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Rapid Influenza Antigen Tests: Specific But Not Sensitive

By Amesh A. Adalja, MD, FACP, March 2, 2012

During peak influenza activity, many clinicians diagnose influenza solely on clinical criteria. However, it can be difficult to distinguish influenza from other respiratory viral illnesses, and specific influenza testing can confirm clinical suspicion.

A clinical diagnosis of influenza can be confirmed with a number of different tests: PCR, viral culture, immunofluorescence, serology, and rapid antigen testing. Of these methods, rapid antigen testing is the one employed at many hospitals because of its cost, ease of use (CLIA-waived), and rapid results. Indeed, the CDC still recognizes its value, even as PCR has become the gold standard for its high degree of specificity and sensitivity.

Comprehensive Assessment of Rapid Testing

A comprehensive assessment of rapid testing has just been published in Annals of Internal Medicine by Chartrand and colleagues, who conducted a meta-analysis of the accuracy of rapid testing as compared to PCR or viral culture.

The researchers included 159 studies comparing rapid influenza testing to viral culture or PCR in their analysis. Both adults and children were included in the studies, 35% of which were from the 2009 H1N1 pandemic season. Rapid assays used in the tests included 26 different products manufactured by several different companies.

Poor Sensitivity but High Specificity of Value

The researchers found that rapid tests exhibit a low sensitivity of 62.3%. When restricted to children, who generally exhibit higher viral shedding levels, sensitivity increased to 66.6%. Additionally, rapid tests were found to be more sensitive at detecting influenza A than influenza B. When compared to viral culture, rapid tests perform marginally better than when compared to PCR, because of the latter’s superiority at detecting influenza.

In contrast to their low sensitivity, rapid tests have a high degree of specificity (>98%). That translates to a positive likelihood ratio of 34.5, which means that in a patient with symptoms consistent with flu, a positive rapid test is highly diagnostic.

The Clinical Role of Rapid Antigen Tests

The results of this analysis are not surprising. Clinicians have long known that a negative rapid test is not useful for categorically ruling out influenza, while a positive result can be tremendously useful. Given their low cost and CLIA-waived status, many healthcare facilities employ these tests and will likely continue to do so. In these settings, clinicians often disregard negative test results and treat for influenza based on symptoms and supporting lab data. However, for infection control purposes, or if the need for definitive diagnosis arises, back-up culture or PCR is performed. Once CLIA-waived rapid influenza PCR tests are developed and FDA-approved, the effect they have on clinical influenza testing, antiviral treatment decisions, and infection control procedures will be important areas for study.

Reference

Chartrand C, Leefland MMG, Minion J, et al. Accuracy of rapid influenza diagnostic tests. Ann Intern Med 2012. http://www.annals.org/content/early/2012/02/27/0003-4819-156-7-201204030-00403?aimhp