Q2 BACTERIOLOGICAL DIAGNOSIS. 



(fig. 17, b). This is the complete positive reaction; it 

 consists of two parts, clumping and paralysis, and is 

 given only (in the dilution used) by the blood of a 

 patient who is suffering or who has suffered from 

 typhoid fever. If this is not the case the bacilli will 

 continue to move about just as before, and will not 

 collect into clumps. 



In the process which has been described above, the 

 blood has been diluted to thirty times its volume, and 

 this is the best dilution to use for diagnostic purposes. 

 But the reaction is given earlier if a lesser dilution (one 



b 



FIG. 17. 

 rt. Negative Widal's reaction. 6. Positive Widal's reaction. 



in ten) is used, though there is then a greater chance of 

 fallacy. 



Sometimes the reaction takes place almost as soon as 

 the serum is added. At other times it is delayed, and 

 for these it is necessary to fix a time limit. With a 

 dilution of one in thirty, one hour is a safe time limit to 

 adopt, and if the reaction takes place after this the re- 

 sult should be looked upon with great suspicion, and the 

 test reapplied after a day or two. 



Exactly similar processes are carried out if dead cul- 

 tures of typhoid bacilli are used, but here the emulsion 



