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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 Universi
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