Quantitative estimation of diphtheria and tetanus toxoids. 4. Toxoids as international reference materials defining Lf-units for diphtheria and tetanus toxoids.

The Lf-unit, which is used in the control of diphtheria and tetanus toxoid production and in some countries also to follow immunization of horses for production of antitoxins, has hitherto been defined by means of antitoxin preparations. A diphtheria toxoid and a tetanus toxoid preparation, both freeze-dried, were examined in an international collaborative study for their suitability to serve as reference reagents in the flocculation tests and for defining the Lf-units. It was shown that flocculation tests using the reference toxoids are very reproducible and reliable and the WHO Expert Committee on Biological Standardization established: the toxoid called DIFT as the International Reference Reagent of Diphtheria Toxoid for Flocculation Test with a defined content of 900 Lf-units of diphtheria toxoid per ampoule; and the toxoid called TEFT as the International Reference Reagent of Tetanus Toxoid for Flocculation Test with a defined content of 1000 Lf-units of diphtheria toxoid per ampoule.