A lumbar puncture for cerebrospinal fluid is usually performed to diagnose which condition?

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Sampling cerebrospinal fluid by lumbar puncture is used to diagnose meningitis because it provides a direct look at the fluid bathing the brain and spinal cord. Analyzing CSF lets us detect infectious organisms (via Gram stain, culture, or PCR) and characterize the inflammatory response, which helps distinguish bacterial from viral meningitis and guides treatment. CSF findings in meningitis typically include an elevated white blood cell count (neutrophils in bacterial meningitis; lymphocytes in viral meningitis), low glucose, and high protein, often with an elevated opening pressure.

Stroke is usually diagnosed with brain imaging such as CT or MRI; brain tumors are identified by imaging and biopsy rather than CSF analysis, and lumbar puncture is not a primary diagnostic tool for tumors and can even be risky if intracranial pressure is high. Thus, the lumbar puncture is most informative for diagnosing meningitis.