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Rapid Swine Flu Test Misses Many Infections

Doctors should diagnose based on symptoms and strains in circulation, experts say,,

THURSDAY, Aug. 6 (HealthDay News) -- Doctors can't rely on rapid tests to diagnose the pandemic H1N1 swine flu, say U.S health officials who evaluated three kits and found that they miss many infections.

The tests do a better job detecting seasonal flu than H1N1 flu, the U.S. Centers for Disease Control and Prevention reported. Sensitivity for the H1N1 swine flu was just 40 to 69 percent.

"These are rapid tests that the physician would do in the office while the patient is waiting," said Michael Shaw, associate director for laboratory science in CDC's influenza division and the report's co-author. "These tests can sometimes provide misleading results."

A quick flu test is just one diagnostic tool, Shaw said. "You shouldn't rely on it alone," he said. "There is no substitute for the judgment of the clinician."

Because people might test negative but actually have the flu, Shaw said, "we want to emphasis that the clinician should also go by the patient's symptoms and what they know is circulating in the community."

Positive test results are accurate, however. "But a positive result only tells you it's flu, not what kind," Shaw said. "It could be seasonal, it could be the pandemic strain." In either case, he said, doctors could start antiviral treatment with a drug such as Tamiflu.

For people at high risk for flu complications, doctors should start treatment with antiviral medication and also get a test to confirm the results, which can take 24 hours, Shaw said.

For the report, which is published in the Aug. 7 issue of the CDC's Morbidity and Mortality Weekly Report, CDC researchers tested three commercially available rapid influenza diagnostic tests that can identify influenza A or B antigens in about 15 minutes. Respiratory samples were used from 65 people known to have swine flu or seasonal flu.

The tests were able to detect the H1N1 swine flu only when a high percentage of the virus was in the respiratory sample, which means that many infections would be missed, according to the CDC.

Shaw said that people will shed the most virus shortly after symptoms start so, for the most accurate results, it's important to give the test early.

Fast flu tests have been in use for a couple of decades. "It's not news that these tests are not as sensitive as we would like them to be," Shaw said. The question for the CDC was whether they would work with the new pandemic strain, he said.

Dr. Marc Siegel, an associate professor of medicine at New York University Langone Medical Center in New York City, said that anyone currently suffering from the flu has H1N1 swine flu.

Right now, the test is not clinically important, Siegel said. "If you got flu now, this is what you got," he said.

Siegel agrees with the CDC that a flu diagnosis is best made on the basis of a person's symptoms and the flu strains in circulation.

"I make the diagnosis on clinical grounds. I am comfortable doing that," Siegel said. "The test is just an adjunct. It's helpful if it's positive, but a negative flu test does not rule it out. Go by your clinical judgment."

Getting a more definitive test, Siegel said, would have two benefits: It could identify what viruses are circulating, and it could confirm a diagnosis for high-risk patients susceptible to complications such as pneumonia.

More information

The U.S. Centers for Disease Control and Prevention has more on H1N1 swine flu.

SOURCES: Michael Shaw, Ph.D., associate director for laboratory science, influenza division, U.S. Centers for Disease Control and Prevention, Atlanta; Marc Siegel, M.D., associate professor, medicine, New York University Langone Medical Center, New York City; Aug. 7, 2009, Morbidity and Mortality Weekly Report

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