DIAGNOSIS OF ACUTE BACTERIAL MENINGITIS
The lumbar puncture is critical to the diagnosis of acute bacterial meningitis. Several case series have looked at the presence of various CSF abnormalities of the lumbar puncture in cases of acute bacterial meningitis. Opening pressure is typically greater than 18 cm H20 and can be very high when cerebral edema is present. Generally, 80% of the time, the fluid is turbid. Gram stain is positive in 50% to 60% of cases. CSF cultures are positive in 65% to 75% of cases, although less often if antibiotics precede the lumbar puncture. The CSF cell count often exceeds 1000 cells/mm3. Neutrophilic predominance, with more than 80% neutrophils, is present in 80% of cases, although there are cases of lymphocyte predominance, seen most commonly with Listeria infection. The total protein usually exceeds 45 mg/dL in more than 95% of cases and is greater than 200 mg/dL in more than 50% of cases. The CSF glucose level is less than 50 mg/dL in approximately 70% of cases. However, these initial indices can be relatively normal in immunocompromised and neutropenic patients, and excluding the diagnosis of acute bacterial menigitis may necessitate a follow-up lumbar puncture. Partial antibiotic treatment, which is defined as the administration of intravenous antibiotics within 6 hours, oral antibiotics within 12 hours, or a prior recent course of antibiotics, does not usually not have a significant effect on CSF indices. Bacterial antigen tests in the CSF have been shown to have sensitivities of less than 75% and cannot be used to rule out disease. CSF analysis, combined with blood culture data, determines the diagnosis of acute bacterial meningitis.
Gram stain and culture of the CSF results should be performed immediately after the lumbar puncture and can direct antibiotic therapy. Since blood cultures are positive in more than 50% of cases of acute bacterial meningitis, two sets of blood cultures should be drawn prior to the administration of any antibiotic therapy.
*4/348/5*
