R. L. KAHN 853 



equally good results. All antigens, furthermore, were standardized to a titre of i cc. 

 of antigen plus i.i cc. of saline. Antigen once standardized will keep without change 

 at least four years, and perhaps indefinitely. 



TREATMENT OF SERUM 



Serum requires heating at 56° C. for optimum results. Unheated serum gives re- 

 sults markedly weaker than heated serum. Furthermore, the longer a syphilitic 

 serum is heated at 56° C, up to two hours, the more sensitive are the reactions in most 

 cases. For conservative results, a thirty-minute period of heating was chosen as the 

 standard. Sera that are "anticomplementary" in the Wassermann reaction give 

 specific Kahn reactions. Sera that are hemolyzed (if not excessively) are also satis- 

 factory in the Kahn reaction. 



Recent experiments indicate that the precipitin substances are associated with the 

 globulin (euglobulin and pseudoglobulin) fraction and not with the albumin fraction 

 of the serum.' 



QUANTITATIVE RELATION BETWEEN SERUM AND ANTIGEN SUSPENSION 



For optimum precipitation results, it is essential that the number of antigenic 

 reacting units correspond to the number of serum reacting units. If the number of 

 antigenic units is excessive, precipitation is inhibited. If the number of serum units 

 is excessive, precipitation is not inhibited, but is somewhat reduced in intensity. 

 Furthermore, the presence of an extra amount of serum over that required to com- 

 bine with the antigen suspension has the same effect as dilution with salt solution and 

 tends to weaken precipitation. Complete precipitation thus depends on the correct 

 oroportion between the antigen (precipitinogen) and serum (precipitin) substances. 



In deciding upon the proportion of serum and antigen suspension to be employed 

 in the routine diagnostic test, many factors come into play. Syphilitic sera of un- 

 usually marked potency may give precipitation reactions when mixed with as much 

 as an equal amount of antigen suspension. With the majority of syphilitic sera, how- 

 ever, this amount of suspension is inhibitory to precipitation. The same is true when 

 half the amount of suspension in relation to serum is employed. As the amounts of 

 suspension are reduced to one-third, one-fourth, one-eighth, and one-twelfth of the 

 serum amounts, proportionally more precipitation reactions are obtained. A serum 

 which is positive with a comparatively large amount of suspension is also positive 

 with lesser amounts of suspension. Based on clinical and serological studies in com- 

 parison with the Wassermann test, a three-tube test was decided upon, employing 

 three proportions of serum to antigen suspension — one part of serum to one-third 

 part of suspension; one to a sixth and one to a twelfth part of suspension. The strong- 

 ly potent sera produce precipitation in all the three serum-suspension proportions; 

 the moderately potent sera are negative or weakly positive in the proportion of serum 

 to suspension of one to one-third but strongly positive in the remaining proportions, 

 while the weakly potent sera produce precipitation only in the proportion of serum 

 to suspension of one to one-twelfth. To render the final results conservative, the 

 readings in the three tubes are averaged. In order that 0.5 cc. of serum be sufficient 



' Kendrick, P. L., and Kahn, R. L.: ibid., 39, 202. 1926. 



