With human infections of bird flu (avian influenza) reported in Asia and Eastern Europe, public health officials have moved swiftly to develop policies on surveillance, diagnosis, and treatment. The virus in question is the H5N1 subtype of influenza A, which has become epidemic in poultry and wild fowl in numerous countries. First in 1997 and now recently, some humans in close contact with infected birds have contracted the virus. Now that the H5N1 subtype has crossed the species barrier to humans, there is concern for human-to-human transmission. Given the presumed paucity of human resistance to the H5N1 subtype, infections could then reach pandemic proportions. This is particularly worrisome since the observed case fatality rate for human H5N1 infections has been over 50%. Furthermore, the H5N1 subtype shows resistance to two mainstays of influenza therapy, amantadine and rimantadine. While medications of choice currently include oseltamivir and zanamivir, there have also been reports of oseltamivir resistance (de Jong, MD et al, New Engl J Med 2005, 353:2667-2672). As news on avian flu evolves, local medical personnel naturally have questions about avian flu, even if they live in unaffected countries. A key question is the applicability of rapid or point-of-care tests.
Since up-to-date information on this topic is crucial, interested readers should keep current with the CDC or WHO (websites below). The WHO has published detailed guidelines on the use of rapid flu tests, including commentary on countries where avian flu is confirmed or suspected. These guidelines (link below) convey a number of important points. Overall, information is limited on the characteristics of rapid assays toward avian flu. A confirmatory test, such as RT-PCR or viral culture, must be performed on all suspected patients. One must bear in mind that avian flu may exist in a community amid other subtypes such as A/H1, A/H3, and B. At the present time, no rapid flu test can differentiate avian flu from other subtypes. Therefore, despite the proven ability of some rapid flu tests to detect H5N1 (see below), these tests have no positive predictive value for H5N1 versus other subtypes.
An equally important question is the negative predictive value of rapid tests. The WHO points out that a negative result on a rapid test does not rule out avian flu infection. Obviously, failure to detect even one infected person could have disastrous public health consequences. For areas where avian flu is confirmed or suspected, any patient who meets the WHO case definition must have confirmatory testing. For surveillance of H5N1 in non-affected areas, rapid tests are never acceptable. Other drawbacks to rapid tests include potential compromises in biosafety, lesser performance early or late in the infection, and loss of isolates or surveillance information.
Despite these cautionary statements, rapid (point-of-care) tests can aid early decisions on whether to initiate antiviral therapy, isolate patients, and notify public health authorities. Laboratories, hospitals, and clinicians should have a plan in place to obtain confirmatory (so-called H5) studies. Since it remains unclear how much H5N1 virus humans shed, different types of respiratory samples (nasal or throat swabs, washes, or aspirates) should be tested on multiple days.
The FDA has approved a number of rapid tests for influenza A. In theory, any of these products can detect the H5N1 subtype, but it should be emphasized that sensitivity is imperfect (see Technology Spotlight, below, for sensitivity and specificity figures). Most of these tests employ immunoassay methods, while the Zstat test assesses neuraminidase activity. Since H5N1 is a subtype of influenza A, tests for influenza B will not detect it. Some of the rapid tests ( Binax NOW Flu A, QuickVue, SAS, and Zstat) are CLIA-waived. For convenience, products are available to test for both influenza A and B. For those who desire a product previously validated on H5N1, there is peer-reviewed literature on the Directigen assays (Yuen KY et al, Lancet 1998, 351:467-471; Chan KH et al, J Clin Microbiol 2002, 40:1675-1680). The product insert of FLU OIA describes favorable in-house results for H5N1.
As worldwide authorities respond to the new threat of avian flu, further improvements and information on rapid tests will accumulate. Meanwhile, if used thoughtfully and selectively, they may provide precious advance notice for laboratories, hospitals, and public health departments.
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