Replacement of the Toxin Neutralisation Test in Mice by in vitro Serological Assay Systems for Potency Testing of Tetanus Toxoid Vaccines for Veterinary Use: Results of a European Collaborative Study.

This paper describes the results of a collaborative study to the use of in vitro serological assay systems in the assessment of the potency of tetanus toxoid in single and multicomponent vaccines for veterinary use. According to the procedure described in monograph 697 of the European Pharmacopoeia (method A) these products are currently tested by toxin neutralisation tests in mice. The collaborative study was performed in seven laboratories throughout Europe. Nine commercial vaccines, representing the range of products available, and one experimental tetanus toxoid preparation were tested by immunisation of groups of rabbits and guinea pigs. Individual and pooled serum samples were titrated for levels of tetanus antitoxin by indirect enzyme-linked immunosorbent assay (ELISA), bowin binding inhibition (ToBI) test, passive haemagglutination (HA) test and by the toxin neutralisation (TN) test in mice. It was found that estimates of potency by in vitro tests and by the TN test were in good agreement for the various vaccines tested and for antitoxin titres of individual serum samples. Significant interlaboratory variation occurred less frequently for ELISA and ToBI than for HA. The frequency of significant interlaboratory variation was acceptable for ELISA and for ToBI but larger variation was observed for HA. It is concluded that ELISA and ToBI are suitable in vitro assay systems for assessing the potency of tetanus toxoid in batches of single and multicomponent vaccines for veterinary use. Rigid standardisation of HA test is essential before this test can be used for the same quality control purpose.