Sat, Aug 08, 2009 - Page 9 News List

The swine flu testing trade-off: speed and economy, or accuracy

By Andrew Pollack  /  NY TIMES NEWS SERVICE

As the swine flu spreads, many doctors and hospitals are turning to rapid tests that can determine within minutes whether an anxious patient has the flu. Sales of such tests are soaring.

But the tests have a severe limitation: They may fail more than half the time to detect swine flu infections, according to newly published studies and to experts in medical testing.

The low sensitivity of the tests is becoming a concern to health authorities because a false negative reading might prompt a doctor not to prescribe anti-flu drugs.

It is also one of the big issues that laboratory directors face as they prepare for what is expected to be a crush of flu testing this fall and winter. Numerous diagnostics companies are hoping to capitalize on demand for influenza testing.

The rapid tests “are missing a ton of flu,” said Christine Ginocchio, director of the division of microbiology, virology and molecular diagnostics at the North Shore-Long Island Jewish Health System in Lake Success, New York.

For seasonal flu, experts have long known about the low detection ability of the rapid tests. The new studies suggest the tests are no better, and possibly worse, at detecting the swine flu strain now spreading around the world, known formally as the novel H1N1 virus.

In a study published recently in the Journal of Clinical Virology, Ginocchio found that one rapid test detected only 10 percent of the swine flu infections that could be picked up by a more sophisticated laboratory culture. A different rapid test detected 40 percent. (Ginocchio is a consultant to Luminex, a company that makes a more accurate but slower test.)

The US Centers for Disease Control and Prevention (CDC) is expected to publish its own study of the rapid tests soon. Last week, it updated guidance urging doctors to be cautious in relying on the tests.

“We’re saying you need to understand the limitations of these tests,” Timothy Uyeki, an author of the CDC guidance, said in an interview. “The clinician should not base a decision to treat or not treat on the basis of a negative result.”

But some doctors say there is no good substitute for the simplicity, speed and low cost of the rapid tests. Manufacturers of the tests say the products are helpful if used appropriately.

“When these tests are used properly, the performance is very, very good,” said John Tamerius, senior vice president for clinical and regulatory affairs at Quidel, which describes itself as the leading maker of such tests.

He said the company’s QuickVue flu test could detect 80 percent of infections if nasal samples were taken correctly and if the test was given early in the course of the disease, when more virus was present.

But in a letter to the New England Journal of Medicine in June, US Navy researchers said the Quidel test missed half the swine flu infections detected by a more sensitive technique.

Spurred by flu test sales, Quidel’s revenue from infectious disease testing rose 70 percent in the second quarter from a year earlier to US$16.1 million. Among the big buyers was Mexico, which had an early swine flu outbreak.

Quidel, of San Diego, is now manufacturing tests as fast as it can. The company’s shares, which closed at US$15.28 on Wednesday, have doubled since the flu outbreak started in April.

Other rapid test makers include Inverness Medical Innovations, 3M, Thermo Fisher Scientific, Meridian Bioscience and Becton, Dickinson. With the exception of Meridian, these companies are much larger than Quidel and less dependent on flu tests.

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