The frequency profile of tuberculin skin testing among students in nursing school

Abstract

The frequency profile of tuberculin skin testing (TST) among students in nursing school was studied. Students received a TST upon matriculation. The TST was done by the method of Mantoux, in which 0.1 ml of PPDs was administered intradermally, and the diameters of skin rash and induration were read by the medical doctor at 48 hours. When TST results are negative--that is, the diameter of skin rash is below 10 mm (in Japan, the TST results are judged by skin rash diameter rather than that of induration)-BCG vaccination is given. Those receiving the BCG vaccination are retested with a TST one year later. When the second TST was also negative both the BCG vaccination and TST were followed for two more years. Those students testing TST-negative are not permitted to take clinical training in the tuberculosis ward. Student's mean age on entrance was 18.6 +/- 2.1 years old, and all but three were female. About 70% of students entering in 1996 to 1998 had a history of previous BCG vaccination. In 14% their positive TSTs could be attributed to probable infection with tuberculosis in childhood. In the remaining 16%, details as to TST and BCG vaccination status are unknown. The frequency distribution of TST results was bimodal, showing one peak at 6 mm and another at 12 mm (skin rash diameter). The percentage of negative and positive reactors are 47.1% and 52.9%, respectively. The TST-negative students entering in 1994 to 1996 were given the BCG vaccination. Twenty-four of 134 students (17.9%) remained negative at the second TST, and 6 students (4.5%) at the third year, even after two repeated BCG vaccinations. The TST results were chronologically observed in the above 6 students after BCG vaccination. The TST results of two students showed positive in September, 1996 and June, 1997. While four students showed positive in September, 1996, all ultimately reverted to negative when retested in June, 1997. Those students had negative results for TST at the initial test in 1998 had the two step-tuberculin skin testing. All eight students with negative TST had the history of BCG vaccination. The second TST showed positive except one student whose scar after BCG vaccination was not observed on the arm. The TST is currently recommended in hospital tuberculosis-control programs. If TST-negative, medical staff and students may not work in the tuberculosis ward. However, after BCG vaccinations is given, and subsequent TST conversion is confirmed, they are then able to work or to have training in the ward. From our results, there is 4.5% non-convertors even after 2 years of repeated BCG vaccinations. However, these non-converters turned positive four months after BCG vaccination, only to revert to negative nine months later. These students are considered to have delayed hypersensitivity to PPD after BCG vaccination. However, their reactivity waned in the short period of nine months after the conversion of their TST's. Therefore, it is concluded that non converters after repeated BCG vaccinations are able to have clinical training in the tuberculosis ward as long as their BCG vaccinations are correctly administered and any immunological deficiencies are ruled out.