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Diagnosis of the flu (influenza) by a physician may involve a medical history and physical examination. The medical history will likely include questions on symptoms, including their duration, severity and progression. The physical examination will check for additional signs and symptoms.

During periods of typical flu outbreaks, the disease is often diagnosed directly from its symptoms. Because other respiratory diseases produce symptoms similar to the flu, tests may be performed to confirm the diagnosis. Tests are also likely if the flu is suspected in periods outside of typical outbreaks.

Most cases of the flu can be confirmed by culturing the virus. This is the process of growing virus particles in a laboratory from samples of material from a patient (in this case, from a patient’s respiratory mucus). These samples are usually obtained through nasal or throat swab. Viral culture may take several days but it allows the type of virus to be determined.

In addition, molecular and immunological techniques are available to confirm flu virus infection. These may test for the genetic material of the virus or antibodies produced by the body to fight the infection. Both may be performed using the same respiratory samples taken for virus culture. These techniques typically provide results faster than culture, but in general are not as sensitive and lack the specificity to determine the type of flu virus.

Throat cultures, nasal smears (analysis of nasal excretions under a microscope) and blood tests may also be performed to rule out other causes of a patient’s symptoms (e.g., strep throat, allergies).