Late boosting phenomenon in TST conversion among health care workers

Background Available information is insufficient to guide determination of whether tuberculin skin test (TST) conversions of health care workers (HCWs) within 2 years of two-step testing are related to occupational exposures or to other causes, including late boosting. Aims To describe the epidemiologic factors of TST conversion in HCWs, comparing early TST conversion (≤ 2 years after two-step testing) with late conversion to possibly distinguish late boosting phenomenon from occupational TST conversion. Methods Retrospective analysis of a database of TSTs of HCWs from 1 January 1998, through 31 May 2014, in the United States Midwest. Results In total, 40 142 HCWs had 197 932 tests over the 16 years, with 123 conversions (conversion rate: 0.3%; 95% CI 0.3-0.4%). Among 61 HCWs with a negative two-step TST, 30 (49%) were found to have early TST conversion within 2 years; 31 (51%) had late conversion, with likely occupational exposure but no identifiable community risks. Persons with early conversion were more likely to be born outside the USA (89% versus 57%; P < 0.05), had a higher rate of prior bacille Calmette- Guérin (BCG) vaccination (89% versus 52%; P < 0.05) and had no identifiable risk factors for conversion (63% versus 58%; P ≤ 0.05). Conclusions Early conversions among HCWs after negative two-step TST are associated with various nonoccupational factors, including international birth and BCG vaccination history. Therefore, conversion is not a reliable indicator of recent tuberculosis contact in this population, and two-step TST is insufficient to discount a delayed boosting response for HCWs.