[Cutaneous reactions to tuberculin and BCG vaccine]

The only available method for quantitative exploration of skin allergy to tuberculin is intradermal injection, since there is no other satisfactory method to evaluate the induration observed. Below 5 mm the induration has no significance. Between 5 and 10 mm it corresponds to an allergic reaction induced by BCG vaccine or by usually inapparent infections due to atypical mycobacteria. A diameter of 10 mm or more suggests either M. tuberculosis infection or allergy to BCG. The intensity of allergic reaction varies under the influence of numerous factors which may be genetic or nutritional, or associated with viral infections, antiviral vaccines and medicinal treatments. Exploring this allergy is of interest on the scores: (1) for epidemiological purposes, to obtain information on the annual risk tuberculosis infection or on the quality of the protection conferred by the vaccine in a given population; (2) for diagnostic purposes, to detect tuberculosis, and especially primary tuberculosis in children; (3) for vaccinal purposes, to determine the limitation of BCG. In subjects who have been vaccinated tuberculosis is rare but remains possible, and in such cases reactions to tuberculin may be difficult or even impossible to interpret.