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Signs and symptoms
Symptoms of influenza,[43], with fever and cough the most common symptoms.[44]
Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills or a chilly sensation, but fever is also common early in the infection, with body temperatures ranging from 38-39 °C (approximately 100-103 °F).[45] Many people are so ill that they are confined to bed for several days, with aches and pains throughout their bodies, which are worse in their backs and legs.[1] Symptoms of influenza may include:
Body aches, especially joints and throat
Extreme coldness and fever
Fatigue
Headache
Irritated watering eyes
Reddened eyes, skin (especially face), mouth, throat and nose
In children, gastrointestinal symptoms such as diarrhoea and abdominal pain,[46][47] (may be severe in children with influenza B)[48]
It can be difficult to distinguish between the common cold and influenza in the early stages of these infections,[2] but a flu can be identified by a high fever with a sudden onset and extreme fatigue. Diarrhoea is not normally a symptom of influenza in adults,[44] although it has been seen in some human cases of the H5N1 "bird flu"[49] and can be a symptom in children.[46] The symptoms most reliably seen in influenza are shown in the table to the right.[44]
Most sensitive symptoms for diagnosing influenza[44]
Symptom:
sensitivity
specificity
Fever
68-86%
25-73%
Cough
84-98%
7-29%
Nasal congestion
68–91%
19–41%
Notes to table:
The ranges given represent different studies that were reviewed.
Sensitivity is the proportion of people having influenza who exhibit the symptom.
Specificity is the proportion of people not having influenza who do not exhibit the symptom.
All three findings, especially fever, were less sensitive in patients over 60 years of age.
Since anti-viral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. Of the symptoms listed above, the combinations of fever with cough, sore throat and/or nasal conjestion can improve diagnostic accuracy.[50] Two decision analysis studies[51][52] suggest that during local outbreaks of influenza, the prevalence will be over 70%,[52] and thus patients with any of these combinations of symptoms may be treated with neuramidase inhibitors without testing. Even in the absence of a local outbreak, treatment may be justified in the elderly during the influenza season as long as the prevalence is over 15%.[52]
The available laboratory tests for influenza continue to improve. The United States Centers for Disease Control and Prevention (CDC) maintains an up-to-date summary of available laboratory tests.[53] According to the CDC, rapid diagnostic tests have a sensitivity of 70–75% and specificity of 90–95% when compared with viral culture. These tests may be especially useful during the influenza season (prevalence=25%) but in the absence of a local outbreak, or peri-influenza season (prevalence=10%[52]).