90 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



will become paralyzed, and remain quite motionless. Fur- 

 ther, they will collect into clumps, each clump consisting of a 

 larger or smaller number of bacilli, arranged in a felted net- 

 work, resembling that seen in a heap of " spellicans " 

 (Fig. 20, b). This is the complete positive reaction ; it con- 

 sists of two parts, clumping and paralysis, and is given only 

 (in the dilution used) by the blood of a patient who is suffer- 

 ing 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 



FIG. 20. 

 a, Negative Widal's reaction ; b, positive Widal's reaction. 



given earlier if a lesser dilution (i in 10) 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 i in 30, 

 one hour is a safe time-limit to adopt, and if the reaction 

 takes place after this the result should be looked upon with 

 great suspicion, and the test reapplied after a day or two. 



INTERPRETATION OF RESULTS. 



A positive result may mean : 



1. That the patient is suffering from typhoid fever. 



2. That he has suffered from typhoid fever within a certain 

 period before the blood was taken. The hypothetical sub- 

 stance which we believe to be the cause of the reaction 

 (agglutinin) continues to be formed, or remains in the blood, 



