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 :—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 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 
