Twenty-year tuberculin testing in an institution for persons with mental retardation

Tuberculin Purified Protein Derivative skin test conversions and reversions were followed over 20 years in 363 patients in an institution for persons with mental retardation. Of these, 88.7% remained negative and 5.2% remained positive. Changes included 2.7% reverting to negative, 0.3% converting to positive, and 3.1% showing two to five negative-positive changes, with varying degrees of positivity over the years. Some patients showed only transient positive reactions. Most changes could not be explained by clinical evidence of infection, but multiple causes were felt to be likely. The discovery of converters was minimal, and the usefulness of routine skin testing in such institutions is doubtful.