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Diagnosis of Flu
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Diagnosis is usually clinical, that is, based on symptoms the person has which are compatible with flu virus infection, or based on coincidence with the flu season or epidemiological data, such as known contact with people infected by the flu virus.
Diagnostic tests are not necessary for most people who have the flu, since their results will not change or modify the treatment, which generally consists of hydration, rest and measures to control fever and other symptoms.
Those at risk of developing complications from influenza (e.g. pregnant women, children under 5 years of age and those over 65 years of age, with chronic diseases, such as asthma, heart or lung disease, diabetes or immune system deficiencies) may be given diagnostic tests (PCR or rapid antigen tests). This is to establish a safe diagnosis, rule out other diseases such as COVID-19 or RSV infection, and to consider starting antiviral treatment to reduce symptoms and the severity or risk of serious or fatal complications.
Diagnostic tests for Flu
Diagnostic tests for the influenza virus are carried out on respiratory samples which should be collected at the earliest opportunity. These samples include:
Nasopharyngeal swab. Samples are collected from the throat and nose area. The sampling process is harmless, with only a slight discomfort or urge to sneeze experienced.
Nasopharyngeal washes. These are generally used in young children because they are straightforward and easy to do. A small amount of sterile saline solution is administered into one nostril with a syringe while the other is covered. Suction is then applied to recover the saline solution along with nasal secretions.
Bronchoalveolar lavage. This is performed on patients admitted to the ICU.
Several diagnostic tests can be used to identify the type of influenza virus:
Viral isolation. This is not usually performed. This involves cultivating the respiratory sample and isolating the virus.
Detection of viral proteins. These tests entail binding antibodies with proteins on the virus. If viral proteins are detected in the respiratory sample, it confirms the patient has the virus.
Detection of viral genetic material. This is the most common. It is a PCR test that extracts the genetic material of the virus from respiratory samples. The RNA of the virus is isolated and, if found in the sample, the presence of the virus is confirmed.
During the flu season and due to the number of patients in emergency services, rapid antigen detection tests with results in 30 minutes are performed. This technique only differentiates Influenza virus types A and B. Its limitation is its low sensitivity and it must be interpreted along with the patient's clinical data.
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Substantiated information by:
Antoni Trilla
Senior Consultant Head of the Department of Preventive Medicine and Epidemiology
Gemina Santana
Nurse
Marta Aldea
Epidemiologist
Published: 10 March 2020
Updated: 10 March 2020
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