Flu Diagnosis

Appropriate treatment of patients with respiratory illness depends on accurate and timely diagnosis. Early diagnosis of influenza can reduce the inappropriate use of antibiotics and provide the option of using antiviral therapy. However, because certain bacterial infections can produce symptoms similar to influenza, bacterial infections should be considered and appropriately treated, if suspected. In addition, bacterial infections can occur as a complication of influenza.

Influenza surveillance information and diagnostic testing can aid clinical judgment and help guide treatment decisions. The accuracy of clinical diagnosis of influenza on the basis of symptoms alone is limited because symptoms from illness caused by other pathogens can overlap considerably with influenza. Influenza surveillance by state and local health departments and Centers for Disease Control and Prevention (CDC) can provide information regarding the presence of influenza viruses in the community. Surveillance can also identify the predominant circulating types, influenza A subtypes, and strains of influenza.

Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, polymerase chain reaction (PCR), and immunofluorescence assays. Sensitivity and specificity of any test for influenza might vary by the laboratory that performs the test, the type of test used, and the type of specimen tested. Among respiratory specimens for viral isolation or rapid detection, nasopharyngeal specimens are typically more effective than throat swab specimens. As with any diagnostic test, results should be evaluated in the context of other clinical and epidemiologic information available to health care providers.

Commercial rapid diagnostic tests are available that can detect influenza viruses within 30 minutes. Some tests are approved for use in any outpatient setting, whereas others must be used in a moderately complex clinical laboratory. These rapid tests differ in the types of influenza viruses they can detect and whether they can distinguish between influenza types. Different tests can detect only influenza A viruses; both influenza A and B viruses, but not distinguish between the two types; or both influenza A and B and distinguish between the two.

None of the tests provide any information about influenza A subtypes. The types of specimens acceptable for use (i.e., throat, nasopharyngeal, or nasal aspirates, swabs, or washes) also vary by test. The specificity and, in particular, the sensitivity of rapid tests are lower than for viral culture and vary by test. Because of the lower sensitivity of the rapid tests, physicians should consider confirming negative tests with viral culture or other means because of the possibility of false-negative rapid test results, especially during periods of peak community influenza activity.

In contrast, false-positive rapid test results are less likely, but can occur during periods of low influenza activity. Therefore, when interpreting results of a rapid influenza test, physicians should consider the positive and negative predictive values of the test in the context of the level of influenza activity in their community. Package inserts and the laboratory performing the test should be consulted for more details regarding use of rapid diagnostic tests.

Despite the availability of rapid diagnostic tests, collecting clinical specimens for viral culture is critical, because only culture isolates can provide specific information regarding circulating strains and subtypes of influenza viruses. This information is needed to compare current circulating influenza strains with vaccine strains, to guide decisions regarding influenza treatment and chemoprophylaxis, and to formulate vaccine for the coming year. Virus isolates also are needed to monitor the emergence of antiviral resistance and the emergence of novel influenza A subtypes that might pose a pandemic threat.

Laboratory Tests
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.

A number of tests can help in the diagnosis of influenza. 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, however, testing for influenza can be very helpful in determining if influenza is the cause of the outbreak.

Appropriate samples for influenza testing can include a nasopharyngeal or throat swab, nasal wash, or nasal aspirates, depending on which type of test is used. Samples should be collected within the first 4 days of illness. Rapid influenza tests provide results within 24 hours; viral culture provides results in 3-10 days. Most of the rapid tests that can be done in a physician’s office are approximately 70% sensitive for detecting influenza and approximately 90% specific. Thus, as many as 30% of samples that would be positive for influenza by viral culture may give a negative rapid test result. And, some rapid test results may indicate influenza when a person is not infected with influenza.

Serum samples also can be tested for influenza antibody to diagnose recent infections. Two samples should be collected per person: one sample within the first week of illness and a second sample 2-4 weeks later. If antibody levels increase from the first to the second sample, influenza infection likely occurred. Because of the length of time needed for a diagnosis of influenza by serologic testing, other diagnostic testing should be used if a more rapid diagnosis is needed.

During outbreaks of respiratory illness when influenza is suspected, some samples should be tested by both rapid tests and by viral culture. The collection of some samples for viral culture is essential for determining the influenza subtypes and strains causing illness, and for surveillance of new strains that may need to be included in the next year’s influenza vaccine. During outbreaks of influenza-like illness, viral culture also can help identify other causes of illness when influenza is not the cause.