Inflammation of the meninges. Diagnosis of meningitis is made from examination of a specimen of cerebrospinal fluid that is found in the subarachnoid space. The fluid sample is obtained by lumbar puncture (spinal tap) or, less commonly, by a ventricular tap, which requires passage of a needle into the brain tissue. Certain types of meningitis are associated with distinctive abnormalities in the cerebrospinal fluid. With certain types of meningitis, especially bacterial, the causative organism can usually be recovered from the fluid.