Although still used in doctors' offices and emergency departments, "rapid influenza diagnostic tests" actually do a fairly poor job of sniffing out H1N1, a growing body of evidence shows.

Scientists reported last week in The Journal of the American Medical Association that one-third of California patients hospitalized with H1N1 flu had a negative rapid test, which looks for influenza A virus in a sample swabbed from the nose and gives results in a half-hour or less.

However, a different test that uses the more sophisticated polymerase chain reaction - or PCR - technology, which can take a single piece of DNA and generate thousands to millions of copies, confirmed they had influenza A or H1N1 - an A strain - in particular.

The PCR test gives results in six to eight hours. It can help doctors decide whether to use antiviral medications to treat sick, high-risk patients.

Because virtually all flu cases have been H1N1, doctors can be fairly sure that anyone with flulike symptoms who's positive for influenza A on the rapid or PCR tests has H1N1 flu.

At this point, says Dartmouth pediatrician Hank Bernstein, who's on the American Academy of Pediatrics' infectious diseases committee, if symptoms look like the flu, "it's H1N1 until proven otherwise, almost."

The rapid test for influenza A is highly specific, basically ignoring everything else, so positive results are truly positive. But it isn't very sensitive: It often overlooks influenza A, especially in adults, who don't shed as much virus as kids. In August, the Centers for Disease Control and Prevention said rapid tests may be worse at detecting H1N1 than seasonal flu.

"I think people are growing a little less confident when they get the results of those tests," says Stephen Baum, an infectious-disease physician at New York's Albert Einstein College of Medicine. "If you test positive, you got it. If it's negative, you may still got it."

Before the pandemic had hit her town, family practice doctor Lori Heim admitted a woman to her Laurinburg, N.C., hospital for what she thought was pneumonia. A rapid test was negative, says Heim, president of the American Academy of Family Physicians, so she assumed the patient didn't have the flu.

She moved the woman to a hospital that could give higher-level care, where a PCR test showed she did have H1N1 flu, so she might have benefited from early antiviral treatment. The experience drove home a lesson learned in her training, Heim says: Treat the patient, not the test.