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Fast, Accurate Influenza A/B Antigen Detection


According to CDC data, influenza infections in this country cause more than 114,000 hospitalizations and 36,000 deaths annually. Among healthy adults, bronchitis and pneumonia are the most common complications. Together, influenza and pneumonia are the sixth leading cause of death in the United States. Influenza is particularly dangerous in elderly and immunocompromised people; however, infections involving fulminant influenza strains are serious illnesses in any patient. Prompt diagnosis of influenza supports improved treatment outcomes, now supported by the availability of rapid testing for point of care use.

Early diagnosis of influenza can reduce the use of inappropriate antibiotics and support the option of antiviral therapy. However, the accuracy of clinical diagnosis of influenza on the basis of symptoms alone is limited because symptoms from illness caused by other pathogens can overlap considerably with influenza.

Quick test kits from three manufacturers are currently CLIA-waived, providing an important diagnostic resource for point of care use. All of these kits are characterized by the College of American Pathologists as moderately complex.

Binax, Inc., Portland, Maine, produces the NOW Flu A Test (Influenza A) and NOW Flu B Test (Influenza B), both of which, according to the company, are the most widely used products of their type in the market.

The NOW Flu A test is an in vitro rapid immunochromatographic assay for the qualitative detection of Influenza A nucleoprotein antigen in nasal wash and nasopharyngeal swab specimens from symptomatic patients.

Using a nasal wash sample, sensitivity is rated at 82 percent and specificity at 94 percent. Using an NP swab sample, sensitivity is 78 percent and specificity is 92 percent

For the NOW Flu B test using a nasal wash sample, sensitivity is rated at 71 percent and specificity at 97 percent. Using an NP swab sample, sensitivity is 58 percent and specificit y is 97 percent.

Quidel Corporation, San Diego, Calif., produces the QuickVue Influenza A+B test, a point-of-care lateral flow immunodiagnostic test designed to detect the presence of the influenza A and B virus in a patient nasal wash or nasal swab sample within 10 minutes. A positive test result is visually detected as a red colored test line. The test has an internal control that will appear as a visually detected blue colored line.

The QuickVue Influenza test technology utilizes the principle of antibody specificity for the influenza virus nucleoprotein (NP) to selectively interact with the influenza A and B viruses. The NP is evolutionarily conserved and not subject to the same year-to-year changes that make it necessary to be vaccinated each year to prevent influenza infection. For this reason the QuickVue test is not adversely affected by the appearance of different influenza strains.

For influenza A, using a nasal wash sample, sensitivity is rated at 77 percent and specificity at 99 percent. Using an NP swab sample, sensitivity is 72 percent and specificity is 96 percent.

For influenza B, nasal wash sample sensitivity is 82 percent and specificity is 99 percent. Using an NP swab sample, sensitivity is 73 percent and specificity is 96 percent.

ZymeTx Inc., Oklahoma City, Okla. manufactures the ZstatFlu Rapid Test for Influenza A and B, the first point-of-care testing platform for the rapid diagnosis of Influenza A and B. It has proven specificity in diagnosing current strains of Influenza A and B, in addition to all of the strains on record for the past 30 years.

ZstatFlu utilizes a throat-swab specimen and employs approximately two minutes of tech time and 20 minutes processing time. It has a three-year shelf life, can be stored at room temperature, and detects all A & B strains with 99 percent specificity.

Several companies manufacture tests that are also considered moderately complex by the Colleg e of American Pathologists, but are not currently CLIA-waived and, thus, require specific laboratory certification for their use. These firms include:

Becton-Dickinson, Franklin Lakes, N.J., which produces the BD Directigen Flu A+B test kit, which the company says is the first rapid assay on the market that distinguishes influenza A viral antigens from those of influenza B. Specimen types that the kit can assess include nasopharyngeal wash, nasopharyngeal aspirate, nasopharyngeal swab, lower nasal swab, throat swab and bronchoalveolar lavage. The company reports influenza A sensitivity of 95.7 percent and specificity of 91.4 percent for nasopharyngeal aspirates. Influenza B sensitivity is 87.5 percent and specificity of 98.1 percent for nasopharyngeal aspirates. Test results time is 15 minutes.

Thermo Electron Corp., Waltham, Mass., produces the FLU OIA test, a high performance assay for the undifferentiated diagnosis of Influenza A and B in just over 15 minutes. Assay advantages include superior performance with mucoid samples, specimen flexibility and performance equivalent to 14-day viral culture, thus decreasing overall diagnostic costs by making additional ancillary testing unnecessary.

While these quick test products mark a significant advance in diagnostic options, the CDC notes that, The specificity and, in particular, the sensitivity of rapid tests are lower than for viral culture and vary by test. Because of the lower sensitivity of the rapid tests, physicians should consider confirming negative tests with viral culture or other means.
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