Rapid Influenza Tests Often Fail to Detect H1N1

Doctors’ offices and hospitals are using “rapid influenza diagnostic tests” to identify the presence of the H1N1 flu in patients; however, these tests actually do a poor job of sniffing out H1N1 because the rapid test does not detect H1N1, only influenza A. A confirmatory test must be done to identify H1N1 – a strain of influenza A.

Scientists confirmed this theory recently in a report published in The Journal of the American Medical Association, which found that one-third of California patients hospitalized with H1N1 flu were given a rapid test that came back negative. A different test that uses more sophisticated technology confirmed they had H1N1.

The discrepancy is caused by the high specificity (80-90 percent) and the low sensitivity (10-70 percent) of the rapid test for influenza A. The test often does not identify influenza A, especially in adults, who don’t shed as much virus as children. Therefore, correctly identifying those with influenza A, which could be either H1N1 or seasonal flu, is the real issue with the rapid tests. Since about 90 percent of circulating influenza A is H1N1, doctors can be fairly sure that anyone with flu-like symptoms who is also positive for influenza A – either on the rapid or more advanced tests – has H1N1 flu. But, there is still a 10 percent chance that it is seasonal flu.

Regardless, many are not taking the chance that their illness could be something other H1N1. Dartmouth pediatrician Hank Bernstein, who’s on the American Academy of Pediatrics’ infectious diseases committee, said in a recent USA Today article, if symptoms look like the flu, “it’s H1N1 until proven otherwise, almost.”

Stephen Baum, an infectious-disease physician at New York’s Albert Einstein College of Medicine, in the same USA Today story, says: “If you test positive, you got it. If it’s negative, you may still got it.”

Therefore, it is important to understand the symptoms of flu versus a common cold. Those symptoms, not the test results, should be the trigger to take relevant action – either treatment or protecting others from exposure. Further, a negative test shouldn’t make one complacent in taking steps to prevent spreading the flu. Learn what you can do to keep your family healthy this winter by visiting the “Prevention and Treatment” section on the Centers for Disease Control and Prevention H1N1 website.

(Jerod M. Loeb, PhD, is Executive Vice President, Division of Quality Measurement and Research, The Joint Commission, and a member of the Water Quality & Health Council).