Combined estimation of tetanus and diphtheria antitoxin in human sera by the in vitro Toxin-Binding Inhibition (ToBI) test

The use of the principle of inhibition of toxin binding to an antitoxin coated immunoassay plate as described in a previous paper for tetanus antitoxin titration, was adapted for the estimation of diphtheria antitoxin in human sera. With a few modifications, a Toxin-Binding Inhibition (ToBI) test was developed which could be used for a combined estimation of both tetanus and diphtheria antitoxin levels. The application of streptavidin-biotinylated peroxidase complex when using small serum samples (<50 μl) is discussed. Antitoxin titres (both diphtheria and tetanus) of 0·002 IU ml⁻¹ were detectable by the ToBI test, this being far below the level considered to be protective in man.

Sera from 140 adults with different vaccination histories were titrated for both tetanus and diphtheria antitoxin. Good correlations were found between the estimates obtained by the ToBI test and those obtained by the toxin-neutralization (TN) test in mice (tetanus antitoxin) and those obtained in the in vitro neutralization test in VERO cells (diphtheria antitoxin).

It is concluded that the ToBI test is a simple and reliable alternative to the functional models currently in use for the estimation of diphtheria and tetanus antitoxin levels. In addition, the ToBI test eliminates the need for laboratory-animal or cell-culture facilities and can be performed with small quantities of serum as required in field trials.