Evaluation of influenza virus antiviral susceptibility testing in Europe: Results from the first external quality assessment exercise

Background: The first antiviral susceptibility testing external quality assessment (EQA) was held for European influenza reference laboratories during winter 2010/11. Objectives: To assess European network influenza antiviral susceptibility testing capability and provide participants with an independent performance evaluation. Study design: The EQA panel contained ten coded specimens of inactivated human influenza A and B viruses with reduced susceptibility to neuraminidase inhibitors (NAI), or adamantanes. Twenty-four laboratories from 19 member states of the WHO European region analysed the panel using phenotypic (determination of 50% inhibitory concentration (IC₅₀) values by neuraminidase (NA) enzyme inhibition assay) and/or genotypic methods. Results: All 24 laboratories returned genotypic data for A(H1N1)pdm09 influenza virus, 18 (75%) for former seasonal A(H1N1), 16 (67%) for A(H3N2) and 15 (63%) for influenza B virus, correctly identifying NAI or adamantane reduced susceptibility-associated substitutions in the NA (mean 84%; range 52-100%) or M2 (mean 85%; range 73-94%), respectively. Thirteen laboratories (54%) returned phenotypic NAI susceptibility data. Despite inter-laboratory and inter-assay IC₅₀ value variation, all 13 laboratories correctly identified oseltamivir reduced susceptibility/resistance in pure preparations of A(H1N1) oseltamivir-resistant viruses. However, only 11 (85%) identified oseltamivir reduced susceptibility/resistance in a mixture of A(H1N1)pdm09 oseltamivir-sensitive/-resistant viruses. Furthermore, 3 laboratories (23%) considered oseltamivir-sensitive influenza B virus reduced susceptible/resistant. Conclusions: Detection of NA-H275Y in A(H1N1) viruses was achieved by most laboratories. IC₅₀ values and interpretation thereof varied for a sensitive/resistant virus mixture and for influenza B virus. The results of this exercise will assist harmonisation of antiviral susceptibility testing, interpretation and reporting within the European network through targeted training.