BD Directigen Flu A + B

For the Differentiated Detection of Flu A and B antigens

Laboratory News December 2000

The laboratory now has the capability to perform direct detection of Influenza A and B antigens on a single specimen using a single test kit. It is suggested that physicians order both test as follows:

FLU A & FLU B

The Directigen ^TM Flu A+B test is a rapid in vitro enzyme immunoassay (EIA) membrane test for the direct and qualitative detection of influenza A and B viral antigens from nasopharyngeal wash, nasopharyngeal aspirate, nasopharyngeal swab, throat swab and BAL specimens of symptomatic patients. The Directigen Flu A+B test is a differentiated test, and therefore influenza A viral antigens can be distinguished from influenza B viral antigens in a single test. The test is to be used as an aid in the diagnosis of influenza A and B viral infections. Negative test results should be confirmed by cell culture. The Directigen Flu A+B in not intended for the detection of influenza C.

Influenza is an acute viral disease that is seasonal in incidence. The illness classically presents with sudden onset of fever, chills, headache, myalgias, and a non-productive cough. Clinical manifestations usually resolve within one week unless complications develop. Influenza A and B virus cause the majority of clinically significant disease, with influenza C virus being responsible only for mild, predominantly upper respiratory tract illness.

Patients who present with suspected influenza might benefit from treatment with antiviral agents. Amantidine and rimantadine are available for both the prevention and treatment of influenza A only. Zanamivir and oseltamivir are available for the treatment of both influenza A and B disease. In adults, therapy with these agents may reduce the severity and duration of illness if given within the first 48 hrs of onset of illness. Since the therapeutic options have expanded to include options for the treatment of influenza B disease, it is important to rapidly distinguish influenza A from influenza B in order to allow physicians a choice in selective antiviral intervention. Moreover, since only amantidine and rimantadine have indications for influenza prophylaxis, it is important to determine if influenza A is causing symptomatic disease in a particular institution (e.g. nursing home) or community , so that the appropriate preventative intervention can be taken for susceptible individuals. It is therefore important to not only rapidly determine whether influenza is present, but which type of influenza virus is present.

The Directigen Flu A+B antigen detection test is an enzyme immunomembrane filter assay to detect influenza A or B antigens extracted from suitable specimens of symptomatic patients. Total test time is less than 15 minutes. Antigenic drift is not an issue with the test because the target antigens are the nucleoproteins, which are type-specific and are highly conserved.

The Directigen Flu A+B make it applicable as a “STAT” influenza A and B antigen detection test, relevant information to assist with the diagnosis of influenza. The use of Directigen Flu A+B to differentiate Flu A from Flu B infection can provide the opportunity for greater selectivity of antiviral intervention.

A total of 1262 specimens were evaluated. The overall sensitivity of the Directigen Flu A+B test for influenza A when compared to culture was 86.2% and for influenza B was 80.8%. The overall specificity for influenza A when compared to culture was 90.7% and for influenza B was 99.5%. The uninterpretable rate for Directigen Flu A+B was 0.08% for both influenza A and influenza B results.

1. The test is capable of detecting both viable and nonviable influenza A and B virus particles.
2. Performance depends on the antigen load and may not correlate with cell culture performed on the same specimen.
3. The etiology of respiratory infection caused by other organisms will not be established with this test.
4. Inadequate specimen collection, improper sample transport or low-level viral shedding can yield a false-negative result.
5. The test has not been approved for ID or confirmation of cell culture isolates.
6. Performance of this test has not been established for monitoring antiviral treatment of influenza.

The rate of positivity observed depends on method of collection, handling/transport of specimen, time of year, age of patient, geographic location and most importantly, local disease prevalence, which varies from year to year. The prevalence in the past 3 years has ranged from 28 to 34%. In 1999—2000 season in the U.S. the prevalence of influenza A in those patients diagnosed with the flu was 99.6% as compared to a prevalence of 0.4% for influenza B.