title: Influence of bacille Calmette-Guérin vaccination on size of tuberculin skin test reaction: to what size?
abstract: BACKGROUND
  Previous bacillus Calmette-Guerin (BCG) vaccination can confound the results of a tuberculin skin test (TST). We sought to determine a cutoff diameter of TST induration beyond which the influence of BCG vaccination was negligible in evaluating potential Mycobacterium tuberculosis infection in a population of health care workers with a high vaccination rate and low incidence of tuberculosis.
  
  
  METHODS
  From 1991 through 1998, all new employees at the University Hospital of Lausanne, Switzerland, underwent a 2-step TST at entry visit. We also gathered information on demographic characteristics, along with factors commonly associated with tuberculin positivity, including previous BCG vaccination, history of latent M. tuberculosis infection, and predictors for M. tuberculosis infection.
  
  
  RESULTS
  Among the 5117 investigated subjects, we found that influence of BCG vaccination on TST results varied across categories of age (likelihood ratio test, 0.0001). Prior BCG vaccination had a strong influence on skin test results of <or=18 mm in diameter among persons <40 years old, compared with the influence of factors predictive of M. tuberculosis infection. Prior latent M. tuberculosis infection and travel or employment in a country in which tuberculosis is endemic also had significant influences.
  
  
  CONCLUSIONS
  Interpretation of TST reactions of <or=18 mm among BCG-vaccinated persons <40 years of age must be done with caution in areas with a low incidence of tuberculosis. In such a population, except for persons who have never been vaccinated, TST reactions of <or=18 mm are more likely to be the result of prior vaccination than infection and should not systematically lead to preventive treatment.
authors.affiliation: Service of Infectious Diseases, University Hospital, Lausanne, Switzerland.
authors.forename: F
authors.initials: F
authors.lastname: Tissot
authors.forename: G
authors.initials: G
authors.lastname: Zanetti
authors.forename: P
authors.initials: P
authors.lastname: Francioli
authors.forename: J-P
authors.initials: JP
authors.lastname: Zellweger
authors.forename: F
authors.initials: F
authors.lastname: Zysset
mesh_terms: D000293:Adolescent; D000328:Adult; D000375:Aging; D001500:BCG Vaccine / Q000008:administration & dosage* / Q000276:immunology*; D015331:Cohort Studies; D004641:Emigration and Immigration; D005260:Female; D006282:Health Personnel; D006801:Humans; D008297:Male; D008875:Middle Aged; D016017:Odds Ratio; D011446:Prospective Studies; D012867:Skin / Q000276:immunology / Q000473:pathology; D014195:Travel; D014374:Tuberculin Test / Q000592:standards*; D014376:Tuberculosis / Q000175:diagnosis* / Q000276:immunology / Q000517:prevention & control
chemical_list: D001500:BCG Vaccine