Replacement of the international standard for tetanus antitoxin and the use of the standard in the flocculation test.

Since 1935 the International Unit for Tetanus Antitoxin has been defined as the activity contained in a certain weight of the first International Standard for Tetanus Antitoxin. As stocks of this standard had become depleted, 11 laboratories in 8 countries were requested to participate in a collaborative assay of a preparation proposed as a replacement.The assay results were analysed and presented to the WHO Expert Committee on Biological Standardization in 1969 which established the preparation studied as the second International Standard for Tetanus Antitoxin and defined the International Unit for Tetanus Antitoxin as the activity contained in 0.03384 mg of the second International Standard for Tetanus Antitoxin. This definition would ensure the continuity of the size of this international unit.The analysis of the collaborative studies also showed that the second International Standard for Tetanus Antitoxin has suitable properties for use in the flocculation test for the determination of the antigen content of tetanus toxoids in Lf values. The designation Lf-equivalent is described and the problems relating to the use of this term for the expression of results of in vitro assays are analysed in relation to the use of international units for expressing results of in vivo assays. As the second International Standard for Tetanus Antitoxin has an in vivo/in vitro ratio of 1.4, the Lf-equivalent of this antitoxin is 1.4 times less than its unitage.