The effect of Schick testing on diphtheria antitoxin status.

Twenty-nine healthy persons were Schick tested as part of their occupational health examination. All but three of them had been previously immunised against diphtheria. One unimmunised person had a history of diphtheria. Blood samples were taken before and at varied intervals after the Schick tests in order to determine whether the Schick test antigen was immunogenic. Of the 29 persons tested, 21 were Schick-negative, three were Schick-positive. Four pseudoreactors were Schick-negative, one pseudoreactor was Schick-positive. Only four of the 29 had any significant rise in antitoxin titre after Schick testing. Three of these were Schick-negative, while one was a pseudoreactor who was negative at a later reading. We conclude that Schick testing is not reliably immunogenic and that, contrary to expectation, it cannot be assumed to elicit a useful booster response in previously immunised persons.