It is a well accepted truth among laboratory workers that biological methods of diagnosis are only relatively specific. This limited usefulness of biological tests is true not only for such as the Abderhalden reaction, for instance, which is still in process of development, but equally true when applied to such widespread tests as Widal and Wassermann reactions.
Theoretically, inasmuch as these reactions depend on the specific biological processes taking place under the conditions of the disease, the biological methods of diagnosis should be strictly specific. The ideal method will be the one by which positive results are obtained in 100 per cent, of cases presenting symptoms of the disease, at the same time giving uniformly negative results on examination of individuals not affected by the disease.
The accumulated experience of a number of investigators through a period of years has shown, however, that practically every method is subject