The cerebrospinal fluid sample is obtained by a physician usually via lumbar puncture in the L3-L4 region.

The opening pressure is first measured (nl 90-180 mm of water in lateral position) and if it is elevated greater than 200 mm, no more than 2 ml of CSF should be withdrawn. Sterile technique is always used to reduce the risk of infection. Care must be taken to avoid injury to neural tissue.