Seasonal influenza occurs in outbreaks mainly from December through March in the northern hemisphere and between May and September in the southern hemisphere. Knowledge of local community disease activity is important when assessing the likelihood that a patient has influenza. The Centers for Disease Control and Prevention (CDC) publishes a weekly influenza surveillance report for the US. CDC: FluView - weekly influenza surveillance report external link opens in a new window

Patients at high risk of developing complications, including those with a history of chronic lung, heart, or renal disease, infants and young children, and older adults, require special attention.[13] Testing for influenza should be done if it will influence the decision to begin antiviral therapy, to order additional diagnostic tests, to institute infection control measures, and for community surveillance of influenza circulation.[76]

Diagnosis is usually made clinically during an outbreak in the community, although testing assays are available in the form of culture, rapid diagnostic tests, and serology. Diagnostic testing may be important to establish the diagnosis quickly, as in hospitalized patients with acute onset of severe pulmonary disease.

History and exam

Seasonal influenza presents most commonly as an acute respiratory illness during the winter season. After an incubation period of approximately 2 days, there is an abrupt onset of high fever, chills, headache, and myalgia. These systemic symptoms may be associated with upper and lower respiratory tract symptoms similar to a common cold, such as cough and sore throat.[77] Viral shedding in influenza peaks within 48 hours of the illness, and most uncomplicated cases resolve within 1 week.[78] Influenza does not present commonly with primary gastrointestinal symptoms such as nausea and vomiting, except in the pediatric population. Diarrhea is rare with influenza and would suggest a viral gastroenteritis, commonly referred to as stomach flu.

During a known influenza outbreak, any person with acute fever and respiratory symptoms should be considered to possibly have influenza. However, if the person has been exposed to influenza or a situation where influenza may be spread quickly (e.g., international travel, cruise ships), the diagnosis of influenza should be considered at any time of the year. 

Although there are no clear pathognomonic features of influenza, it affects the upper and lower respiratory tract in association with systemic symptoms. Fever, headache, myalgia, and fatigue are often associated with upper respiratory symptoms such as sore throat and lower respiratory symptoms of cough. Not all patients with influenza exhibit these symptoms, and those that do may not always have influenza. Overall, up to 85% of patients with influenza will exhibit clinical symptoms of influenza illness. Manifestations of influenza infection also depend on patient age and previous history of immunization.[79][77]

With sporadic cases of influenza, it may be difficult to differentiate influenza clinically from infections caused by other respiratory viruses. In this scenario, influenza virus infection may account for only a small number of such cases. In a review of 497 episodes of upper respiratory infection in older patients living in the community during the winters of 1992 to 1994, a pathogen was identified in 43% of the cases. The most common pathogens were rhinoviruses (52%) and coronaviruses (26%); influenza A or B accounted for only 10%.[80]

Clinical findings are helpful, but do not confirm or exclude the diagnosis of influenza.[77] Examination may yield nonspecific findings, since physical findings generally are few in cases of uncomplicated influenza. The patient may appear hot and flushed, and the oropharynx may demonstrate hyperemia, with complaints of severe sore throat. Mild cervical lymphadenopathy may be present and is more frequent in younger patients.

Laboratory testing

The role of laboratory testing is to reduce the inappropriate use of antibiotics and to provide the option of using antiviral therapy. Diagnostic testing, in conjunction with surveillance, can also identify the predominant circulating types, subtypes, and strains of influenza.[76]

  • Outpatient testing should be considered for any person who is at high risk of developing complications of influenza and who presents with an acute (up to 5 days) febrile illness.

  • Outpatient immunocompromised people, older adults, and infants and children with febrile respiratory illness of any duration should be screened for influenza during an outbreak.

  • Hospitalized patients with fever or who develop fever during hospitalization for respiratory infection should be screened for influenza.

Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, PCR, and immunofluorescence assays.[76][81][82][83]

Nasopharyngeal specimens are recommended for a respiratory specimen for viral isolation. They are more effective than throat swab specimens. Viral culture remains the definitive test, despite the availability of rapid diagnostic tests. It is not often used for initial clinical management as results may take up to 72 hours to be reported. Rather, it is used for confirming screening tests and for public health surveillance. Only culture isolates can provide specific information regarding circulating strains and subtypes of influenza viruses. Virus isolates also may provide information about the emergence of antiviral resistance and the development of novel influenza A subtypes that may potentially cause a pandemic.

In the outpatient setting a nasal swab, wash, or aspirate should be collected within the first 4 days of illness. Rapid influenza tests provide results within 30 minutes or less; viral culture provides results in 3 to 10 days. Rapid tests are approximately 70% sensitive and 90% specific for detecting influenza.

Routine serologic testing for influenza requires paired acute and convalescent sera. It is not recommended for accurate clinical decision-making.

During outbreaks of influenza, respiratory samples should be tested by both rapid tests and viral culture. Viral culture is essential for determining the influenza A subtypes and influenza A and B strains causing illness, and for surveillance of new strains that may need to be included in the next year's influenza vaccine. Viral culture also can help identify other causes of illness.


If a patient has an underlying chronic medical condition or falls into a high-risk category, viral or bacterial pneumonia should be considered. These patients will experience persistence of their symptoms beyond the usual time frame for resolution of uncomplicated seasonal influenza. There may be high fever, cough, and dyspnea. If there is an exacerbation of fever and cough with purulent sputum, a secondary bacterial pneumonia is most likely. A chest x-ray confirms infiltrates.

Diagnosis in children

Signs and symptoms of upper and/or lower respiratory tract involvement are common, but seasonal influenza may present more variably in children, depending on age and previous exposure.

The typical symptoms of uncomplicated seasonal influenza virus infection are still often present and include the abrupt onset of fever, headache, myalgia, and malaise associated with manifestations of respiratory tract illness, such as cough, sore throat, and rhinitis.

However, young children frequently struggle to vocalize such symptoms as myalgia and headache. They may have higher fevers than adult patients, experience febrile seizures, and have more gastrointestinal complaints (e.g., nausea and vomiting, poor appetite).[84][85] Respiratory symptoms may be less prominent in children at the onset of illness than in adolescents and adults.[84]

Clinical findings in children may include fever, tachypnea, conjunctival erythema, nasal edema and discharge, hyperemia of oropharynx, and cervical adenopathy.[86]

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