On the basis of reasonable clinical suspicion, a lumbar puncture must be performed to make or exclude the diagnosis of meningitis. The WBC is not an adequate screen for meningitis. In the absence of fever, a computed tomography (CT) scan of the brain may be necessary to exclude intracranial mass lesion prior to lumbar puncture. If meningitis is strongly suspected, the CT scan should not delay antibiotic therapy. CSF should be obtained for culture and sensitivity, protein and glucose levels should be determined, and cell count and Gram stain should be rapidly performed. CSF leukocytosis with a predominance of polymorphonucleocytes, CSF protein level greater than 100 mg/mL, and CSF glucose level less than 50 percent of blood glucose level are considered positive screening test results for bacterial meningitis. The Gram stain has a 70 percent sensitivity for preliminary identification of the offending organism and should be reported to the emergency physician as soon as available. Because of the emergence of penicillin-resistant strains of S. pneumoniae, it is prudent to perform CSF latex agglutination for specific bacterial antigens on all patients with abnormal CSF to facilitate early identification of high-risk patients. If prior treatment with antibiotics has occurred, the emergency physician should have a lower threshold for performing a lumbar puncture, and rapid antigen techniques are routinely indicated as these are often essential to diagnosis.39 A bedside blood glucose level, complete blood count, electrolyte panel, and cultures of blood and urine should be obtained before performing the lumbar puncture. Indications for deferring lumbar puncture in the ED include cardiorespiratory compromise or risk of increased intracranial pressure. In such cases, antibiotic therapy should be given in the eD and lumbar puncture performed as soon as possible in the inpatient setting.

The differential diagnosis for bacterial meningitis includes the same spectrum of systemic disease as described for sepsis. Aseptic meningitis refers to evidence of meningeal inflammation with negative CSF cultures and is far more common than bacterial meningitis. Most frequently, this is due to viral meningeal infection, but other causes such as tuberculosis or syphilis should be considered. Parameningeal infection or brain abscess may rarely mimic the presentation and laboratory features of meningitis. Central nervous system (CNS) mass lesion, nonaccidental head injury, and toxic drug ingestion should also be considered in afebrile patients.

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