G. W. McCOY 



953 



tration of a toxin to be used in the testing against an antitoxic serum of known potency. 

 When the amount of toxin which flocculates with i unit of antitoxin has been deter- 

 mined, the flocculating value of the toxin expressed as units of antitoxin flocculated 

 per centimeter of toxin is readily ascertained. Fixed amounts of the toxin are then 

 mixed with varying amounts of the antitoxin to be titrated, the mixtures incubated 

 at 55° C, and the first tube to show flocculation is the indication of the neutralizing 

 value of the serum and from this the unitage is calculated. 



TETANUS ANTITOXIN 



The United States unit for this product was originally based ^ on a certain 

 amount of a toxin used in the early work, this amount of the toxin being in turn 

 selected as a convenient multiple of its minimal lethal dose. The actual unit^ 

 is, as with diphtheria antitoxin, the combining or neutralizing power contained in a 



TABLE III 



stated volume of the official standard antitoxin which is distributed by the Hygienic 

 Laboratory. A dried toxin is also distributed, with the test dose indicated, and is used 

 for comparing the combining or neutralizing power of the antitoxin to be tested with 

 the standard. As with diphtheria antitoxin, the criterion of exact balance between the 

 doses of toxin and of antitoxin (o.i unit for tetanus) is death at ninety-six hours after 

 injection of the mixture, but heavier guinea pigs are used, weighing about 350-80 gm., 

 instead of the 250-80 gm. pigs used for diphtheria. 



The test dose of toxin, having been roughly ascertained by preliminary tests, is 

 determined more exactly as in the protocol shown in Table III. 



The middle dose (0.00082 gm.) employed is selected as it is as near as is ordi- 

 narily practicable to secure to the ideal of one that will kill in just under ninety-six 

 hours. It is better, of course, to err on the safe side by having a dose of toxin slightly 

 in excess rather than to risk one too low which would result in deceptively high titres 

 for serum. 



Using the dose selected as above, a test is carried out on commercial samples to 

 be tested as shown in Table IV. 



' Rosenau, M. J., and Anderson, J. F.: Hygienic Laboratory Bull. 4j. 1908. 

 » United States Public Health Service: Miscellaneous Publ, 10, par. 72. 1919. 



