Spinal fluid flows through a channel between the middle and inner layer of tissues (meninges) that cover the brain and spinal cord. To remove a sample of this fluid, a doctor inserts a small, hollow needle between two bones (vertebrae) in the lower spine, below the point where the spinal cord ends. Usually, people lie on their side with their knees curled to their chest. This position widens the space between the vertebrae, so that the doctor can avoid hitting the bones when the needle is inserted.
Doctors collect spinal fluid in test tubes and send it to a lab for testing.