Diagnosis of influenza
Influenza is caused by influenza A and B viruses. The annual influenza season peaks during the winter months in temperate climates, although cases occur throughout the year (eg in tropical regions1, infections acquired overseas).
The influenza virus is spread through droplets and contact with fomites (virus-contaminated objects). Infection with avian influenza should be considered if there are epidemiological risk factors, particularly returned travellers from an area with an outbreak or patients who have had recent contact with poultry.
During periods of high influenza activity (eg influenza season), influenza can usually be diagnosed based on clinical features because the probability of influenza is high. Clinical features includeWorld Health Organization (WHO), 2024:
- sudden onset of cough, headache or muscle and joint pain
- fever
- severe malaise
- sore throat
- rhinorrhoea.
Cough and fever are the most indicative features of influenza but are nonspecific; influenza may be clinically indistinguishable from other respiratory viral infections (eg coronavirus disease 2019 [COVID-19]).
Do not rule out influenza in patients with a current history of seasonal influenza vaccination.
Most influenza infections are uncomplicated and self-limiting, typically lasting less than 7 daysWorld Health Organization (WHO), 2024. However, complicated illness (eg sepsis, pneumonia, acute respiratory distress syndrome [ARDS]) or atypical presentations (eg neurological manifestations, myositis) without significant fever or respiratory features can occur.
Consider diagnostic testing when the results may either change management or inform decisions about infection prevention and control. Diagnostic testing is recommended for symptomatic patients who are admitted to hospital for management, and for symptomatic patients with immune compromise.
If diagnostic testing for influenza A and B viruses is performed, nucleic acid amplification testing (NAAT) (eg polymerase chain reaction [PCR]) is preferred.
- NAAT (eg PCR) of a nose and throat or nasopharyngeal swab have high sensitivity and specificity; multiplex PCR testing panels of respiratory pathogens (including severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) are useful for differential diagnosis2.
- In comparison to NAAT, rapid antigen tests (RATs) are specific but less sensitive. For patients who are admitted to hospital for management, and those with immune compromise, if influenza is not detected on RAT, perform NAAT if available.
Bacterial infections (eg Streptococcus pneumoniae or Staphylococcus aureus infection) are well-recognised complications of influenza and can be difficult to distinguish from influenzaHealth Protection Agency (HPA), 2011Klein, 2016Sarda, 2019. Coinfection with SARS-CoV-2 may be associated with increased mortalityAdams, 2022Stowe, 2021Swets, 2022.
For detailed recommendations about diagnosis of influenza and avian influenza, see the Communicable Diseases Network Australia (CDNA) guidelines on influenza.