SERUM DIAGNOSIS 373 



it may safely be said that it lias been derived from a case of 

 typhoid fever, provided that the patient has not been inoculated 

 against typhoid, and has not previously suffered from the 

 disease. Suspicion should be entertained as to the diagnosis 

 if a lower dilution, or if a longer time is required. 



The methods by which the test can be applied have already 

 been described (p. 116). 



(1) It will be there seen that the loss of motility and clumping 

 may be observed microscopically. If a preparation be made by 

 the method detailed (typhoid serum in a dilution of, say, 1 : 30 

 having been employed), and examined at once under the micro- 

 scope, the bacilli will usually be found actively motile, darting 

 about in all directions. In a short time, however, these move- 

 ments gradually become slower, the bacilli begin to adhere to one 

 another, and ultimately become completely immobile and form 

 clumps by their aggregation. 'When this occurs the reaction 

 is said to be complete. If the clumps be watched still longer a 

 swelling up of the bacilli will be observed, with a granulation 

 of the protoplasm, so that their forms can with difficulty be 

 recognised. In a preparation similarly made with non-typhoid 

 serum the individual bacilli can be observed separate and 

 actively motile. 



(2) A corresponding reaction visible to the naked eye is 

 obtained by the " sedimentation test," the method of applying 

 which has also been described (p. 11*). The test in this form 

 has the disadvantage of taking longer time than the microscopic 

 method, but it is useful as a control ; in nature it is similar. 



Such is what occurs in the case of a typical reaction. The 

 value of the method as a means of diagnosis largely depends on 

 attention to several details. 



The race of typhoid bacillus employed is important. All races do not 

 give uniformly the same results, though it is not known on what this differ- 

 ence of susceptibility depends. A race must therefore lie selected 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 main- 

 tained 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. 



