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.
Quidel estimates that about 8 million rapid flu tests in total were sold in the US in the 2007-2008 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.
“When parents come in with a kid with fever, they want to know what the fever is from and is there something they can do about it,” said Dorothy Levine, a pediatrician in Connecticut who uses the tests.
The rapid tests also cost only about US$10 to US$20, though some doctors might charge more, versus about US$100 for a test using a sophisticated technique called the polymerase chain reaction (PCR). While a negative result on a rapid test might not be reliable, a positive result, at least during flu season, usually does indicate a person has the flu.
The rapid tests do not tell if a patient has the swine flu. They say only if flu is present, or in some cases whether it is type A or type B influenza. The swine flu is type A, but so are many seasonal flu strains.
So more sophisticated tests, beside being used to double check a negative rapid test result, are also needed to see if a positive test result is the swine flu.
Demand by patients to know if they had swine flu overwhelmed some public health and hospital laboratories in the spring, and some experts fear a repeat in the fall.
“I definitely think there will be another crunch,” said Rosemary Humes, senior adviser for scientific affairs at the Association of Public Health Laboratories.
The CDC has developed and distributed its own test for swine flu that runs on equipment made by Life Technologies. But the agency says it would be costly and unnecessary to determine whether every flu case is swine flu.
Quest Diagnostics, the nation’s largest clinical laboratory company, recently received an emergency authorization from the Food and Drug Administration to distribute a swine flu test to hospitals and other laboratories.
Luminex and Prodesse also sell tests to hospital laboratories that detect flu, though they do not specifically test for the swine flu. Because of the pandemic, “business is extraordinary for us,” said Andrew Shrago, chief marketing officer of Prodesse, a privately held company in Waukesha, Wisconsin.
In May, the Food and Drug Administration sent warning letters to Prodesse, Luminex and Becton, Dickinson, saying they were improperly claiming their tests were for swine flu. The companies removed the offending language from their Web sites.
The tests from the CDC, Quest, Luminex and Prodesse use the PCR technique, which can amplify minute amounts of the viral genes, making those tests highly sensitive.
The rapid tests are more like pregnancy tests. They use antibodies to detect a protein from the virus. If that protein is present, a colored stripe appears on the test strip. But even a nasal sample from a patient with the flu might not contain enough of the protein to register a positive result.
DxNA, a company in St. George, Utah, says it hopes to have a one-hour PCR swine flu test approved for emergency use this winter. Enigma Diagnostics of Britain hopes to introduce a similar test in 2011.
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