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 experts in medical testing.

Viral check: Doctors examine a person for swine flu infection at Ram Manohar Lohia Hospital in New Delhi. Manish Swarup / AP

It is also one of the big issues 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 C. 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.

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

The US Centers for Disease Control and Prevention 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.

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 D. Tamerius, senior vice-president for clinical and regulatory affairs at Quidel Corp., which describes itself as the leading maker of such tests.

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

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

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

Quidel estimates that about 8 million rapid flu tests in total were sold in the US in the 2007-08 flu season. The number is likely to jump this year. More accurate tests are available but they generally require sophisticated laboratories. And results might not come for a day or more, making the tests of little use in deciding whether to prescribe drugs like Tamiflu, which are supposed to be started within 48 hours of the appearance of symptoms.

The rapid tests, by contrast, take only a few minutes to half an hour, and most can be done in the doctor’s office or emergency room, without a laboratory. That is appealing to some doctors.