The ppd, normatively written as PPD, is a tuberculin test, a method of detecting infection with Mycobacterium tuberculosis by means of a tuberculin test, which is one of the auxiliary diagnostic indicators for the diagnosis of tuberculosis. Through the principle of immune response, tuberculin is injected into a person who may be infected with tuberculosis for 72 hours and the results are observed.
Generally, the body infected with tuberculosis bacillus will produce corresponding sensitized lymphocytes that can recognize tuberculosis bacillus, and when encountering a small amount of tuberculin or tuberculosis bacillus, the corresponding sensitized lymphocytes will be stimulated again by the same antigen, and the sensitized lymphocytes will secrete a variety of soluble lymphokines, leading to high vascular permeability, which will lead to the phenomenon of macrophage concentration in the local area, and thus the positive reaction of redness, swelling and hardness will appear. A positive reaction of erythema and nodules may occur.
When there is no redness, swelling or nodules, the reaction is negative. A negative reaction indicates that there are no active TB foci in the body or that the body is in the latent stage; a positive reaction indicates that the body has been vaccinated against TB or that there may be active TB foci in the body. A strong positive test indicates an active Mycobacterium tuberculosis infection in the body, but does not necessarily prove tuberculosis.
The PPD test can be used for diagnosis and differential diagnosis of infection with Mycobacterium tuberculosis, and can also provide a basis for BCG vaccination, as well as a basis for the effectiveness of immunization.