Comparison of conventional lateral-flow assays and a new fluorescent immunoassay to detect influenza viruses

Sofia, a novel, fluorescent lateral-flow immunoassay was compared with two conventional colorimetric assays, Quickvue Influenza A+B and Directigen FLU A+B, to identify influenza viral antigen from patient nasopharyngeal specimens. A total of 118 frozen original influenza-positive specimens and 57 prospective specimens were examined. Using rt-PCR as a referee assay, sensitivity values (%) for influenza A/B of 80.0/74.8, 73.3/59.3 and 73.3/40.7 were obtained using the Sofia, Quickvue and Directigen assays, respectively. All assays demonstrated reduced sensitivity for influenza B as compared with influenza A virus. With respect to the Sofia assay, the sensitivity of influenza B for the Directigen assay was significantly diminished. False positive results were not observed in the Sofia and Directigen assays. The Quickvue assay produced 3 false-positive results (2 influenza A and 1 influenza B) resulting in a specificity (%) of 96 and 98 for influenza A and B, respectively. Cross-reactivity to other respiratory viruses was not observed among immunoassays. A sensitivity rank (highest to low) of rt-PCR>culture>Sofia>Quickvue>Directigen was established using dilutions of influenza A and B. Sofia provides enhanced sensitivity and objective result interpretation over conventional colorimetric immunoassays.