490 APPLICATION OF METHODS OF BACTERIOLOGY 



"Widal's Reaction." — When employed conversely — ?'. e., 

 for deciding if the serum used is from a case of typhoid fever 

 or. not — ^the reaction constitutes " Widal's serum diagnosis 

 of typhoid fever." In beginning these tests it is often neces- 

 sary to try several cultures of genuine typhoid bacilli from 

 different sources and of varying degrees of vitality, before 

 a strain is procured that reacts conspicuously and quickly 

 with genuine typhoid serum. 



Widal's Reaction with Dried Blood. — This reaction 

 can also be obtained with redissolved dried blood — i. e., 

 by the Johnston method: a drop of the blood to be tested, 

 obtained by a needle-prick in the cleansed finger or lobe of 

 the ear, is collected on a bit of clean, unglazed paper and 

 allowed to dry. The paper is then folded, kept free from 

 contamination, and taken to the laboratory. With a 

 medium-size platinum-wire loop a drop of sterile bouillon, 

 water, or physiological salt solution is gently rubbed upon 

 the loop of dried blood until the contents of the loop are of 

 a dark amber color; this is then mixed with a drop of 

 bouillon culture of typhoid bacilli on a cover-glass, which is 

 mounted upon the hollow-ground slide as a hanging drop, 

 when the effect of the diluted blood upon the culture can 

 be observed with the microscope. The reaction, if positive, 

 should occur within a half hour. Many object to this 

 method because it is impossible accurately to dilute the 

 blood by the plan used. A number of tests have shown us 

 that preparations made in this way correspond roughly 

 with a fresh-blood dilution of from 1 : 15 to 1 : 20, as deter- 

 mined by the hemoglobinometer. In a small number of 

 cases in which parallel tests were made with this and with 

 fresh fluid serum the results were concordant. We are 

 inclined to the opinion, however, that in doubtful cases, in 



