Lumbar puncture
Formal Definition
A procedure in which a needle is inserted into the subarachnoid space of the lumbar spine, typically at L3-L4 or L4-L5, to obtain cerebrospinal fluid for analysis and to measure opening pressure; used to diagnose meningitis, subarachnoid hemorrhage, MS, and other CNS conditions; performed with the patient in lateral decubitus or sitting position under aseptic technique.
How It's Used on the Ward
LP or spinal tap — sticking a needle into the lower back to get spinal fluid; the classic test for meningitis.
Example
"Suspected bacterial meningitis: LP performed at L4-L5. Opening pressure 28 cmH2O (elevated). Fluid: cloudy, WBC 2,400/μL (85% neutrophils), glucose 18 mg/dL (serum 95), protein 380 mg/dL. CSF gram stain shows gram-positive diplococci. Diagnosis: pneumococcal meningitis. Empiric vancomycin + ceftriaxone started immediately before LP results, continued based on sensitivities."
Clinical Context
Contraindications: increased ICP with risk of herniation (focal neuro deficits, papilledema, abnormal CT), coagulopathy (INR >1.5, platelets <50k), skin infection at puncture site, spinal hardware. CT before LP: altered mental status, focal neuro deficits, papilledema, new seizure. CSF analysis: WBC, RBC, glucose, protein, gram stain, culture, PCR panels. Bacterial meningitis: high WBC (neutrophil predominant), low glucose, high protein. Viral: lymphocytic, normal/slightly low glucose, mildly elevated protein. SAH: xanthochromia on supernatant (centrifuge), RBCs. Opening pressure: normal 10-20 cmH2O, >20 elevated, >30 markedly elevated (possible cryptococcal or meningeal carcinomatosis).