Routine two-step skin testing for tuberculosis in the staff of a geriatric hospital in Israel: booster and conversion rates

Abstract

The objective of this study was to determine the prevalence of positive skin tests amongst the staff of a 200 bed geriatric hospital in Haifa, Israel. By comparing the findings with those of a study performed five years previously, we hoped to ascertain the number of conversions which had occurred in the period studied. This was undertaken in order to assess a new policy from the Israel Ministry of Health regarding skin testing for health care workers. We also hoped to decide upon the frequency of skin testing required and to compare data from recent immigrants from countries with a high prevalence of TB. In 1997, we performed two-step skin testing (TSST) on 318 health care workers. We ascertained the number of positive reactions on the first and second testing and calculated the number of subjects who showed significant boosting. We also compared the results to those obtained in a study in 1992 and calculated the rate of conversion. We used multivariate analysis to examine the effects of age, gender, country of origin, years in Israel, previous BCG vaccination, previous exposure to contagious TB, work site and area of residence in the city, on the response to TSST. Between 1990 and 1996, 655 000 immigrants from the former USSR arrived; ‘recent immigration’ was defined from that date onward. The final number of positive reactions out of 282 subjects, who were either positive or negative on step 1 and presented for step 2, was 171 (60%).Booster effect was not significantly associated with any of the variables examined. The size of reaction in TSST was related to country of origin and recent immigration. The 83 recent immigrants from the former USSR had more frequent (61%) and larger reactions (mean (sd): 9.0 (6.46) mm) than the 114 native-born Israelis with 39% positive reactions (6.2 (5.89) P= 0.009). Comparison with 1992 revealed 26 (31%) of previous negatives as positive. Conversion was associated with age. All conversions save one were in individuals younger than 50 years (P= 0.07). In conclusion, TSST, performed to enable detection of recent infection after exposure to contagious TB, was relevant for 40% of health care workers (HCWs). Second step testing contributed an additional 23% positive reactions. New immigrants had larger initial reactions. Conversion occurred mostly in younger workers and could be either due to unrecognized TB in the hospital or to exposure in the community.