Diagnosis of Influenza
Flu symptoms are similar to those caused by other respiratory viruses. When the influenza virus is present in a local area, acute respiratory illness that also causes fever, muscle aches, malaise (feeling unwell), and extreme tiredness usually is the flu.
Secretions from the throat or nose can be used to make a definitive diagnosis of the flu. In these tests (e.g., nasal swab, nasopharyngeal swab, nasal wash), a sample of secretions from the nose and/or throat is taken using a cotton swab and is examined under a microscope for the presence of the influenza virus.
In some cases, cells from the sample are grown in a laboratory setting (called a viral culture). Results of a viral culture usually are available in about 3–10 days. A viral culture is necessary to determine the strain of the flu (i.e., subtype of the influenza A or B virus) and to identify other causes of the illness. Information about the strain of flu virus is used to develop the flu vaccine for the following year.
More recently, rapid tests have been developed that can detect the influenza virus in as little as 30 minutes to 24 hours. These tests can accurately detect the flu virus in more than 70 percent of cases and can identify the strain of the flu in about 90 percent of these cases.
Blood tests also can be used to detect recent influenza infection. A small amount of blood usually is drawn within the first week of the illness and another sample is drawn 2–4 weeks later. If the level of flu antibodies in the blood increases from one sample to the next, influenza infection is likely.