BACILLI OF THE COLON-TYPHOID-DYSENTERY GROUP 665 



and agglutinins will markedly affect the completeness or incompleteness 

 of the reaction. In high dilutions an excess of typhoid bacilli may bring 

 about complete absorption of all the agglutinins present, without agglu- 

 tinating all the microorganisms. 



The blood of the patient to be used for a Widal test may be obtained 

 in a number of ways. The most convenient method is to bleed the 

 patient from the ear or ringer into a small glass capsule, in the form of 

 that used in obtaining blood for the opsonin test, or into a small centri- 

 fuge tube. About 0.5 to 1 c.c. is ample. These capsules or tubes, 

 after clotting of the blood, may be placed in the centrifuge which in 

 a few revolutions will separate clear serum from clot. The dilutions 

 of the serum are then made. It is best to use sterile physiological salt 

 solution as a diluent, but neutral broth may be used. The dilutions 

 may be made either by means of an ordinary blood-counting pipette or 

 by means of a capillary pipette upon which a mark with a grease pencil, 

 made about an inch from the tip, furnishes a unit of measure, and upon 

 which suction is made by means of a rubber nipple. It is convenient 

 to have at hand a small porcelain palette such as that used by painters, 

 in which the various cup-like impressions may be utilized to contain the 

 various dilutions. Dilutions of the serum are made, ranging from 1 : 10 

 to 1 : 50. A drop of each of these dilutions is mixed with a drop of the 

 typhoid culture or emulsion upon the center of a cover-slip and the cover- 

 slip inverted over a hollow slide. A control with normal serum and with 

 the same culture should always be made and also one with the culture 

 alone to exclude the possibility of spontaneous clumping. Mixture 

 with the typhoid culture, of course, each time doubles the dilutions 

 so that, for instance, a drop of serum dilution 1 : 10, plus a drop of 

 the typhoid culture, gives the final dilution of 1 : 20. The preparation 

 may be examined with a high power dry lens or an oil immersion lens. 

 In a positive reaction, the bacilli, which at first swim about actively, 

 singly or in short chains, soon begin to gather in small groups and lose 

 much of their activity. Within one-half to one hour, they will be 

 gathered in dense clumps between which the fluid is clear and free from 

 bacteria, and only upon the edges of the agglutinated masses may slight 

 motility be observed. The degree of dilution and the time of exposure 

 at which such a reaction may be regarded as of specific diagnostic value 

 have been largely a matter of empirical determination. It is generally 

 /iccepted at present that complete agglutination within one hour in 

 dilutions from 1 : 40 to 1 : 60 is definite proof of the existence of typhoid 

 infection. Exceptions, however, to this rule may occur. Agglutina- 

 tions of typhoid bacilli in dilutions of 1 : 40, and over, have occasionally 



