76 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



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 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 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. 



Exactly similar processes are carried out if dead cultures of 

 typhoid bacilli are used, but here the emulsion should be sent out 

 from the laboratory free from clumps, and containing exactly the 

 right number of bacilli, so as to be ready for immediate use. But 

 the practitioner is urged not to trust to such an emulsion without 

 making a hanging-drop, and examining it just before making the 

 test. 



If dead cultures are used, it is advisable to use a rather less 

 degree of dilution than in the above process. A dilution of i in 

 20 will answer perfectly. The time-limit is the same. 



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 substance 

 which we believe to be the cause of the reaction (agglutinin) con- 

 tinues to be formed, or remains in the blood, for some time after 

 complete convalescence from typhoid fever ; the reaction has 

 been known to persist for seven or eight years, and probably 

 usually does so for about two. This fact must be remembered 

 in interpreting the results obtained from Widal's test. If the 

 patient has suffered from typhoid fever, or from an obscure 

 illness, which might possibly have been typhoid fever, a year or 

 two previously, the positive reaction should be regarded with 

 suspicion. 



In such cases the test should be carried out so that the smallest 

 dilution which will cause clumping can be ascertained, and the 

 test repeated in two or three days. If, for instance, we found 



