title: Tuberculin skin testing in patients with HIV infection: limited benefit of reduced cutoff values.
abstract: BACKGROUND
  When determining eligibility for isoniazid preventive therapy of human immunodeficiency virus (HIV)-infected patients, the cutoff value of the tuberculin skin test (TST) is often reduced from an induration of 10 mm in diameter to one of 5 mm in diameter to compensate for loss of sensitivity. The effectiveness of this reduction depends on the underlying mechanism: a gradual decrease in skin test responsiveness with decreasing immunocompetence or an all-or-nothing switch to complete anergy. No published studies have assessed this directly in patients with tuberculosis.
  
  
  METHODS
  We performed a cross-sectional study of TST responses and HIV infection among patients with sputum smear-positive pulmonary tuberculosis in 6 hospitals in Tanzania. Skin test anergy was defined as a TST reaction < or =2 mm in diameter.
  
  
  RESULTS
  Of 991 patients with complete results, 451 (45.5%) had HIV infection. Anergy was observed in 111 (24.6%) of 451 HIV-infected patients and 18 (3.3%) of 540 HIV-uninfected patients (P<.001). The reaction size distributions among nonanergic HIV-infected and uninfected patients showed a limited difference (mean diameter +/- standard deviation, 15.9 +/- 5.0 mm and 16.8 +/- 3.8 mm, respectively; P=.048). The sensitivity of the TST among HIV-uninfected patients was 91.1% at a cutoff value of 10 mm and 95.2% at a cutoff value of 5 mm. The sensitivity of the TST among HIV-infected patients was 64.3% at a cutoff value of 10 mm and 71.2% at a cutoff value of 5 mm; the sensitivity of the TST was 67.6% and 74.5%, respectively, after adjustment for tuberculosis-specific anergy.
  
  
  CONCLUSIONS
  In subjects with tuberculosis disease and HIV infection, loss of TST sensitivity is predominantly attributable to anergy (i.e., an all-or-nothing phenomenon). The decrease in the proportion of false-negative TST results obtained by reducing the cutoff value from 10 mm to 5 mm is limited.
authors.affiliation: KNCV Tuberculosis Foundation, The Hague, The Netherlands. cobelensf@kncvtbc.nl
authors.forename: Frank G
authors.initials: FG
authors.lastname: Cobelens
authors.forename: Saidi M
authors.initials: SM
authors.lastname: Egwaga
authors.forename: Tessa
authors.initials: T
authors.lastname: van Ginkel
authors.forename: Hemed
authors.initials: H
authors.lastname: Muwinge
authors.forename: Mecky I
authors.initials: MI
authors.lastname: Matee
authors.forename: Martien W
authors.initials: MW
authors.lastname: Borgdorff
mesh_terms: D000293:Adolescent; D000328:Adult; D003430:Cross-Sectional Studies; D005260:Female; D015658:HIV Infections / Q000150:complications* / Q000453:epidemiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D013636:Tanzania / Q000453:epidemiology; D014374:Tuberculin Test / Q000592:standards*; D014376:Tuberculosis / Q000175:diagnosis* / Q000453:epidemiology