Tuberculin skin testing in patients with HIV infection: limited benefit of reduced cutoff values.

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. CONCLUSION: 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.

The impact of the HIV epidemic on tuberculosis transmission in Tanzania.

OBJECTIVE: To assess the impact of the HIV epidemic on tuberculosis transmission in Tanzania by estimating the trend in annual risk of tuberculosis infection (ARTI) over the period 1983-2003. DESIGN: Tuberculin survey among school children aged 6-14 years, randomly selected by cluster sampling. METHODS: Primary outcome was the ARTI among children without a BCG vaccination scar. To obtain time trends, data were reanalysed from three previous surveys carried out at intervals of 5 years since 1983, using identical methods and definitions. RESULTS: Of 96,226 children included in the analysis (74% of those enrolled), 10,239 (11%) had no BCG scar. The ARTI was 0.68% (95% confidence interval 0.55-0.81). Despite a doubling of notification rates of smear-positive tuberculosis since 1983, this represents an average annual decline since the first survey of 2.7% (P < 0.001). The declining trend in ARTI was observed in 17 of 20 regions, with no association between this trend and region-specific prevalence of HIV infection among patients with tuberculosis (P = 0.575). A similar decline in ARTI was observed among children with a BCG scar and for various ways of estimating the prevalence of tuberculosis infection from the distribution of skin test reactions. CONCLUSION: Despite substantial increases in tuberculosis incidence, the overall population-level effect of the HIV epidemic on tuberculosis transmission in Tanzania has been limited. This suggests that in the presence of a strong control programme, the HIV epidemic has limited impact on tuberculosis transmission.