CSF evaluation is the single most important aspect of the laboratory diagnosis 
of meningitis. Analysis of the CSF abnormalities produced by bacterial, 
mycobacterial, and fungal infections may greatly facilitate diagnosis and direct 
initial therapy. Basic studies of CSF that should be performed in all patients 
with meningitis include measurement of pressure, cell count and white cell 
differential; determination of glucose and protein levels; Gram's stain; and 
culture. In bacterial meningitis, Limulus lysate assay and tests to identify 
bacterial antigens may allow rapid diagnosis. Where there is strong suspicion of 
tuberculous or fungal meningitis, CSF should also be submitted for acid-fast 
stain, India ink preparation, and cryptococcal antigen; unless contraindicated 
by increased intracranial pressure, large volumes (up to 40-50 mL) should be 
obtained for culture. If a history of residence in the Southwest is elicited, 
complement-fixing antibodies to Coccidioides immitis should also be ordered. 
Newer tests based on immunologic methods or gene amplification techniques hold 
great promise for diagnosis of infections caused by organisms that are difficult 
to culture or present in small numbers. Despite the great value of lumbar 
puncture in the diagnosis of meningitis, injudicious use of the procedure may 
result in death from brain herniation. Lumbar puncture should be avoided if 
focal neurologic findings suggest concomitant mass lesion, as in brain abscess, 
and lumbar puncture should be approached with great caution if meningitis is 
accompanied by evidence of significant intracranial hypertension. Institution of 
antibiotic therapy for suspected meningitis should not be delayed while 
neuroradiologic studies are obtained to exclude abscess or while measures are 
instituted to reduce intracranial pressure.