Influenza Symptoms and Laboratory Diagnostic Procedures

Symptoms

Influenza illness can include any or all of these symptoms: fever, muscle aches, headache, lack of energy, dry cough, sore throat, and possibly runny nose. The fever and body aches can last 3-5 days and the cough and lack of energy may last for 2 or more weeks. Influenza can be difficult to diagnose based on clinical symptoms alone because the initial symptoms of influenza can be similar those caused by other infectious agents including, but not limited to, Mycoplasma pneumoniae, adenovirus, respiratory syncytial virus, rhinovirus, parainfluenza viruses, and Legionella spp.

Laboratory Diagnostic Procedures

A number of tests can help in the diagnosis of influenza (see table). But, tests do not need to be done on all patients. For individual patients, tests are most useful when they are likely to give a doctor results that will help with diagnosis and treatment decisions. During a respiratory illness outbreak in a closed setting (e.g. hospitals, nursing home, cruise ship, boarding school, summer camp) however, testing for influenza can be very helpful in determining if influenza is the cause of the outbreak.

Preferred respiratory samples for influenza testing include nasopharyngeal or nasal swab, and nasal wash or aspirate, depending on which type of test is used (see table). Samples should be collected within the first 4 days of illness. Rapid influenza tests provide results within 15 minutes or less; viral culture provides results in 3-10 days. Most of the rapid tests that can be done in a physician"e;s office are approximately 50-70% sensitive for detecting influenza and approximately greater than 90% specific. Therefore, false negative results are more common than false positive results, especially during peak influenza activity.

Routine serological testing for influenza requires paired acute and convalescent sera, does not provide results to help with clinical decision-making, is only available at a limited number of public health or research laboratories and is not generally recommended, except for research and public health investigations. Serological testing results for human influenza on a single serum specimen is not interpretable and is not recommended.

During outbreaks of respiratory illness when influenza is suspected, some respiratory samples should be tested by both rapid tests and by viral culture. The collection of some respiratory samples for 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"e;s influenza vaccine. During outbreaks of influenza-like illness, viral culture also can help identify other causes of illness.

Influenza Diagnostic Table