AGGLUTINATION WITH DRIED BLOOD. 343 



that low dilutions sometimes 1 give a reaction with non-typhoid 

 sera, great discussion has taken place as to what is the minimum 

 dilution at which, when complete clumping occurs, it may safely 

 be said that the reaction is due to the specific action of a typhoid 

 serum. 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. 1 



TJie Dried-blood Method. This method was introduced by 

 Wyatt Johnston of Montreal, and is especially useful for routine 

 work in health departments, where obviously critical methods 

 cannot be readily employed. As practised by health boards, 

 the physician is instructed to cleanse the lobe of the ear or the 

 tip of a finger of the patient with soap and water and alcohol, 

 to draw blood by pricking the part with a needle and permitting 

 a drop or two of blood to be deposited separately upon a pro- 

 vided sterile glass slide or aluminium strip and allowing the 

 blood to dry (in lieu of glass or aluminium, non-absorbent paper 

 may be used). Upon reaching the laboratory the sample is 

 covered by approximately five times its amount of water and 

 allowed to stand two minutes, then one loopful is removed to a 

 cover-slip and to it is added one loopful of the preparation of 

 typhoid bacilli, and the whole is treated as a hanging drop in 

 the manner carried out in quantitative examination. 



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 day, 

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

 Usually it gradually becomes more marked as the disease ad- 

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

 typhoid, but cases occur in which it may permanently disappear 

 before convalescence sets in. How long it lasts after the end of 

 the disease has not yet been fully determined, but in many cases 

 it has been found after several months at least. As a rule, up 

 to a certain point, the reaction is more marked where the fever 

 is of a pronounced character, whilst in the milder cases it is less 



1 By American observers it is usually conceded that a diagnosis may be made, 

 using a dilution of 1-50, with a time limit of two hours, without falling into error. 



