[Analysis on the results of TB skin test in medical students]

To obtain baseline, medical students are recommended to make two-step tuberculin skin tests when they are in good physical health, as the baseline information to detect later tuberculosis infection. We investigated a method to obtain appropriate baseline data, because the accurate method of this test was not yet established.

The subjects were the tuberculin skin test results of 1066 medical students who were subjected to different methods of tuberculin skin test (58% tested once, 37% tested twice, and 5% tested three times). We retrospectively made multi-dimensional analysis about these data.

(i) In the first tuberculin skin test, 20% of the results were negative. (ii) When repeated with intervals between one to four years, the diameters of erythema gradually increased due to the effect of prior tests. (iii) The difference in size of erythema between the first tuberculin skin test and the repeated tests was less than 10 mm. (iv) When two-step tuberculin skin test was repeated, significant increase in the diameters of erythema were demonstrated in the second test (P = 0.0048). (v) Regarding booster phenomenon, it apparently lasted for one year, and it also remained after two years or over. CONSIDERATION: Thus, repeated tuberculin skin tests performed in good physical health was difficult to interpret measuring the diameters of erythema to detect tuberculosis infection. Thus, the diagnostic value of a tuberculin skin test was reduced while it requires unnecessary time and expenditure for its implementation.

It seems to be appropriate for medical students to make a two-step tuberculin skin tests, soon after their enrollment, and the results should be used as a baseline to detect possible later tuberculosis infection.