For a spinal tap (lumbar puncture), you lie on your side with your knees drawn up toward your chest. This position helps widen the spaces between the bones of the lower spine so that the needle can be inserted more easily. A numbing medicine (local anesthetic) is put in the skin. Then a long, thin needle is put in the spinal canal to collect a sample of cerebrospinal fluid (CSF). Your doctor may need to move to another area of your spine if it is hard to get to the spinal fluid.

The colour, blood cell counts, and amounts of protein, glucose, and other substances are measured in the CSF sample. Some of the sample may be added to a substance that promotes the growth of germs (such as bacteria or a fungus). This is called a culture. The pressure of the CSF also is measured during the procedure.