Late boosting phenomenon in TST conversion among health care workers

W. H. Farah, L. E. Breeher, R. D. Newcomb, Mohammad H Murad, A. I.S. Vaughn, P. T. Hagen, R. G. Molella

Research output: Contribution to journalArticle


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.

Original languageEnglish (US)
Pages (from-to)484-489
Number of pages6
JournalOccupational Medicine
Issue number6
StatePublished - Aug 1 2017



  • Boosting phenomenon
  • Health care workers
  • Latent tuberculosis
  • Skin test

ASJC Scopus subject areas

  • Public Health, Environmental and Occupational Health

Cite this

Farah, W. H., Breeher, L. E., Newcomb, R. D., Murad, M. H., Vaughn, A. I. S., Hagen, P. T., & Molella, R. G. (2017). Late boosting phenomenon in TST conversion among health care workers. Occupational Medicine, 67(6), 484-489.