--is also known as a spinal tap, during which a needle is inserted into the lower spinal column and a small amount of cerebrospinal fluid is extracted. It is then examined for blood, autoimmune markers, and infection.

It's not a pleasant procedure, but it doesn't have to be really bad, either. It often depends on the skill of the performer, and how easy it is to find your spinal canal, given your individual anatomy.

You will be told to lie flat and unmoving for a several hour period of time afterwards, in order to avoid headache that often results from the reduced spinal fluid pressure after extraction.