WIDAL'S METHOD. 93 



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 one in twenty will answer perfectly. The 

 time limit is the same. 



INTERPRETATION OF RESULTS. 



A positive result may mean: i. 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 hypothe- 

 tical substance which we believe to be the cause of the 

 reaction (agglutinin) continues 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 that the blood clumps only in 

 a dilution of one in twenty on one day and in a dilution 



