R. L. KAHN 857 



been mixed with salt solution in such a way as to produce a lipoid emulsion or suspen- 

 sion.' Further study of the relationship between the method of mixing antigen with 

 saline and the physical appearance and reactive properties of the resulting lipoid sus- 

 pension brought to light a second zone phenomenon. Only when the antigen-saline 

 proportion is restricted to a certain zone (determinable by trial for each antigen) are 

 visible particles observable in the mixture. Furthermore, the antigen-saline propor- 

 tion giving optimum precipitation reactions in the Kahn test is Hmited to a still nar- 

 rower zone within the confines of the other. A third zone effect is observed in the 

 relationship between serum and antigen suspension, where excess of antigen suspen- 

 sion over serum and — to a less marked degree — of serum over antigen suspension is 

 inhibitory to precipitation. 



Turning from zone phenomena to a consideration of certain other factors, it is 

 found that agitation, which is known to play an important part in hastening the 

 coagulation of precipitates in colloid systems, is an essential feature of the Kahn 

 reaction. Finally, it has been emphasized that dilution of the reacting mixture delays 

 the precipitation reaction, as would indeed be expected from an analysis of the kinetics 

 of precipitation; this observation was among the first to be made in the series of 

 studies that led to the development of this precipitation test. Numerous problems in 

 connection with the nature of the reaction are yet to be solved. It is hoped that the 

 relative simplicity of the Kahn test will permit its ready utilization in connection 

 with further work in this field. ^ 



THE KAHN PROCEDURES 



All the procedures in this system are carried out by shaking for three minutes the tubes 

 containing the antigen suspension with the serum, cerebrospinal or other fluid. After the 

 shaking period an arbitrary amount of normal saline is added to the tubes to render the 

 precipitates readily visible, and the results are read. 



Routine or diagnostic test with sera. — This is a three-tube test consisting of three differ- 

 ent proportions of serum and antigen suspension. The amounts of the latter are 0.05, 

 0.025, <^^^ 0.0125 cc, respectively, while the amounts of serum are 0.15 cc. throughout. 

 Experiments have shown that 0.05 cc. is an e.xcessive amount of suspension when used with 

 0.15 cc. of serum, and will produce precipitation reactions only with serum which is rich in 

 syphilitic reagin. Weaker sera will be negative with the 0.05-cc. suspension amount and 

 positive either with the 0.025- or 0.0125-cc. amounts, depending on the potency of the serum. 

 The less suspension used, the stronger the precipitate with weakly reacting sera. Table I 

 gives an outline of the routine test and the interpretation of the results. The final result of a 

 reaction is the average of the findings in the three tubes, expressed in plus signs. 



Clinical application: As an aid in the diagnosis of syphilis and as a check on anti- 

 syphilitic treatment. 



Quantitative procedure with scrum. — This procedure consists of a series of serum dilutions 

 in 0.15-cc. amounts in which each dilution is tested with o.oi cc. of standard antigen suspen- 

 sion. The reactions in the individual tubes are interpreted as positive or negative, depending 

 on the presence or absence of a definite precipitate. The final result is expressed in syphilitic 



' The importance of antigen-salt-solution emulsions in connection with the Wassermann test 

 was pointed out by Sachs and Rondoni as early as 1908. 



' For the application of this precipitation reaction in syphilis to agglutination reactions in other 

 diseases, see Huddleson, I. F., and Carleson, E. R.: /. A. Vet. M. A., 70, 229. 1926; Noble, A.: /. 

 Bacterial., 14, 287. 1927. 



