Titration of diphtheria and tetanus antitoxins in sera of low titre.

Available methods for titrating diphtheria and tetanus antitoxin at low concentrations in human or animal blood are surveyed, with special attention to the amount of serum required for the test.In vivo methods, especially the rabbit or guinea-pig intradermal test for diphtheria and the mouse test for tetanus, are precise and reliable. If, however, serum levels as low as about 0.001 IU/ml have to be determined, rather large amounts of serum are required: moreover, the tests are rather time-consuming and expensive.Tissue culture methods are available only for diphtheria antitoxin titration. The titres found coincide very well with those from animal tests. The titrations are less time-consuming and more economical. These methods seem to be very promising for the replacement of animal tests. Of the real in vitro methods, the haemagglutination procedure has been investigated most thoroughly and used most frequently. Low titres can be measured using small amounts of serum, but the titres thus obtained may differ considerably from those obtained in animal tests, at least for individual sera. For mass screening, the method is very suitable.A number of other methods are briefly discussed, but they are either less suitable or have not yet been sufficiently investigated.