Influenza A and B Antigen
Influenza viruses are spread from person to person primarily through large particle respiratory droplet transmission that requires close contact between source and recipient, because droplets do not remain suspended in the air and generally travel only a short distance of 1 meter or less. The typical incubation period for influenza is 1 to 4 days. Adults can be infectious from the day before symptoms begin until approximately 5 days after illness onset. Young children might shed virus several days before illness onset and remain infectious up to 10 days after onset of symptoms.
Uncomplicated influenza is characterized by the abrupt onset of constitutional and respiratory signs and symptoms such as fever, myalgia, headache malaise, nonproductive cough, sore throat and rhinitis. Uncomplicated illness usually resolves after 3 to 7 days, but cough and malaise can persist for more than 2 weeks. More serious complications include viral pneumonia, encephalopathy, transverse myelitis, myositis, myocarditis, pericarditis and Reye’s syndrome. Influenza infections can also lead to secondary bacterial pneumonia, sinusitis or otitis.
Influenza testing includes rapid antigen detection and conventional virus culture. The rapid flu test differentiates between types A and B. The differentiation allows a choice of therapy between the newer neuraminidase inhibitors, which are active against both strains, or use of the older, less expensive antiviral medication for influenza A only.
The ideal time to collect respiratory specimens is within 48 to 72 hours after onset of symptoms when the mean viral titers in nasopharyngeal specimens are highest. Because respiratory viruses replicate in the epithelium of the nasopharynx, nasopharyngeal washes or swabs provide more reliable specimens than throat swabs.
Sensitivity is reported to be 89-97%, with the best performance obtained from nasopharyngeal (NP) washes or swabs. Sensitivity is lower than specificity for all rapid influenza antigen tests, therefore false negative results are more likely than false positive results. False-positive results are most likely early in the influenza season, when prevalence in the community is low. Rapid tests that are interpreted by an optical reader have the highest sensitivity and specificity.
Nasopharyngeal swab specimens for rapid influenza testing should be collected on FLOQSwabs (flocked swabs) and submitted in M6 viral transport media. Dry swabs cannot be tested. Nasal washes and aspirates are also acceptable for testing, however bronchoscopy specimens cannot be tested. Specimens should be transported to the laboratory as soon as possible, but can be refrigerated for up to 72 hours prior to testing, if necessary.
Reference value is negative. Intra-nasal vaccine (FluMist) may give falsely positive results for influenza antigen A, B, or both within the first week after vaccination. Suspected false negative results should be followed by PCR testing, when illness is severe or when otherwise clinically indicated.
