Infectious disease/brief research report
Performance Characteristics of a Rapid Immunochromatographic Assay for Detection of Influenza Virus in Children During the 2003 to 2004 Influenza Season

Presented in part at the Pediatric Academic Societies annual meeting, May 2005, Washington, DC. Reprints not available from the authors.
https://doi.org/10.1016/j.annemergmed.2005.10.019Get rights and content

Study objective

We evaluate the performance of a rapid assay (Binax NOW) for the detection of influenza A virus in children.

Methods

The performance of an in vitro rapid immunochromatographic assay for detection of influenza A virus was compared to viral culture in 4,383 consecutive respiratory specimens received during the 2003 to 2004 season, which included an influenza A epidemic in October and November of 2003.

Results

The overall test sensitivity was 61.6% (95% confidence interval [CI] 60.3% to 63.2%) and specificity was 95.8% (95% CI 95.1% to 96.3%). In preplanned subset analyses, we found the test more sensitive in infants aged 90 days or younger (sensitivity 70.3%; specificity 96.6%) and less specific during the epidemic (sensitivity 61.7%; specificity 90.4%).

Conclusion

This rapid assay was highly specific for detecting influenza A in children and thus appears useful for confirming this infection. Because of its limited sensitivity, however, a negative test cannot rule out influenza A.

Introduction

Influenza viruses may cause seasonal epidemics of disease, with particular morbidity and mortality at the extremes of age and in those patients with underlying medical conditions. Contemporary medical literature has described epidemics, usually caused by influenza A virus, every 4 to 10 years, as well as multiple pandemics that have killed 20 to 50 million people.1, 2, 3, 4

Early detection of influenza epidemics and pandemics has important public health implications. Rapid and accurate emergency department (ED) diagnosis can permit rooming patients with positive assays together for infection control reasons. Rapid diagnosis can also justify early antiviral therapy for patients and prophylaxis for contacts, allows more judicious antibiotic usage, and emphasizes vaccination of unimmunized individuals in an effort to decrease disease burden.1, 5, 6, 7, 8, 9, 10 Furthermore, there are recent data showing that febrile children younger than 3 years and with influenza A have a lower prevalence of serious bacterial infections than children without influenza A.11 This observation has implications about the diagnostic evaluation of febrile children in the ED setting.

There are at least 7 diagnostic kits approved by the US Food and Drug Administration for rapid detection of influenza virus in respiratory specimens. These tests are widely available, and at least 3 of them are Clinical Laboratory Improvements Act–waived and can be performed in point-of-care settings.6, 12, 13, 14 Reported sensitivities in children range from 39% to 76%, with specificities of 94% to 98%.15, 16

We evaluated the performance of 1 rapid influenza test, Binax NOW Flu A & Flu B Test (Binax Inc, Portland, ME), in comparison to the reference standard of viral culture in pediatric patients during the 2003 to 2004 season, which included an influenza A epidemic.

Editor’s Capsule Summary

What is already known on this topic

The ability to rapidly and accurately identify influenza A in children with acute respiratory symptoms could lead to more selective use of anti-influenza agents and antibiotics.

What question this study addressed

Using viral culture as a criterion standard, what is the test performance of the Binax NOW rapid influenza A assay in children?

What this study adds to our knowledge

The rapid assay was highly specific but only moderately sensitive in detecting influenza A.

How this might change clinical practice

This rapid assay can identify but not exclude influenza A in children.

Section snippets

Materials and methods

The performance of a commercially available, Clinical Laboratory Improvements Act–waived, rapid test kit for influenza virus (Binax NOW Flu A & Flu B Test) was evaluated by comparing it to the reference standard of viral culture. This rapid test, the only one used at our institution during the study period, is an in vitro immunochromatographic assay for the qualitative detection of influenza A and B nucleoprotein antigens.

The specimen population was 4,383 consecutive fresh respiratory specimens

Results

The median age of the patients tested was 1.4 years (range 1 day to 41 years), of whom 911 (20.7%) patients were aged 90 days or younger. Of 4,383 respiratory specimens submitted to the laboratory during the study period, 4,302 (98.1%) were nasal washes; 59 (1.3%), tracheal aspirates; 10 (0.23%), nasal swabs; 8 (0.18%), bronchoalveolar lavage; and 2 (0.05%) each, sputum and sinus fluid. Two thousand four hundred twenty-seven of 4,383 (55.37%) specimens were obtained in the ED. Three thousand

Limitations

A limitation of this study is that we could not determine why patients had rapid viral testing performed (eg, signs and symptoms at presentation, the clinical course of the disease, underlying medical comorbidities). Also, it is possible that rapid assays, which do not depend on viable virus, may be more sensitive than viral culture in certain scenarios. Alternatively, apparent false-positive rapid test results during the peak period of an intense influenza season may be due to inadequate

Discussion

The immunochromatographic assay (Binax NOW Flu A & Flu B Test) was specific in detecting influenza A virus. The sensitivity of the assay was lower than previously reported; however, the performance was better in infants (≤90 days of age), perhaps because the viral load in this population is higher.6

We evaluated the assay during a season with an influenza A epidemic, with very high rates of positive cultures in October and November 2003. Our ED averaged 7,990 visits per month during the peak of

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Supervising editor: Steven M. Green, MD

Author contributions: ATC and LJM were responsible for database creation. ATC, ACC, and GJD conducted data analysis. ATC and ACC were responsible for manuscript creation. JMG conducted database maintenance, performance of rapid assays, and viral cultures. GJD conducted manuscript review. ATC takes responsibility for the paper as a whole.

Funding and support: This work was partially supported by grant D43 TW01036 from the National Institutes of Health-Fogarty International Center (ACC) and by the National Institutes of Health-Society for Pediatric Research Summer Student Research Program (LJM). Neither Binax nor any other pharmaceutical or corporate sponsor supported the study or any of the authors in any way.

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