Accuracy of a Flu Test
Rapid flu tests, otherwise known as influenza rapid antigen tests, offer a quick way to see if your body is harboring a particular strain of influenza virus. Performed in a clinical setting, including a doctor’s office or urgent care center, rapid flu tests are not as accurate as viral cultures grown in a laboratory to identify the precise organism causing the infection.
The U.S. Food and Drug Administratio approved more than 10 rapid influenza tests, according to the Centers for Disease Control and Prevention All are designed to identify antigens produced by the body in response to the influenza A and B viruses that cause flu symptoms in the U.S., but only some of them distinguish between those viruses.
Each rapid flu test comes with instructions about the type of specimen needed and the best way to obtain that specimen. According to Lab Tests Online, a nasal aspirate is the best way to test for the virus but most specimens come from nasopharyngeal swabs. A nasal aspirate involves pushing a small amount of sterile saline into the nose and then pulling back on the syringe to collect the mucus-laden saline. Nasopharyngeal swabs are done with a Dacron swab that’s inserted into the nostril and rotated to collect mucus.
A positive rapid flu test indicates the presence of an influenza virus. Even if a rapid test can distinguish between influenza A and influenza B viruses, however, it can’t identify a particular strain, such as the H1N1 strain of influenza A responsible for the 2009 flu epidemic.
Rapid flu tests can miss up to 30 percent of influenza infections, yet can give a false positive in people who really don’t have the flu, according to Lab Tests Online. When it comes to confirming infection with the 2009 H1N1 flu virus, rapid tests may be right only 10 percent to 70 percent of the time.
The CDC says false-positive rapid flu tests are more common at the beginning and end of flu season, when relatively few cases are in the community. False-negative tests, on the other hand, occur most often at the peak of the influenza season.
Rapid flu tests are most accurate when the specimen is collected as early as possible, but especially within four to five days of symptom onset in adults.