Monitoring influenza virus oseltamivir resistance - our experience to date.

Objective: The monitoring of influenza virus resistance is a routine part of influenza virus surveillance conducted by the National Reference Laboratory for Influenza and Non-Influenza Respiratory Viral Diseases (NRL/INI) at the National Institute of Public Health (NIPH). The aim is to detect neuraminidase inhibitor (oseltamivir) resistance in patients diagnosed with influenza.

Material and methods: A total of 326 influenza virus isolates from tissue culture were included in the study. They were obtained from inpatient and outpatient nasopharyngeal swabs which were referred to the NRL/INI during the seasons 2013/2014 to 2019/2020 and turned out to be RTPCR (reverse transcription polymerase chain reaction) positive for RNA (ribonucleic acid) of influenza virus A or B. The MDCK (Madin-Darby canine kidney) tissue culture cells were used for virus isolation from nasopharyngeal swabs. Oseltamivir resistance was tested using the NA-Star Influenza Neuraminidase Inhibitor Resistance Detection Kit (Applied Biosystems, Foster City, CA).

Results: Nine of 326 positive specimens were oseltamivir resistant. Resistant strains showed IC50 values 100 times as high on average as those in oseltamivir sensitive strains.

Conclusions: Monitoring influenza virus resistance is helpful in controlling reasonable prescription of antivirals and thus becomes an integral part of influenza virus surveillance. Antiviral resistance monitoring is necessary not only in hospitalized patients on antivirals but also in symptomatically treated outpatients as the detection of antiviral drug resistant strains in the latter group can suggest the emergence and/or spread of antiviral drug resistance in the population.