TYPHOID FEVER. 499 



or even i : 200. Widal, however, found 1 : 60 sufficient. 

 Dilutions of 1 : 10 are satisfactory for routine work, and 

 it is only necessary to use the high dilutions in doubtful 

 cases. 



Welch says : " From the observations thus far reported, although 

 they are insufficient in number for definite conclusions, there 

 would seem to be only a small liability of failure to recognize 

 genuine typhoid cases by resorting to dilutions of 1 : 40 or 1 : 50, 

 but unquestionably a few would escape recognition, and for this 

 reason lower dilutions should also be used, and if those between 

 1 : 10 and 1 : 50 give decided reaction there should be at least 

 suspicion of typhoid. It is not, therefore, to be recommended 

 that one make the test with only high dilutions, such as 1 : 50. 

 The negative result of the preliminary test with equal parts of 

 serum and culture suffices to exclude typhoid reaction. The 

 examination, if positive, may then be made with a low dilution 

 of the serum, and for this Widal's recommendation of 1 : 10 or 

 1:15 may well be adopted. If with this dilution the microscopic 

 reaction is complete and almost immediate, as is often the case, 

 there is practically no risk in making a positive diagnosis. But 

 for absolute certainty, and above all, in cases in which the result 

 of the reaction is not prompt, complete, and unmistakable, higher 

 dilutions should be employed ; if the amount of serum permits 

 only one such, it maybe 1 : 50, but preferably intermediate dilutions 

 should also be made, and it is desirable, if not absolute^ neces- 

 sary, to try dilutions higher than 1 : 50." 



A time limit is also to be fixed, and when, with the 

 ordinary dilution of 1 : 10, a reaction does not come on 

 within fifteen minutes, any subsequent agglutinative 

 phenomena should be looked upon with suspicion. We 

 have, however, seen reaction delayed an hour and longer 

 in 1 : 10 dilutions in cases known to be typhoid. With the 

 weak dilutions, 1 : 50, etc., the time limit must be ex- 

 tended to at least an hour. Usually the reaction occurs 

 so promptly that no difficulty is experienced. 



XI. The condition of the culture is also to be carefully 

 considered from several standpoints, else the validity of 

 the test may be called into question. 



1. Age. — Young cultures of the typhoid bacillus are 

 actively motile, and are apt to contain rather elongate 

 individuals. Both these features are essential to success. 



