Monaldi Archives for Chest Disease vol:57 issue:5-6 pages:237-41
We investigated reactivity to tuberculin in a group of 91 elderly subjects (65 years old and more) and in a control group of 53 younger subjects between 18 and 59 years old, by means of the tuberculin skin test (TST) and by an in vitro lymphocyte stimulation test (LST). The LST had been validated in preliminary tests to accurately reflect T cell reactivity towards tuberculin. We defined a LST with a stimulation index (SI) (ratio of the tuberculin stimulated cultures to the non-stimulated cultures) of 10 or more at a concentration of 0.4 microgram/ml as indicative for sensitisation to tuberculin. The correlation between TST and LST was r = 0.58 (p = 0.0001) in the elderly subjects and r = 0.64 (p = 0.0001) in the younger age group; age did not affect those correlations (p = 0.39). Of the subjects with initially negative TST, 44.4% (16 LST-positive and 16 LST-negative) reacted positive on TST as well as on LST after repeated testing. We conclude that (a) there is a statistically significant but rather weak correlation between in vivo and in vitro assays of cell-mediated immune response to tuberculin, (b) age did not influence this correlation, (c) the in vivo and in vitro tests give complementary information but cannot substitute for each other and (d) repeated TST can induce an immunological boosting phenomenon on TST as well as on LST.