456 IMMUNOLOGY 



tube test requiring 0.45 c.c. of serum and three dilutions oi 

 antigen. Weakly positive sera react in the highest ratio of 

 antigen-serum (1/12). 



Presumptive Test. — A presumptive test, due to its high sensi- 

 tivity, is used chiefly in the ruling out of .syphilis. In rare cases 

 it may be used as a check on treatment and the diagnosis of ob- 

 scure manifestations of syphilis, but great care must be used in 

 the interpretation of the results. In general, negative presumptive 

 reactions can be accepted and positive reactions considered 

 doubtful. The presumptive test requires 0.15 c.c. of serum and 

 0.01 c.c. of antigen. 



Quantitative Test. — The quantitative procedure is an eight- 

 tube test, using dilutions of serum varying from undiluted to a 

 1 :60 dilution. Each precipitating tube is assumed by definition to 

 contain four i-eacting units. The titer of the serum is determined 

 by substitution in the formula: 



S = 4 X D 



where D is the highest reacting dilution and S is the titer. This 

 quantitative procedure is of particular value in estimating the 

 response of cases to treatment. It can also be used in the evalua- 

 tion of new agents for treatment. 



Sinnal Fluid Procedure. — In spinal fluid procedure the stand- 

 ard Kahn antigen may be used but a new salt titration must be 

 made. This is necessary because of the changed concentration 

 of electrolytes resulting from the ammonium sulphate used in the 

 test. The titration technique is similar to that of the routine Kahn 

 salt titration from which it differs by increasing the amount of 

 salt solution used to disperse the eholestcrolized antigen (1.1 c.c. 

 to 1.5 c.c. usually) and the addition of 0.15 c.c. of 5 per cent am- 

 monium sulphate solution in place of 0.15 c.c. of sodium chloride. 



The remainder of the test is conducted in a manner similar to 

 the presumptive test or by using known dilutions of spinal fluid, 

 like the quantitative procedure. One hundredth c.c. of antigen, 

 prepared as indicated by the modified salt ammonium sulphate 

 titration, is used in each tube. 



Application of Quantitative Tests. — The quantitative Kahn, 

 both with serum and spinal fluid, is applicable only when strong 



