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Significant Increase of Sensitivity on Rapid Influenza Antigen Assays Using Silver Amplification Immunochromatography.

Introduction
Influenza virus has been recognized as one of the most pandemic infectious disease agent. Rapid diagnosis of influenza is commonly performed with immunochromatography method (IC) using specimens taken from upper respiratory tract. Although IC is easy and relatively cheap, its sensitivity is not perfect especially in early stage of disease because of low concentration of virus antigens. Applying a newly developed “silver amplification” principle, we generated a new assay method to increase sensitivity. The aim of this study is if our method has higher sensitivity and specificity than conventional IC assays.

Materials and Methods
One hundred and twenty cases of influenza-like symptoms; fever, rhinorrhea, cough, and/or general fatigue who visited pediatrics department of our hospital from November 2011 to April 2012 were entitled. Cotton swab specimens were applied to Fuji Drychem IMMUNO AG1TM (FUJIFILM Medical, Tokyo, Japan). Simultaneously, a conventional IC method was performed with Quick Chaser FLU A/BTM by Mizuho Medy (Tosu, Japan). A real time PCR with TaqMan probe was also done for gold standard. This study was authorized by local ethic committee and written informed consent was obtained from all the cases.

Results
With silver amplification method, 23, 15 cases were influenza A, B positive, respectively. On the other hand, 23, 8 cases were A, B, positive with conventional assays. All the cases, which showed positive in new, negative in conventional influenza B assays, PCR results were positive. Their virus concentration ranged 105 to 109 copies/ml.

Discussion
Specificity and sensitivity for influenza A was not different between the two assays. However, the silver amplification method showed higher sensitivity for influenza B. Because the new method applies photo-developing principle, its sensitivity was reported to increase 8 to 16 times, theoretically. This analyzer is small (18x20x11 cm) and light (1.8kg), it is suitable for bedside testing. Since number of cases is limited, more data are required to confirm the result.

Conclusion
Our novel assay method using silver amplification has high potentiality to increase sensitivity of rapid bedside testing especially for influenza B.
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