Influenza is an acute respiratory illness caused by infection with influenza A and B viruses. The illness can affect both the upper and lower respiratory tract and is often accompanied by systemic signs and symptoms, such as: abrupt onset of fever, chills, non-productive cough, myalgias, headache, nasal congestion, sore throat, and fatigue. Diagnosis: Not everyone infected with influenza viruses will become symptomatic, and not everyone with the above symptoms will have influenza. This is because different viral and bacterial circulating agents cause an influenza-like illness with a clinical picture each year that is indistinguishable from influenza. Between 40% and 85% of infections with influenza result in clinical illness, depending on age and pre-existing immunity to the virus. One systematic review (search date 2004; 6 RCTs in Europe, North America, and the southern hemisphere; 7164 people) of symptoms of influenza found that, in all age groups, the likelihood of influenza was decreased by the absence of fever (OR 0.40, 95% CI 0.25 to 0.66), cough (OR 0.42, 95% CI 0.31 to 0.57), or nasal congestion (OR 0.49, 95% CI 0.42 to 0.59). It found that, in people aged 60 years or older, the probability of influenza was increased by the combination of fever, cough, and acute onset (OR 5.4, 95% CI 3.8 to 7.7); fever and cough (OR 5.0, 95% CI 3.5 to 6.9); fever alone (OR 3.8, 95% CI 2.8 to 5.0); malaise (OR 2.6, 95% CI 2.2 to 3.1); or chills (OR 2.6, 95% CI 2.0 to 3.2); the review also found that influenza was less likely if sneezing was present (OR 0.47, 95% CI 0.24 to 0.92). Although influenza is usually diagnosed clinically, genuine influenza infection can only be diagnosed with laboratory confirmation, either by culture, by serological responses, or by bedside testing. The rapid bedside diagnostic tests available on the market are mainly antigen detection immunoassays, and (unlike laboratory tests, such as culture or reverse transcription–polymerase chain reaction) can be carried out within 30 minutes. However, the results must be interpreted with caution. During times of low influenza viral circulation, the positive predictive value is low, leading to an increased proportion of false-positive results. In times of high viral circulation, the negative predictive value is low, leading to an increased proportion of false negatives. It is also impractical to test all potential influenza cases. If a good surveillance system is in place, with quick feedback, the positive predictive value of clinical diagnosis alone (based on high fever and a cough) will be similar to the bedside test (79–87%). Population: For the purpose of this review, we have included trials that assessed both influenza-like illness and influenza, which are clinically indistinguishable, in people with no comorbid conditions. Where appropriate, the applicability of data to influenza pandemics has been discussed. ...read more.
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