The flu can be deadly — and deceptive. Rapid flu tests used in clinics and doctors’ offices can show that patients don’t have the flu when they actually do.
These false negatives aren’t uncommon. The Centers for Disease Control and Prevention note that rapid influenza testing has a sensitivity ranging from approximately 50% to 70%. That means that in up to half of influenza cases, the flu swab results will be negative.
“Doctors test for flu with a swab that goes up the nose,” said Jeremy Allen, MD, of American Family Care, in a statement. “A certain amount of antigen or the substance that’s linked to the virus is needed to get a positive flu result. Sometimes the sample isn’t broad enough and the flu isn’t detected.”
Insufficient specimens happen for various reasons, said Daniel Eiras, MD, assistant professor of infectious diseases at NYU Langone Health.
“You have to go pretty deep with the swab,” he told the Daily News. And because that’s uncomfortable, it can be “a challenge” for babies and children, as well as for some adults.
Another reason specimens come up short is that people have the rapid flu test too soon, said Eiras. If you go to the doctor within a day or two of having symptoms “it may be too early” to test positive.
Returning for another test if flu symptoms persist can get expensive — and isn’t actually necessary. That’s because if you come in with flu-like symptoms such as fever, chills, aches and exhaustion and test negative for flu, doctors treat you like you have it anyway.
Both the CDC and the Food and Drug Administration note that once influenza is established in a community, false negative rates of rapid influenza testing increase and as such a negative flu swab should not be used to exclude influenza as a diagnosis.