In the past several years, the FDA has approved an array of rapid diagnostic tests that may improve medical decision-making. Approved tests are now available for Clinical Laboratory Improvement Act (CLIA)-waived (QuickVue Influenza A/B; ZstatFlu) and nonwaived labs (BD Directigen Flu A+B; BD Directigen Flu A).
Nasal washes or swabs, not throat swabs, are the best method for obtaining specimens.
Compared with viral culture (the gold standard), reported sensitivities for these tests are 62%–73%, and specificities are 80%–99%.5
A study conducted in a private practice reported sensitivities of 72%–95% and specificities of 76%–84%.3 In this mainly pediatric population, the prevalence of influenza was about 50% by culture, and the positive predictive value (the likelihood that a positive test indicates true disease) ranged from 80%–86%, with a negative predictive value (the likelihood that a negative test indicates absence of disease) of 77%–90%.
In both studies, QuickVue was the best performing CLIA-waived test; the ZstatFlu test did not perform as well as the others. The tests generally give results in under 15 minutes, except ZstatFlu, which was more cumbersome to use. Since the prevalence of a condition in the population influences the predictive value of tests, all the tests perform better at finding true disease during active flu outbreaks than at the beginning or end of outbreaks when patients are less likely to have influenza.
The tests range in price from $15 to $25.
Antiviral treatments
Amantadine and rimantadine can reduce the duration of uncomplicated influenza A by about 1 day when started within 2 days of the onset of illness.
The newer drugs, zanamivir and oseltamivir, can reduce the duration of uncomplicated influenza A and B by about 1 day compared with placebo. Data are limited regarding the benefits of these drugs for patients at high risk of serious complications, or for children, although 1 study has shown a decrease in the incidence of otitis media among children taking oseltamivir.
Zanamivir is approved for adults and for children older than 7 years. It is administered via inhalation twice a day and costs $48 for the standard 5-day treatment.
Oseltamivir is approved for adults and for children older than 1 year. It is taken orally twice daily, with dose based on age and weight. It costs $60 for a 5-day treatment.
Prophylaxis. Amantadine and rimantadine are approved for prophylaxis against influenza A, and prevent 70%–90% of cases. Oseltamivir is approved for prophylaxis in adults and children older than 13 years. When used prophylactically, these drugs must be taken daily for the duration of influenza activity in the community. This can mean taking medication for weeks, which would be quite expensive in the case of oseltamivir.
Correspondence
1601 Parkview Avenue, Rockford, IL 61107. E-mail: ehenley@uic.edu.