False-positive tuberculin skin tests in a state prison system

OBJECTIVE: To assess the existence and extent of false-positive tuberculin skin test (TST) results in a regularly tested population subsequent to switching to a different skin testing product. METHOD: Over 9300 state prison inmates were tuberculin skin tested as part of a routine annual testing program. A shortage of Tubersol(R) caused the prison system to purchase and test with Aplisol(R). Prison health services staff reported an apparent increase in skin test positivity using Aplisol(R) compared to Tubersol(R). Record reviews were conducted in 34 prisons where inmates read as positive (greater than or equal to5 mm) were retested with Tubersol(R). RESULTS: Among 368 inmates recorded as positive (greater than or equal to5 mm) with Aplisol(R) who were retested with Tubersol(R), 150 (40.8%) were read as negative (0-4 mm). CONCLUSION: The tuberculin skin test is a valuable tool in evaluating patients for TB infection. However, variations in planting and reading the test, and in the consistency between lots and manufacturers, can contribute to errors in determining an individual's infection status. The entire clinical and epidemiological picture for each patient must always be evaluated using the TST as a tool, and not an indisputable answer.