Lumbar puncture, artwork. A lumbar puncture is a diagnostic procedure used to collect a sample of cerebrospinal fluid (CSF), which surrounds the spinal cord and brain, for analysis. During the procedure the patient lies on their side with their neck bent forward and their knees pulled to their chest. A needle is then inserted between two of the lumbar (lower) vertebrae, through the outer and middle meninges (the membranes that surround the spinal cord and brain) and into the subarachnoid space that contains the CSF. A sample of the liquid is then drawn into the syringe.