Study Finds Rapid Flu Tests Fairly Accurate, but Should Be Done Soon After Symptoms Appear

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December 12, 2012

The Food and Drug Administration (FDA) has approved 11 rapid influenza diagnostic tests (RIDTs) that are currently used in doctors' offices and emergency rooms to confirm influenza ("the flu"), but until recently there was no comprehensive evaluation of their analytical performance. Accurate diagnosis of influenza is important for patient management, prevention of health care-associated infections as well as public health efforts to avoid the spread of infection in a community.

Influenza is a viral respiratory infection that can be spread from person to person through coughing, sneezing, and contact with contaminated surfaces. Symptoms, such as headache, fever, chills, muscle pains, exhaustion, congestion, sore throat, and cough, tend to be more severe and longer-lasting than those caused by the common cold and can lead to serious complications, especially among the very young, the elderly, pregnant women, and those with compromised immune systems or pre-existing lung disease. Doctors often use RIDTs to differentiate between the flu and other respiratory illnesses. If influenza is detected, there are effective antiviral medicines that can be prescribed to shorten the length and severity of flu symptoms.

A recent study in Morbidity and Mortality Weekly Report (MMWR), published by the Centers for Disease Control and Prevention (CDC), evaluated all of the RIDTs on the market during the 2011-2012 flu season. The tests were performed on 23 different strains of flu viruses, including pandemic and seasonal strains. RIDTs use samples from patients that doctors take with a nasopharyngeal (NP) swab or by aspirating nasal secretions, depending on the test they use. The tests use antibodies to the influenza virus to detect antigens in the patient's sample. A positive test for flu antigen means that the patient is infected with influenza. Test results are usually available in 15-30 minutes.

The study was conducted by researchers at the CDC, the Medical College of Wisconsin, and the Biological Advanced Research and Development Authority of the U.S. Department of Health and Human Services. The investigators used identical viral concentrations for each of the 23 viruses at 5 different concentration levels. They found that the rapid tests were generally very effective when the viral concentrations in the samples were high, but less effective when the concentrations were lower.

These results led the researchers to advise that health care providers should be aware of this variability when interpreting negative results on RIDTs. A negative result could mean the person does not have the flu, but it could also mean that the sample was inadequate for the virus to be detected. Therefore, it is critical that samples are collected when the virus is at its peak in a patient – within 24 to 72 hours of symptoms beginning – and that health professionals conducting the tests use best practices and the appropriate method for sample collection.

Dr. Daniel Jernigan, MD, MPH, who is deputy director of the Influenza Division in the National Center for Immunization and Respiratory Diseases at CDC, told Lab Tests Online, "We're encouraged that a number of new tests are in development and that a number of manufacturers are looking to develop greater accuracy," adds Dr. Jernigan. "We hope the MMWR study will help prompt better tests and better practice."

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Evaluation of 11 Commercially Available Rapid Influenza Diagnostic Tests – United States, 2011-2012. MMWR, November 2, 2012 / 61(43);873-876. Available online at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6143a3.htm?s_cid=mm6143a3_w through http://www.cdc.gov. Accessed November 2012.

Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/flu/professionals/diagnosis/clinician_guidance_ridt.htm through http://www.cdc.gov. Accessed November 2012.

Strategies for Improving Rapid Influenza Testing in Ambulatory Settings. The Joint Commission. Available online at http://www.jointcommission.org/siras.aspx through http://www.jointcommission.org. Accessed November 2012.

Phone interview with Daniel Jernigan, MD, MPH. Deputy Director, Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention. November 21, 2012.