Lumbar puncture

Role

Lumbar puncture obtains cerebrospinal fluid for diagnosis of suspected meningitis, encephalitis, subarachnoid haemorrhage, and other central nervous system pathology - a core procedural skill for infectious diseases, with cell count, protein, glucose, and specific microbiological/molecular testing on the sample driving diagnosis and treatment
One of the most frequently performed diagnostic procedures in suspected CNS infection, given CSF analysis's central diagnostic role

Safety assessment before LP is essential - clinical or imaging signs of raised intracranial pressure (focal neurological deficit, reduced consciousness, papilloedema, new seizure) should prompt CT head first to exclude a mass lesion/shift that would make LP unsafe (risk of cerebral herniation with pressure release below a mass lesion)
Do not delay empirical antibiotics for suspected bacterial meningitis while awaiting CT or LP - if imaging is required first, empirical treatment should be started immediately, since delayed antibiotic administration worsens outcomes

CSF analysis: cell count/differential (neutrophilic pleocytosis suggests bacterial, lymphocytic suggests viral/TB/fungal), protein (typically markedly elevated in bacterial meningitis), glucose (CSF:serum ratio reduced in bacterial/TB meningitis), Gram stain and culture, and PCR panels for specific organisms (HSV, enterovirus, and others depending on clinical context)
Opening pressure measurement is relevant to specific diagnoses (idiopathic intracranial hypertension, cryptococcal meningitis where pressure management itself is therapeutic)

Result pattern directly guides empirical-to-targeted antibiotic/antiviral rationalisation once organism and susceptibility are known
A traumatic tap (blood-contaminated sample from the procedure itself) can confound interpretation, particularly of red cell count when subarachnoid haemorrhage is also being considered - clinical correlation and, where needed, repeat sampling or alternative diagnostic approaches are used to resolve ambiguity

Bacterial meningitis (neutrophilic, high protein, low glucose), viral meningitis/encephalitis (lymphocytic, normal/mildly reduced glucose), raised intracranial pressure as a contraindication requiring pre-LP imaging, traumatic tap interpretation

Empirical antibiotics for suspected bacterial meningitis should never be delayed to obtain a CT head or perform the lumbar puncture - if imaging is needed first to assess safety, give antibiotics before the scan, since the mortality cost of delayed treatment substantially outweighs any diagnostic benefit of a pre-treatment CSF sample

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