Doctors' offices and laboratories offer several types of tests for influenza, which vary by accuracy, availability and the time required to get a result. All tests for influenza detect both influenza A and B viruses, which cause the flu. Not everyone suspected of having the flu needs an influenza test. Doctor's decide whether to order an influenza test based on the level of influenza activity in the community and the severity of the patient's illness.
Rapid Tests
Rapid tests for influenza detect the presence of influenza A and B viruses in nasal, throat and, less commonly, phlegm specimens. Designed for use in doctors' offices and medical clinics, these tests provide results in less than 15 minutes, according to the U.S. Centers for Disease Control and Prevention. Some rapid influenza tests distinguish between influenza A and B viruses but others do not. None of the rapid influenza tests provides information about the specific strain of virus present.
Rapid influenza tests are highly specific, that is, a positive test result indicates the presence of influenza viruses with a high level of certainty. A negative rapid influenza test result, however, does not rule out the possibility of influenza. A variety of factors can cause a false negative test result, including performing the test too late after the onset of flu symptoms and poor sample quality.
Direct Fluorescent Antibody Staining
Direct fluorescent antibody staining is a technique used by laboratories to detect the presence of influenza A and B viruses in nasal, throat or airway mucus specimens. Test results are typically available in two to four hours, notes the CDC. Direct fluorescent antibody staining more accurately detects the presence of influenza viruses than do the rapid influenza tests. The test also distinguishes between influenza A and B viruses.
Viral Cultures
A viral culture involves placing a nasal, throat or airway mucus specimen on special nutrient media and observing for virus growth. Although viral cultures remain one of the most accurate tests available for detecting influenza, results are usually not available for three to 10 days, reports the CDC. Viral cultures detect influenza A and B viruses and other types of viruses that cause illnesses similar to the flu.
Public health laboratories can take viral cultures from community-based laboratories and perform additional testing to determine the specific strain of the influenza virus present. This information helps public health officials track the level of activity of specific influenza virus strains in the population.
Molecular Tests
Molecular tests for influenza detect the genetic material of influenza A and B viruses using a technique called real-time polymerase chain reaction, or PCR. PCR is the most accurate method for detecting influenza viruses but a limited number of laboratories perform the test, reports Lab Tests Online. The time to receive results varies from one to several days.
References
- U.S. Centers for Disease Control and Prevention: Influenza Symptoms and Laboratory Diagnostic Procedures
- U.S. Centers for Disease Control and Prevention: Questions and Answers, Diagnosing Flu
- U.S. Centers for Disease Control and Prevention: Rapid Diagnostic Testing for Influenza
- Lab Tests Online: Influenza Tests
