During the influenza season patients are labeled as having an influenza-like illness (ILI) which may be either a viral or bacterial infection. We hypothesize that C-reactive protein (CRP) levels among patients with ILI diagnosed with a bacterial infection will be higher than patients diagnosed with an influenza or another viral infection.

We enrolled a convenience sample of adults with ILI presenting to an urban academic emergency department from October to March during the 2008 to 2011 influenza seasons. Subjects had nasal aspirates for viral testing, and serum CRP. Bacterial infection was determined by positive blood cultures, radiographic evidence of pneumonia, or a discharge diagnosis of bacterial infection. Receiver operating characteristic curve, analysis of variance, and Student t test were used to analyze results.

Over 3 influenza seasons there were 131 total patients analyzed (48 influenza infection, 42 other viral infection and 41 bacterial infection). CRP values were 25.65 mg/L (95% CI, 18.88-32.41) for influenza, 18.73 mg/L (95% CI, 12.97-24.49) for viral and 135.96 mg/L (95% CI, 99.38-172.54) for bacterial. There was a significant difference between the bacterial group, and both the influenza and other viral infection groups (P < .001). The receiver operating characteristic curve for CRP as a determinant of bacterial infection had an area under the curve of 0.978, whereby a CRP value of <20 had a sensitivity of 100% and >80 had a specificity of 100%.

C-reactive protein is both a sensitive and specific marker for bacterial infection in patients presenting with ILI during the influenza season.