SEROLOGICAL METHODS 



211 



The ring test. The antigen is diluted serially as in the antigen dilution 

 test, and 0.1 ml layered over 0.1 ml of antiserum in a 6- by 50-mm tube. 

 The serum is usually added first, and the antigen must then be added 

 slowly, holding the tube nearly horizontal so that a sharp interface forms 

 between the two solutions. The tubes are placed in a rack at room tem- 

 perature and observed at 15-min intervals. In a positive test a precipi- 



Table 22. Combination of Serum Dilution and Antigen 

 Dilution Procedure 



tate forms at the interface. It is essential that the serum have a greater 

 density than the antigen, and the serum is often used undiluted, or in the 

 case of more potent serums, the appropriate concentration may be reached 

 by using normal serum as a diluent. A serum-saline control should be 

 included. If economy of reagents is necessary, the ring test may be per- 

 formed in small capillary tubing. 



The ring test is a qualitative or roughly quantitative procedure and 

 thus has limited value for precise work. It does have the advantages 

 that smaller amounts of material are required and that inhibition due to 

 excess antigen is less likely, since the reactants can diffuse into each other. 

 It has considerable value for the detection of qualitative (type or group) 

 differences among different strains of the same species and has been used 

 for this purpose with organisms like the streptococci and pneumococci. 



Optimal proportions. If a precipitin test is set up as described for 

 ''Antigen dilution" and the tubes examined at frequent intervals, one 

 tube will usually be observed to precipitate before the others. This is 

 called the point of optimal proportions. The ratio (dilution of antigen/ 

 dilution of antiserum) is called the optimal ratio, and should the anti- 

 serum dilution be changed, it usually requires a proportional dilution of 

 the antigen to give the most rapid precipitation. 



The optimal-proportions method, then, is a way of measuring the 

 velocity of the precipitin reaction. When antigen is diluted and serum 

 held constant, the test is referred to as the ''alpha procedure"; con- 



