In anticipation of “Flu season”, it is valuable to revisit available testing methods for influenza and discuss when each test should be ordered. Though generally a self-limited infection for otherwise healthy individuals, influenza can be particularly severe for high-risk populations including infants, elderly, pregnant women, and immunocompromised hosts. Influenza incidence is largely seasonal with the majority of cases occurring between December and April of each year.
Prevalence within the community significantly impacts the positive and negative predictive values (PPV and NPV) of testing. When prevalence is low during the summer months, the PPV is reduced. Since false-positive results are more likely, the most specific test available should be used for testing during this time. At Spectrum Health, a molecular method is used under the orderable Test Code 156 “Influenza (Flu) A/B PCR.” Please order this test for current patients as we are not yet in Influenza Season.
Due to the concentration of cases during the winter months, the PPV is high during this period and positive results are more likely to be true. The majority of these cases can be detected using a rapid antigen point-of-care test that identifies and differentiates Influenza A and B by detecting viral antigens collected by nasal and nasopharyngeal swabs from patients during acute presentation of respiratory illness. The use of rapid antigen testing as a screening method during this period of high prevalence reduces both the cost to the patient and turn-around-time as compared to molecular methods. Rapid antigen tests generally have high analytical specificity, but lack the sensitivity of nucleic acid amplification methods. During influenza season, current guidelines recommend reflex testing in which patients that test negative by rapid antigen are automatically tested using a molecular technique.1-2 This reflexive diagnostic algorithm should be ordered during influenza season as Test Code 155 “Influenza (Flu) A/B Rapid. Influenza A/B PCR if Negative.”
As always, the interpretation of positive results should take into account the clinical characteristics of the patient and the current local epidemiology. Additional system-wide communication will be distributed later this fall, likely November, as to when ordering practices should be switched from an initial PCR to the rapid antigen testing algorithm.
- US Centers for Disease Control and Prevention. Guidance for clinicians on the use of rapid influenza diagnostic tests. http://www.cdc.gov/flu/professionals/diagnosis/clinician_guidance_ridt.htm
- Harper SA, Bradley JS, Englund JA, et al. Seasonal influenza in adults and children–diagnosis, treatment, chemoprophylaxis, and institutional outbreak management: clinical practice guidelines of the Infectious Diseases Society of America. Clin Infect Dis 2009; 48:1003.