SERUM DIAGNOSIS 339 



which gives the best result in the greatest number of undoubted 

 cases of typhoid fever, and which gives as little reaction as 

 possible with normal sera or sera derived from other diseases. 

 This latter point is important, as some races react very readily to 

 non-typhoid sera. Again, care must be taken as to the state of 

 the culture used. The suitability of a culture may be impaired 

 by varying the conditions of its growth. Continued growth of a 

 race at 37 C. makes it less suitable for use in the test, as the 

 bacilli tend naturally to adhere in clumps, which may be 

 mistaken for those produced by the reaction. Wyatt Johnson 

 recommended that the stock culture should be kept growing on 

 agar at room temperature and maintained by agar sub-cultures 

 made once a month. For use in applying the test, bouillon 

 sub-cultures are made and incubated for twenty-four hours at 

 37 C. The relation of the dilution of the serum to the 

 occurrence of clumping is most important. It has been found 

 that if the degree of dilution be too small a non-typhoid serum 

 may cause clumping. If possible, observations should always be 

 made with dilutions of 1 : 10, 1 : 30, 1 : 60, 1 : 100. To speak 

 generally, the more dilute the serum the longer time is necessary 

 for a complete reaction. Some typhoid sera have, however, 

 very powerful agglutinating properties, and may in a compara- 

 tively short time produce a reaction when diluted many hundreds 

 of times. With a too dilute serum not only may the reaction 

 be delayed, but it may be incomplete, the clumps formed being- 

 small and many bacilli being left free. These latter may either 

 have been rendered motionless or they may still be motile. No 

 diagnosis is conclusive which is founded on the occurrence of 

 such an incomplete clumping alone. Seeing that low dilutions 

 sometimes give a reaction with non-typhoid sera, it is important 

 to know what is the highest dilution at which complete 

 clumping indicates a positive reaction. The general consensus 

 of opinion, with which our own experience agrees, is that when 

 a serum in a dilution of 1 : 30 causes complete clumping in half 

 an hour, it may safely be said that it has been derived from a 

 case of typhoid fever. Suspicion should be entertained as to 

 the diagnosis if a lower dilution is required, or if a longer time 

 is required. 



The reaction given by the serum in typhoid fever usually 

 begins to be observed about the seventh day of the disease, 

 though occasionally it has been found as early as the fifth clay, 

 and sometimes it does not appear till the third week or later. 

 Usually it becomes gradually more marked as the disease 

 advances, and it is still given by the blood of convalescents from 



