touch your spinal cord.