Calibration of replacement international standards of diphtheria and tetanus toxoids for use in flocculation test

The 1st International Reference Reagents (IRR) of Diphtheria and Tetanus Toxoids for Flocculation Test (DIFT and TEFT) were established by the WHO in 1988. These reagents are essential for the standardization of assays used to calculate Lf units of toxoids. Candidate replacement materials were provided by several European vaccine manufacturers and were formulated and freeze-dried at NIBSC. This paper provides a summary of the results of an international collaborative study including 18 laboratories from 16 countries, which examined the candidate replacement materials in a variety of methods. Materials 02/176 and 04/150 were proposed and adopted by the Expert Committee on Biological Standardization of WHO in October 2007 as 2nd WHO International Standards of Diphtheria and Tetanus Toxoid for use in Flocculation Test. The replacement standards were assigned the value of 1100 and 690 Lf/ampoule, respectively, based on results of flocculation tests carried out using provided reagents. Material coded 02/176 fully complied with the WHO specifications for stability, residual moisture content, precision of fill and sterility. Stability of material coded 04/150 was slightly lower than expected but predictions were based only on 2-year data and were to be further monitored, post-adoption.