Standardizing the Influenza Neuraminidase Inhibition Assay among United States Public Health Laboratories Conducting Virological Surveillance.

•Three U.S. laboratories performed a standardized NI assay initially using reference viruses and drugs prepared by the CDC.•Subsequent preparation of viruses and drugs by each participating laboratory resulted in minimal variation in IC50s.•NI assay standardization was achieved through stepwise uniform procedures with support from the CDC reference laboratory.•WHO-AWVG criteria facilitated reporting of IC50 data generated by other laboratories, in U.S. national surveillance.