Acute phase reactants and risk of bacterial meningitis among febrile infants and children. STUDY OBJECTIVE: To test the hypothesis that quantitation of either C-reactive protein (CRP) or the total peripheral WBC count can improve clinical detection of underlying bacterial meningitis among young febrile children. DESIGN: Cross-sectional survey of selected symptoms of central nervous system infection, signs of meningeal irritation and/or elevated intracranial pressure, levels of CRP in serum, and total peripheral WBC counts among unselected pediatric patients undergoing lumbar punctures for evaluation of acute febrile illnesses. SETTING: Emergency department and acute care "walk-in" clinic of an urban, university-affiliated general hospital. PARTICIPANTS: 160 previously well, acutely febrile infants and children (median age, 6 months). RESULTS: The prevalence of bacterial meningitis was 6%. Sensitivity of symptoms was 1.00 and specificity was 0.17. Sensitivity of signs was 0.70 and specificity was 0.81. Of the acute phase reactants, sensitivity of a CRP level of more than 1.0 mg/dL was 0.80, while that of a total peripheral WBC count of more than 15,000/mm3 was 0.40. The presence of signs and/or a CRP level of more than 1.0 mg/dL correctly identified all children with bacterial meningitis (sensitivity, 1.00). The absence of signs and a CRP level of 1.0 mg/dL or less correctly identified 71 of 150 children without bacterial meningitis (specificity, 0.47). Of 125 children without meningeal signs, the combination of symptoms and a CRP level of more than 1.0 mg/dL correctly identified all three children with bacterial meningitis (sensitivity, 1.00). The absence of these symptoms and/or a CRP level of 1.0 mg/dL or less correctly identified 80 of 122 children without bacterial meningitis (specificity, 0.66). CONCLUSION: Quantitation of CRP but not the total peripheral WBC count can increase the sensitivity of physical examination findings and the specificity of symptoms for the diagnosis of bacterial meningitis. Measurement of CRP in serum is useful as an adjunct to history and physical examination for the detection of acute bacterial meningitis in the acutely febrile child. 