Choosing appropriate criteria for tuberculin positivity and conversion in a long-term care facility

The Centers for Disease Control and Prevention has issued new criteria for conversion of the tuberculin skin test; in persons over 35 years of age, an increase in induration of at least 15 mm is considered indicative of new tuberculous infection. We reviewed our experience in a tuberculosis control program in a long-term care facility to assess the applicability of the new criteria to our patient population.

Retrospective review of seven years of tuberculosis control records and outbreak investigation.

Long-term care Veterans Affairs hospital.

All patients in the facility between 1985 and June 1992 who received routine admission and annual tuberculin skin testing or who were evaluated for possible exposure to active tuberculosis. A total of 2,342 skin tests were performed.

Mean increase in skin test diameter in patients with at least two prior negative tests and known exposure to active tuberculosis was 13.9 +/- 4.7 mm. Frequency distribution histograms of skin test sizes of initial tuberculin testing in the entire population indicated 10 mm induration as a reasonable criterion for initial positivity.

In our long-term care population, an increase in skin test induration of 10 mm may indicate new tuberculous infection. Criteria for skin test conversion derived from ambulatory populations in other geographic areas may not apply in all situations. Prevalence of infection with Mycobacterium tuberculosis and prevalence of skin test reactivity due to nontuberculous mycobacteria are likely to influence the predictive value of criteria for tuberculin conversion in a given population.