Diagnostic methods, chemical, bacteriological and microscopical : a text-book for students and practitioners . he fact, THE BLOOD. 563 first shown by Widal, that cultures of the bacillus typhosus which had beenkilled by heating to 60 ° C. for 45 minutes did not lose their capacity to aggluti-nate when brought into contact with suspected serum. For this reason, deadcultures of typhoid bacilli, which have been killed by heat or by various chemicalagents, have been introduced for the use of the general worker. The use ofsuch dead cultures does not seem in any way advisable to the writer, as such
Diagnostic methods, chemical, bacteriological and microscopical : a text-book for students and practitioners . he fact, THE BLOOD. 563 first shown by Widal, that cultures of the bacillus typhosus which had beenkilled by heating to 60 ° C. for 45 minutes did not lose their capacity to aggluti-nate when brought into contact with suspected serum. For this reason, deadcultures of typhoid bacilli, which have been killed by heat or by various chemicalagents, have been introduced for the use of the general worker. The use ofsuch dead cultures does not seem in any way advisable to the writer, as such amethod entirely eliminates the factor of diminished or lost motility, which issuch an important one in deciding as to the character of the reaction. Thesetests are always macroscopic in character; that is, the suspension of typhoidbacilli gradually settles out in tubes on the addition of the suspected serum,so that the reaction may be observed by the naked eye. It has been shown that the healthy serum of many patients or that ofsubjects suffering with diseases other than typhoid fever frequently shows a. Fig. 153.—A positive Widal reaction. {Da Costa.) reaction similar to that given by typhoid fever. This statement is true onlywhen the question of dilution of the serum is not taken into account. Thecharacteristic Widal reaction is shown when the blood is diluted i to 50 withwater or, in many cases, at very much higher dilutions, while in other diseasesand in healthy subjects the reaction practically disappears at a dilution of i to20. The element of time at which this reaction is evident is also of greatimportance in judging the result. While the reaction in cases of typhoid fevermay occur instantly in a dilution of i to 10 or i to 20, and in dilutions of ito 50 it may take one-half to three-quarters of an hour, yet a period rangingfrom one-half to one hour is necessary with other sera at a dilution of i to 10or I to 20. In order, therefore, to demonstrate the specific typh
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