When doctors need to find information about illnesses that a patient may be suffering from by studying the cerebrospinal fluid they perform a lumbar puncture. It is used to verify whether patients have life- threatening ailments like bacterial meningitis and subarachnoid hemorrhage. While doing this a small amount of fluid is extracted from the spine.
A lumbar puncture is never used as a tool which is performed before initial treatment is started. The patient is first put on antibiotics and steroids to counter any ailments they may have. If there is no response a neurological examination is conducted followed by the lumbar puncture.