286 IMMUNOLOGICAL METHODS OF DIAGNOSIS 



preserved in sterile glass beads in the ice-box. In this manner the 

 material will keep for months, and can be used either for the micro- 

 scopic test, in which case the time of examination should be extended 

 ' to two hours, or it may be employed in the macroscopic test described 

 below. 



Macroscopic Method. This method is just as exact as the micro- 

 scopic method, and is in a manner less apt to lead to confusion; 

 somewhat larger amounts of blood, however, are required (1 c.c.). 

 The serum is diluted in the same proportions as described above. 

 Equal quantities (0.25 c.c.) are then placed in small tubes, such as 

 the collecting tubes figured in Plate III, and treated with equal 

 volumes of the bacterial emulsion. These tubes together with a 

 control of equal volumes of saline and bacterial emulsion are placed 

 in the incubator, or some other warm place, and are examined after 

 twelve to twenty-four hours. If the reaction is positive the bacteria 

 in the serum tubes will have settled to the bottom, leaving the super- 

 natant fluid almost clear, thus contrasting sharply with the control 

 which is still as turbid as it was in the beginning. 



RESULTS. While a positive Widal reaction may be obtained as 

 early as the first day of the disease, meaning thereby the first day 

 that the patient spends in bed, or the fifth of general malaise, such 

 an occurrence must be viewed as a great rarity. In the vast majority 

 of cases a positive result is obtained only after the fifth or sixth day in 

 bed. As the likelihood of positive bacteriological findings is greatest 

 during the first week of the disease, an examination in this direction 

 may at this time well take precedence over the agglutination test. 

 During the second week, when the value of the two methods is on 

 a par, convenience may decide which one is to be employed. After 

 this, however, the agglutination test should be given the preference. 

 Experience has shown that a positive reaction may be obtained in prac- 

 tically all cases of true typhoid fever, but it is clear from what has 

 been said that much depends upon the period of the disease at which 

 the examination is made. The production of agglutinins evidently 

 does not begin at the same time in all cases, and does not become 

 fully established until after the disease has progressed for a certain 

 length of time. It may happen, indeed, that a positive reaction 

 is not obtained until convalescence, or even until a subsequent 

 relapse occurs. For this reason it is advisable to repeat the examina- 

 tion at frequent intervals, if on first trial a negative result is obtained. 



