Influenza affects up to 40% of healthy children each year and carries with it risks of respiratory tract complications, hospitalization and, rarely, death.1 Infants and young children are at increased risk for complications and hospitalization, as are children of any age who have underlying medical problems, especially those with significant cardiopulmonary disease. Timely diagnosis of influenza permits early initiation of specific antiviral treatment as well as prophylaxis of unimmunized, high-risk contacts. Accurate diagnosis of influenza may also help both clinician and patient avoid unnecessary evaluation and treatment for other causes of illness.
However, because the signs and symptoms of influenza are not specific and often resemble those caused by other respiratory viruses, diagnosing it on the basis of clinical findings can be challenging. In one recent study, the clinical diagnosis of influenza in children was correct only about one third of the time. Diagnostic accuracy was especially low in children younger than 3 years; it was also particularly low early and late in the "flu season," when influenza was less common in the community.2 Because clinical diagnosis is often inaccurate (and because the timing of each year's epidemic varies, making determinations of "early in the season" difficult), clinicians sometimes order a laboratory test to confirm a clinical suspicion of influenza.
Studies that can confirm a diagnosis of influenza include rapid antigen testing, immunofluorescent antigen testing, enzyme immunoassay, reverse transcriptase polymerase chain reaction, viral culture, and serology.1 In the setting of a pediatric office or clinic, rapid antigen tests are most commonly used because they are quick (results in less than 30 minutes) and practical.
In this article, we examine how the accuracy of these rapid tests is affected by the prevalence of influenza in the community at the time of testing. We also present a practical approach to determining whether ordering a rapid antigen test is a reasonable option, based on both disease prevalence and clinical suspicion.
HOW PREDICTIVE VALUES VARY
Several rapid antigen detection kits for influenza are commercially available; each has its own sensitivity and specificity. Test sensitivity refers to the proportion of patients with influenza who will have a positive result on a rapid test, and test specificity refers to the proportion of patients without influenza who will have a negative result on a rapid test.
Rapid antigen detection tests for influenza typically have a sensitivity of 70% or greater and a specificity of 90% or greater.3 These numbers sound fairly reassuring, but clinicians are typically more concerned with a test's predictive values. The positive predictive value is the proportion of patients with a positive result on a rapid test who actually have influenza, and the negative predictive value is the proportion of patients with a negative result on a rapid test who do not have influenza. The predictive values depend on the test's sensitivity and specificity, but also on the frequency of disease in the community. For this reason, the positive and negative predictive values of rapid influenza tests depend on the time during influenza season when the testing is done.
The Advisory Committee on Immunization Practices (ACIP) of the CDC has advised physicians who order rapid influenza tests to consider the current level of influenza activity in their community when interpreting the results of these tests.1,4 However, a busy pediatrician may not have time to calculate the predictive values of positive and negative test results at different frequencies of disease in the community. To help place rapid influenza test results in a useful clinical context, we have calculated the positive and negative predictive values of rapid influenza test results at several points during a hypothetical influenza season.
