From Medscape Emergency Medicine > Ask the Experts
Testing for H1N1 Influenza in the Emergency Department
Posted: 11/05/2009
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Question
What is the best way to test for H1N1 influenza in the ED? How do I decide which admitted patients need isolation?
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Response from James Wilde, MD
Associate Professor of Emergency Medicine & Pediatrics, Department of Emergency Medicine; Faculty, Pediatric Emergency Medicine, Medical College of Georgia, Augusta, Georgia |
Rapid tests currently on the market are designed to detect influenza type A, type B, or both. Some can distinguish influenza A from influenza B, but none can distinguish novel H1N1 (swine) flu from seasonal strains of flu. New rapid tests for the specific detection of H1N1 are coming to market in the near future that have been approved for use under the Emergency Use Authorization (EUA) pathway, but they are not yet widely available.
Novel H1N1 flu is a type A influenza virus. If the rapid test is positive for influenza B, H1N1 is not likely. If the result is positive for influenza A, the patient may be infected with H1N1 or with seasonal strains of influenza A. However, current data from the US Centers for Disease Control and Prevention (CDC) indicate that as of October 3, 2009, 99% of the circulating influenza viruses in the United States are H1N1; seasonal strains of flu are virtually absent.[1] Thus, although it is not possible to confirm H1N1 infection with an influenza rapid test, a positive rapid test result for influenza A is assumed to be due to infection with H1N1.
Routine testing for H1N1 using rapid tests is not recommended by the CDC because the sensitivities of the currently available rapid tests for the detection of H1N1 are quite poor. Various studies have shown detection rates between 20% and 70%.[2] This means that the rapid test may fail to detect H1N1 in 30%-80% of cases. It is critical for physicians to be aware that a negative result on an influenza rapid test does not rule out H1N1 infection. For this reason, the CDC recommends that management of patients with suspected H1N1 infection should be based on symptoms and underlying risk factors rather than the result of a rapid flu test.[1] Outpatients with influenza-like illness (ILI) who also have risk factors for complications due to flu infection should be treated with neuraminidase inhibitors whether a rapid test result is known or not. Patients with ILI who do not have risk factors generally do not require antiviral treatment. Symptomatic and supportive treatment is recommended for all patients with ILI. Therefore, rapid tests have little to no role in the management of outpatients with ILI.
Patients who present to the emergency department or clinic with ILI and who are ill enough to be admitted to the hospital should be assumed to have H1N1 infection given the current epidemiology of the pandemic. This makes isolation decisions simple: Isolate everyone who is admitted to the hospital with ILI or complications of ILI. If confirmation of H1N1 infection is necessary, samples can be sent from the inpatient site for testing by reverse transcriptase polymerase chain reaction (RT-PCR), the most definitive method available to clinicians for the identification of H1N1. Treatment with neuraminidase inhibitors should begin presumptively because the results may take several days to return.
Physicians should also be wary of the indiscriminate use of influenza rapid tests during a period when influenza is not circulating at high levels. This is because the specificity of these assays is not as high as for culture or RT-PCR. The positive predictive value of a test for an infectious disease such as influenza depends on the specificity of the test and on the prevalence of the disease in the population tested. If influenza prevalence is high, the positive predictive value of a rapid test is increased, and thus a positive test is more likely to represent a true positive. However, during periods when influenza activity is low, such as during most summer months, a positive result on a rapid test is much more likely to represent a false positive. During periods of low prevalence, physicians who require a definitive diagnosis should order tests with high levels of specificity, such as culture or RT-PCR, because false-positive results are significantly less likely.
References
- CDC. Updated interim recommendations for the use of antiviral medications in the treatment and prevention of influenza for the 2009-2010 season. October 16, 2009. Available at: http://www.cdc.gov/h1n1flu/recommendations.htm Accessed November 1, 2009.
- CDC. 2008-2009 influenza season week 39 ending October 3, 2009. Available at: http://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly39.htm Accessed October 23, 2009.
Authors and Disclosures
Author(s)
James Wilde, MD
Associate Professor of Emergency Medicine & Pediatrics, Department of Emergency Medicine; Faculty, Pediatric Emergency Medicine, Medical College of Georgia, Augusta, Georgia
Disclosure: James Wilde, MD, has disclosed no relevant financial relationships.
Medscape Emergency Medicine © 2009 Medscape, LLC
