The first step in the diagnosis of influenza is clinical suspicion. You or your general practitioner may suspect flu based on your symptoms and the occurrence of the illness in the community.
At the peak of the flu season, laboratory tests will show that up to 70% of everyone suspected of having flu do in fact have it. Other respiratory viruses could, however, cause a similar clinical picture.
Influenza infection can only be confirmed by a laboratory test: throat and/or nasal swabs are taken and the virus is grown in cell culture. After the virus has multiplied in culture, it can be detected in many ways, for instance by adding red blood cells or using a fluorescence microscope.
More recently developed techniques include the use of methods to detect the presence of the genetic information (RNA) of the virus. This is called reverse-transcription polymerase chain reaction (RT-PCR).
In most cases, it isn’t necessary to test for the virus, as the result will not really change the way in which you’re treated. Flu testing may be conducted in the setting of an outbreak of a respiratory illness to determine whether the flu is the cause of the illness.
In pregnant women, or in those with weakened immune systems in whom flu is suspected, the diagnosis of flu through testing can help the treating doctor to make decisions about care.
Reviewed by Cape Town-based general practitioner, Dr Dalia Hack. March 2019.