Collaborative assay of a proposed international standard for tetanus toxoid (adsorbed)

An international collaborative assay was performed with the aim of establishing an international standard for tetanus toxoid (adsorbed). Six laboratories participated. A suitable preparation in the lyophilized state was made available by the Paul Ehrlich Institut, Frankfurt am Main, Federal Republic of Germany. Preliminary experiments showed that there were no significant differences between individual ampoules and that the preparation was highly thermostable.The proposed preparation (A) was compared in guinea-pigs and mice with the existing international standard for tetanus toxoid (B), which is a plain toxoid, with another arbitrarily chosen adsorbed toxoid (C), and with a number of locally prepared preparations (D). In the mouse tests preparation B was omitted. It was found in the guinea-pig tests that 1 mg of the proposed standard contained an activity equivalent to 1.5 IU if compared with the non-adsorbed international standard but since the slopes of the log dose-response lines for the adsorbed and plain types of preparation were not parallel, this is a rough approximation only. When the different adsorbed preparations were compared, there was close correlation between the guinea-pig tests and the mouse tests. In the guinea-pig test 1 ml of preparation C corresponded to 126.5 mg of preparation A and in the mouse test to 129.9 mg of preparation A. In one laboratory comparative tests were made by titrating blood antitoxins after immunization of guinea-pigs with graded doses of the toxoids. The results corresponded very well with the data from challenge tests (1 mg of preparation A contained 1.5 IU of the plain toxoid in challenge tests and|1.4 IU in the titration test).Preparation A was established by the WHO Expert Committee on Biological Standardization as the international standard for tetanus toxoid (adsorbed) and the international unit was defined as the activity contained in 0.6667 mg of the international standard.