Establishment of an equine tetanus antitoxin reference standard for veterinary use in Japan.

To establish the first National Veterinary Assay Laboratory (NVAL) equine tetanus antitoxin reference standard for veterinary use, we manufactured vials of a candidate antitoxin. These were quality tested for moisture content, vacuum, colour, clarity, and the presence of foreign objects. Ultimately, 115 quality-controlled vials were prepared. To estimate the antitoxin potency of the candidate standard, three different laboratories conducted parallel line assays alongside the existing antitoxin standard. These potency estimates ranged from 38 to 42 IU. This activity was maintained for two years after manufacture, as compared with a fresh vial. No statistically significant non-linearity or non-parallelism of the regression lines was observed (p > 0.05). Statistical assessment of inter- and intra-laboratory variability revealed acceptable coefficients of variation of 3.2% and 2.4-3.1%, respectively. Based on these results, the potency of the potential reference standard was calculated at 40 units of antitoxin activity per 1-mL vial. Vials of this preparation were distributed for use as the first equine tetanus antitoxin reference standard for veterinary use in September 2015.