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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 (
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