Cerebrospinal fluid findings in aseptic versus bacterial meningitis

Aseptic meningitis is often reported to be characterized by a mononuclear cell predominance in the cerebrospinal fluid (CSF), whereas bacterial meningitis is characterized by a polymorphonuclear (PMN) cell predominance. In contrast, other studies suggest that PMNs can be the most prevalent cell in early aseptic meningitis followed by a shift to mononuclear cells within 24 hours. These contradictory reports may lead to uncertainty in the diagnosis and treatment of meningitis.

To assess 1) the characteristics of the CSF differential in aseptic versus bacterial meningitis, 2) the influence of duration of illness on the CSF differential, and 3) the role of the CSF differential in discriminating between aseptic versus bacterial meningitis.

A retrospective chart review was conducted of all cases of meningitis in children >30 days of age hospitalized during the peak months for enteroviral meningitis (April to October) between 1992 to 1997. Cases of aseptic meningitis were defined as having at least 20 white blood cells/mm(3) and the absence of bacterial growth on culture. Patients were excluded if they received antibiotic therapy within the previous 5 days. Cases of bacterial meningitis were defined as having a positive culture of the CSF or the presence of a CSF pleocytosis with positive cultures of the blood. CSF variables including white blood cell differential and time from the onset of symptoms to the performance of a lumbar puncture were analyzed. PMNs were considered to be predominant when the percentage of neutrophils added to juvenile forms was >50% of cells.

One hundred fifty-eight cases of meningitis were reviewed: 138 were aseptic and 20 were bacterial. The patients ranged in age from 30 days to 18 years; 61% were male. Fifty-seven percent of cases of aseptic meningitis had a PMN predominance. The percentage of PMNs in the CSF in patients with aseptic meningitis was not statistically different for patients who had a lumbar puncture performed either within or beyond 24 hours of the onset of symptoms. Fifty-one percent of the 53 patients with aseptic meningitis and duration of illness >24 hours had a PMN predominance. The ability of a PMN predominance to differentiate between aseptic and bacterial meningitis was assessed. The sensitivity of a PMN predominance for aseptic meningitis is 57% whereas the specificity is 10%. The positive predictive value of a PMN predominance for aseptic disease is 81% but the negative predictive value is 3%. Alternative definitions of PMN predominance from 60% to 90% were not useful as a clinical indicator of bacterial disease.

The majority of children with aseptic meningitis have a PMN predominance in the CSF. The PMN predominance is not limited to the first 24 hours of illness. Because the majority of children with a PMN predominance during enteroviral season will have aseptic disease, a PMN predominance as a sole criterion does not discriminate between aseptic and bacterial meningitis.