A doctor is usually able to diagnose a case of influenza by simply observing common symptoms and noting the presence of influenza in the local community. Most people will not require tests for diagnosis.
Sometimes, however, laboratory tests are necessary for certain high-risk individuals in order to determine the specific strain of flu present in the community, or to verify that the illness isn’t caused by another pathogen that produces similar symptoms.
Influenza can be difficult to diagnose because its symptoms are similar to other infections, such as, but not limited to, parainfluenza viruses, respiratory syncytial virus, adenovirus, and others.
Laboratory tests may be helpful in determining treatment options, as well as establishing if influenza is the cause of an outbreak in a closed setting such as a hospital, nursing home, or summer camp.