NON-SPORING BACILLI 235 



Or the serum may be diluted by means of a graduated 

 pipette, such as a leucocytometer pipette, or by using a 

 capillary pipette which is rilled to a certain mark and 

 emptied into a watch-glass, and the desired number of the 

 full of the pipette of bouillon added and mixed. The 

 mixture of a pipette-full of the diluted serum and of the 

 bacillary emulsion is then examined, the final dilution 

 being double that of the diluted serum. 



b. Macroscopic method or Sedimentation. Take a range 

 of test tubes, 5 cm. x 0-5 cm., and put into each 1 ex. 

 of various serum dilutions and 1 c.c. of bacillary emulsion. 

 Also put up a control with 1 in 20 normal serum. Plug, 

 and keep vertical, and either incubate for three hours at 

 37 C, or keep at room temperature for twelve to twenty- 

 four hours ; or first incubate and then keep for twenty-four 

 hours, reading the results at both stages. The result can 

 be controlled by microscopic examination of the super- 

 natant fluid. 



The statement of the result should comprise all the 

 conditions of the experiment, namely : kind of test (hanging- 

 drop or tube), dilution of serum, times of observation, and 

 intensity of reaction (complete, medium, or nil agglutin- 

 ation). 



The test is also given by dead bacilli. Young cultures 

 are used to prevent spontaneous agglutination. It is not 

 always convenient to have young cultures, and so the 

 following suspension of dead bacilli (which keeps well but 

 agglutinates tardily though easily), may be used : To a 

 twenty-hours' broth culture, add 1 per cent formalin, 

 incubate for two days at 37 C, pour off the fluid from 

 the precipitate and store in an ice chest. 



Interpretation of Results. A positive Widal reaction 

 may be due to : (1) An attack of typhoid fever (especially 

 if increasing) ; (2) A previous attack ; (3) An attack of 

 paratyphoid fever ; (4) Some other disease (jaundice and 

 tuberculosis) . 



A negative Widal may be due to : (1) Too great or too 

 little infection ; (2) Disease not typhoid ; (3) Test applied 

 too early (before eighth day) ; (4) Inhibition phe- 

 nomena (some sera agglutinate in a dilution 1-100 and 

 not in 1-20). 



