Mycobacterium tuberculosis infection in health care workers in rural India: comparison of a whole-blood interferon gamma assay with tuberculin skin testing

JAMA. 2005 Jun 8;293(22):2746-55. doi: 10.1001/jama.293.22.2746.


Context: Mycobacterium tuberculosis infection in health care workers has not been adequately studied in developing countries using newer diagnostic tests.

Objectives: To estimate latent tuberculosis infection prevalence in health care workers using the tuberculin skin test (TST) and a whole-blood interferon gamma (IFN-gamma) assay; to determine agreement between the tests; and to compare their correlation with risk factors.

Design, setting, and participants: A cross-sectional comparison study of 726 health care workers aged 18 to 61 years (median age, 22 years) with no history of active tuberculosis conducted from January to May 2004, at a rural medical school in India. A total of 493 (68%) of the health care workers had direct contact with patients with tuberculosis and 514 (71%) had BCG vaccine scars.

Interventions: Tuberculin skin testing was performed using 1-TU dose of purified protein derivative RT23, and the IFN-gamma assay was performed by measuring IFN-gamma response to early secreted antigenic target 6, culture filtrate protein 10, and a portion of tuberculosis antigen TB7.7.

Main outcome measures: Agreement between TST and the IFN-gamma assay, and comparison of the tests with respect to their association with risk factors.

Results: A large proportion of the health care workers were latently infected; 360 (50%) were positive by either TST or IFN-gamma assay, and 226 (31%) were positive by both tests. The prevalence estimates of TST and IFN-gamma assay positivity were comparable (41%; 95% confidence interval [CI], 38%-45% and 40%; 95% CI, 37%-43%, respectively). Agreement between the tests was high (81.4%; kappa = 0.61; 95% CI, 0.56-0.67). Increasing age and years in the health profession were significant risk factors for both IFN-gamma assay and TST positivity. BCG vaccination had little impact on TST and IFN-gamma assay results.

Conclusions: Our study showed high latent tuberculosis infection prevalence in Indian health care workers, high agreement between TST and IFN-gamma assay, and similar association between positive test results and risk factors. Although TST and IFN-gamma assay appear comparable in this population, they have different performance and operational characteristics; therefore, the decision to select one test over the other will depend on the population, purpose of testing, and resource availability.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Cross-Sectional Studies
  • Developing Countries
  • Female
  • Health Personnel*
  • Humans
  • India / epidemiology
  • Interferon-gamma / immunology
  • Male
  • Middle Aged
  • Mycobacterium tuberculosis / isolation & purification
  • Mycobacterium tuberculosis / physiology
  • Prevalence
  • Risk Factors
  • Rural Population
  • Tuberculin Test
  • Tuberculosis / diagnosis*
  • Tuberculosis / epidemiology*
  • Virus Latency


  • Interferon-gamma