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Case Study: Influenza
Shira Doron, MD, Assistant Professor, Division of Infectious Diseases, Associate Hospital Epidemiologist, Antimicrobial Management, Tufts Medical Center,
Kirthana Beaulac, PharmD, Clinical Pharmacy Specialist- Infectious Diseases and Antimicrobial Stewardship, Tufts Medical Center, Boston, MA
JP is a 29 year-old female presenting to the Emergency Department with dyspnea, myalgia, and rhinorrhea. Her symptoms began approximately 1 day ago and are continuous, steadily getting worse. She is having significant nasal discharge but minimal cough. Her 4 year-old son has experienced rhinorrhea as well over the past 3 days, but is not as ill as she is. She has no significant past medical history, and takes no routine medications. She reports receiving the flu vaccine when her child first fell ill, 3 days ago. She was a smoker but quit when she became pregnant 4 years ago. Ten point review of systems was negative except for fever, lethargy, nasal discharge, shortness of breath, and muscle soreness.
Radiology: Chest X-ray showed patchy diffuse bilateral infiltrates suggestive of pneumonia.
JP was diagnosed with influenza. The patient was admitted to the hospital for respiratory support and started on the antiviral Tamiflu (oseltamivir). She was not started on antibiotics for bacterial pneumonia, as the patient did not demonstrate typical symptoms of bacterial pneumonia (a notable lack of cough). Also, she had a reasonable alternative explanation for her symptoms and clinical findings. She was discharged after 1.5 days of hospitalization as her ability to oxygenate improved. She completed a 5-day course of oseltamivir at home and returned to usual health within two weeks.
The diagnosis of influenza has been problematic to our healthcare system for quite some time. The gold standard for flu testing has been viral culture; however, this process can have a 3-10 day turnaround. A shell vial culture can reduce the time to results down to 48 hours, with similar accuracy to viral cultures.1 However, even if received only 48 hours after presentation, a shell vial culture for flu is of limited value to practicing clinicians, since the mainstay of treatment, oseltamivir, works best only when initiated within 48h of symptom development.2 Likewise, because the course of treatment with oseltamivir is 5 days, if a patient was empirically initiated on oseltamivir at the time shell vial culture was sent, they would have completed half their antiviral course by the time the diagnosis was confirmed or refuted. Therefore, rapid flu testing provides significant advantages over traditional diagnostics.
The rapid flu swab test is a relatively fast and accurate method for diagnosing influenza. The test takes, on average, 15-30 minutes to complete. The specificity is high, estimated at 90-95%. Therefore, a positive test yields a reliable result. However, the sensitivity is significantly lower, 50-70%, necessitating a confirmatory viral culture for negative rapid flu swabs.3,4
Most commercially available rapid flu tests can differentiate influenza A from influenza B.3 This is beneficial for the purpose of treatment decisions, since the M2 inhibitors amantadine and rimantidine are effective only against influenza A, and because in a given year the commonly circulating strain of influenza A or B may be known to be resistant to one or the other class of antiviral. Rapid flu tests cannot identify virus subtypes, which makes them less useful for influenza epidemiologic surveillance.
Commercially available rapid flu testing kits cost between $10-$25, including supplies and reagents.5 This cost is offset by the wholesale cost of oseltamivir, which is currently approximately $14 per 75 mg capsule.6 The typical treatment regimen for adults with influenza is oseltamivir 75 mg twice daily for 5 days.2 Often, patients at high suspicion of influenza will be empirically initiated on therapy at the time of presentation. A positive rapid flu test can reassure providers that concomitant antibiotics are unnecessary, resulting in mild drug acquisition savings but a major stewardship gain, namely minimizing the use of antibiotics when not indicated.
- Gavin PJ, Thomson RB. Review of rapid diagnostic tests for influenza. Clinical and Applied Immunology Reviews. 2003;4:151-72.
- Harper SA, Bradley JS, Englund JA, File TM, Gravenstein S, Hayden FG, et al; Expert Panel of the Infectious Diseases Society of America. Seasonal influenza in adults and children—diagnosis, treatment, chemoprophylaxis, and institutional outbreak management: clinical practice guidelines of the Infectious Diseases Society of America. Clin Infect Dis. 2009;48:1003-32.
- Centers for Disease Control and Prevention (CDC). Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests. Accessed at http://www.cdc.gov/flu/professionals/diagnosis/clinician_guidance_ridt.htm
- Chartrand C, Leeflang MM, Minion J, et al. Accuracy of Rapid Influenza Diagnostic Tests. Annals of Internal Medicine. 2012; 156: 500-511.
- Rapid diagnostic tests for influenza. Med Lett Drugs Ther. 1999;41:121-2.
- RedBook, Pharmacy's Fundamental Reference, 2010 Edition. PDR Network LLC, Montvale, NJ.
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