94 BACTERIOLOGICAL DIAGNOSIS. 
of one in a hundred three days later, this affords a cer- 
tain proof that the reaction is due to a present attack of 
typhoid fever, and is not due to one which took place 
at a previous date. This investigation should be en- 
trusted to an expert bacteriologist and plenty of blood 
sent on each occasion. 
A negative result may mean :— 
1. That the patient is not suffering from typhoid 
fever. : 
2. That he is suffering from typhoid fever, but the 
date is too early for the appearance of the reaction. 
The reaction sometimes occurs on the fifth or sixth day, 
usually after the tenth day, and in all but a very small. 
number of cases before the end of the second week. If 
the onset of the disease (as far as it can be fixed) is less 
than this the examination should be repeated after two 
or three days. 
3. In a very small number of cases the reaction is 
delayed still further, and if the patient dies may not 
occur at all. These cases are usually severe ones and 
do not present any difficulty in diagnosis. Sometimes 
the reaction is delayed well into the convalescence in 
mild attacks, but this is exceedingly rare. 
Macroscopic MEtTHop. 
The macroscopic method can be carried out witha 
young living culture on agar, or with an emulsion of 
dead bacilli. The technique is not so difficult as in the 
previous process and no apparatus is required other: 
than a piece of narrow glass tubing from which to make 
pipettes. 
Requisites.—1. A young culture on agar and some 
