LP (Lumbar Puncture)
Formal Definition
A procedure in which a needle is inserted between lumbar vertebrae into the subarachnoid space to collect cerebrospinal fluid (CSF) for analysis or to administer intrathecal medications.
How It's Used on the Ward
"We need to do a tap" or "let's LP this patient" — used to diagnose meningitis, subarachnoid hemorrhage, or measure opening pressure.
Example
""24-year-old with fever, stiff neck, and photophobia — LP stat before antibiotics. Opening pressure 28, cloudy CSF with glucose 20, protein 280, WBC 800 with 90% neutrophils. Empiric coverage started immediately.""
Clinical Context
LP should never be delayed for CT in a patient with classic meningitis symptoms unless you suspect elevated ICP or a space-occupying lesion (papilledema, focal neuro deficits, altered mentation, new seizure, immunocompromised). A normal CT does NOT fully exclude elevated ICP.
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