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Don't rely solely on rapid tests for H1N1, CDC warns

September 25, 2009
With the spread of the pandemic HIN1 2009 influenza virus, commonly known as "swine flu," many doctors and hospitals have increased their use of rapid influenza diagnostic tests that can determine within minutes whether you have the flu. However, these rapid tests can not distinguish seasonal influenza from the pandemic H1N1 virus and overall these rapid flu tests often don't detect pandemic H1N1 influenza very well, according to the U.S. Centers for Disease Control and Prevention (CDC).

Rapid tests are those that yield results in less than a half hour and may be done in doctors' offices or emergency rooms in addition to laboratories. More accurate viral cultures or molecular tests require complex laboratory techniques that cannot be done on site in an office setting and require more time to obtain a result, so they can't help doctors decide during office visits whether to prescribe Tamiflu or other antiviral medicines.

In its own preliminary study, detailed in the August 7 Morbidity and Mortality Weekly Report, CDC evaluated three widely used, commercially available rapid tests using 65 clinical samples collected during April and May 2009. The samples had previously tested positive for pandemic H1N1 influenza or seasonal flu virus.

The study showed that the three rapid tests detected the current new strain of H1N1 influenza well only when samples had high concentrations of the virus. The overall sensitivity, the ability to identify people with a specific disease, ranged from only 40% to 69% for detecting pandemic H1N1. The ability of these rapid diagnostic flu tests to detect seasonal flu was slightly better, with sensitivities ranging from 60% to 83%.

CDC urges health care professionals to view results of rapid tests as just one factor among many in making flu diagnoses. For now, all results from rapid tests in suspected H1N1 cases "should be interpreted in the context of circulating influenza virus strains in the patient's community, level of clinical suspicion, severity of illness, and risk for complications," an editorial note concludes.

Meanwhile, updated CDC interim guidance on use of rapid tests to detect pandemic H1N1 urges health care professionals against ruling out flu infections based on a negative rapid test result. The guidance adds that such results should neither be used to justify infected children's return to school or day care nor to exclude flu as cause of a disease outbreak in residential facilities.

Related Pages

On this site

Tests: Influenza Tests
In the News: Laboratory Tests Help Confirm Swine Flu Cases (2009)

Elsewhere on the web

Centers for Disease Control and Prevention: 2009 H1N1 Flu (Swine Flu)

Article Sources

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.

A. Balish et al. Evaluation of Rapid Influenza Diagnostic Tests for Detection of Novel Influenza A (H1N1) Virus --- United States, 2009. Morbidity and Mortality Weekly Report. 58(30); 826-829.

Interim Guidance for the Detection of Novel Influenza A Virus Using Rapid Influenza Diagnostic Tests. Centers for Disease Control. Available online at http://www.cdc.gov/h1n1flu/guidance/rapid_testing.htm through http://www.cdc.gov. Issued August 10, 2009. Accessed August 27, 2009.

David G. Fairchild. H1N1 Update: Rapid Diagnostic Tests Have Low Overall Sensitivity for Novel Influenza A Virus. Physician's First Watch. August 7, 2009.