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Make the Diagnosis: Influenza

Prior Probability

Clinicians must rely on informed clinical judgment to determine the prior probability of influenza, which requires an understanding of the epidemiology weekly reports for influenza, available from the CDC (for the United States,, and for International surveillance,; accessed June 1, 2008) that applies to your population. For US data, the CDC reports the frequency of influenza-like illness during influenza season and whether influenza virus is present in your region. However, they do not report whether a given patient with an influenza-like illness is likely to have influenza.

Population for Whom Influenza Should Be Considered

The clinical evaluation is used to identify influenza-like illness among all patients during influenza season.

Detecting the Likelihood of Influenza

A few findings can lower the likelihood of influenza among patients (all adults or children) when they have influenza-like illness (Table 26-5).

Table 26-5.Likelihood Ratios for Findings Across All Age Groups That Lower the Probability of Influenza

For older adults, the presence of a few findings increases the likelihood of influenza, whereas the presence of 1 finding (sneezing) makes influenza a little less likely (Table 26-6).

Table 26-6.Likelihood Ratios for Findings That Suggest Influenza in Older Adults

During influenza season, a negative result on a commercially available rapid influenza tests greatly decreases the likelihood that a patient has influenza (Table 26-7). These studies may be most useful as the rate of influenza-like illness is increasing on visits to sentinel providers and before data are available from the CDC that suggest influenza is circulating in your area. The CDC provides updated guidance on available rapid influenza tests and their role in screening for influenza (; accessed June 1, 2008). We cannot be absolutely certain that the operating characteristics of these tests will stay constant from one influenza season to the next.

Table 26-7.Likelihood Ratios for Some Rapid Influenza Tests

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