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A diagnosis of acute bacterial meningitis is made by obtaining cerebrospinal fluid by lumbar puncture. Cerebrospinal fluid should be obtained whenever meningitis is suspected.

Lumbar puncture is generally a safe procedure, but there are recognized contraindications: signs of raised ICP, infection of the skin at the lumbar puncture site, the presence of bleeding diatheses, and cardiovascular compromise.

In considering the risks of lumbar puncture in patients with bacterial meningitis, particularly those with suspected meningococcal disease, additional factors should be considered.

Patients with meningococcal disease are often in compensated septic shock. Performing a lumbar puncture may cause an increase in respiratory and cardiovascular workload due to stress and patient positioning, and may result in acute clinical deterioration. In addition, the airway may become obstructed and respiratory function may be compromised by limiting chest expansion. The patient position may also interfere with venous return and thus limit cardiac output. Patients with meningococcal disease are likely to have a coagulopathy, which would be another relative contraindication to lumbar puncture. They may have raised ICP as a consequence of meningitis and reduced cerebral perfusion due to shock. Lumbar puncture in these circumstances may result in acute brainstem herniation (C§,rtwright..1995).

We recommend that lumbar puncture is contraindicated in patients with suspected meningococcal disease until acute cardiovascular compromise has been adequately treated. This would also be true for any patient with signs of shock.

If any features of raised ICP are present, the risk of cerebral herniation following lumbar puncture is considerable and the procedure should be deferred.

In bacterial meningitis the cerebrospinal fluid white cell count is usually over 1000/pl, with a neutrophil predominance. About 10 per cent of patients will have lymphocytic cerebrospinal fluid, particularly in listerial meningitis.

Reduced cerebrospinal fluid glucose concentration (less than 60 per cent of serum glucose measured simultaneously) is found in 70 per cent of cases. The cerebrospinal fluid protein concentration is elevated in almost all cases of bacterial meningitis.

Nearly all cases of bacterial meningitis as compared with viral meningitis have a combination of cerebrospinal fluid glucose below 2 mmol/l, a ratio of cerebrospinal fluid to serum glucose below 0.23, cerebrospinal fluid protein above 2.2 g/l, and cerebrospinal fluid white cell count above 2000/pl or 1180 neutrophils/pl. However, any one of these should raise the suspicion of bacterial meningitis, even if other cerebrospinal fluid results are atypical.

The cerebrospinal fluid Gram stain is positive in 60 to 90 per cent of cases and is extremely specific. However, the sensitivity is only 40 to 60 per cent and depends on the observer, the time spent inspecting the specimen, and whether the patient has had recent antibiotic therapy.

Cerebrospinal fluid culture is positive in up to 85 per cent of patients, but this falls to below 50 per cent in those who have received prior antibiotic therapy.

In patients with cerebrospinal fluid findings typical of bacterial meningitis, but where the Gram stain is negative, latex agglutination testing of the cerebrospinal fluid for bacterial antigen improves identification of the causal organism. Kits are available for the detection of Hemophilus influenzae type b (Hib), Streptococcus pneumoniae, Neisseria meningitidis, Escherichia coli K1, and group B streptococci. The sensitivity of the test varies from 50 to 100 per cent depending on the organism, but the specificity is high.

The polymerase chain reaction has recently been introduced for diagnosis of meningococcal meningitis and will become increasingly useful once it comes into widespread clinical usage (Tunkel..§nd,.„S,chield 1995).

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