Rapid Testing Only First Step in Diagnosis of H1N1 Influenza

According to the CDC, physicians can use rapid diagnostic tests to begin evaluating patients with influenza A (H1N1) symptoms, but updated guidelines from the agency stress that results from such tests should be interpreted with caution.

Confirmation of novel H1N1 flu infection can only be made by reverse-transcription polymerase chain reaction testing or viral culture, said the CDC. The sensitivity and specificity of rapid tests for the new virus are not yet known, and the agency said it has received reports of false-positive and false-negative results.

"Rapid antigen tests are not confirmatory tests. We don't have a test for point-of-care that will give an answer to a clinician early," said Jonathan Temte, M.D., Ph.D., an associate professor in the department of family medicine at the University of Wisconsin, Madison.

Temte said testing during the initial phase of an outbreak is more about surveillance than diagnosis because it can take several days for state or CDC labs to complete their own tests. The CDC said May 4 that is has distributed test kits to every state, which should allow for more rapid diagnosis at the state level.

The CDC also has updated its interim guidance for clinicians on identifying and caring for patients with novel influenza H1N1. The guideline stresses that testing should be prioritized for those with severe respiratory illness and those at highest risk of complications from influenza.

In areas with many confirmed cases of novel H1N1 infection and where the virus is spreading, the CDC said patients who test positive on a rapid diagnostic test may be treated with antiviral medications if clinically indicated.

A negative result from a rapid test, however, does not mean that the H1N1 influenza virus can be excluded. According to the CDC, if a patient has an epidemiologic link to a confirmed case or has traveled to or resides in a community where there are one or more confirmed novel H1N1 cases, further testing and treatment should be based on clinical suspicion, severity of illness and risk for complications. If there is no epidemiologic link and the patient has mild illness, further testing and treatment are not recommended.

If a patient tests positive for influenza A by rapid test, the CDC said that in areas with no or few confirmed cases of novel H1N1 flu, a nasopharyngeal swab/aspirate or nasal aspirate should be collected. The swab should be sent to the state public health laboratory to determine if the patient has H1N1 infection, seasonal influenza A virus infection or a false-positive test result.

Meanwhile, HHS Secretary Kathleen Sebelius said May 5 that the CDC is rescinding its guidance for school closings based on the belief that the virus is not as severe as originally feared.

Federal officials had recommended that schools with a confirmed H1N1 case close for as long as 14 days, and more than 400 schools had closed. However, said Sebelius during a May 5 news conference, "This virus does not seem to be as severe as we once thought it could be." She noted that HHS is "urging parents to take steps -- if your child is sick, please do not send your child to school," and don't send children to the mall, either, she added.

Despite encouraging signs that the situation might not be as severe as once feared, Richard Besser, acting director of the CDC, stressed that individuals should continue to take personal responsibility for precautions, such as hand-washing and staying home from work or school if they have flu-like symptoms.

Elsewhere, the World Health Organization reported May 5 that there were 1,516 laboratory-confirmed cases of novel H1N1 infection in 22 countries, including 822 cases and 29 deaths in Mexico. Besser said May 5 that public health officials in Mexico have indicated that the outbreak appears to have leveled off in that country.