© 2008 American Thoracic Society
What Can the NHANES Data Tell Us about the Tuberculin Skin Test and the Risk for Active Tuberculosis?To the Editor:We read with interest the article by Dr. Bennett and colleagues describing the prevalence of latent tuberculosis infection (LTBI) in the United States based on the NHANES 1999–2000 tuberculin skin-test (TST) data (1), and we wanted to examine the relationship between the population estimates for LTBI and the annual cases reported with active tuberculosis (TB). In 2000, active TB was reported in 8,714 United States–born persons and 7,554 foreign-born persons (2). Using the LTBI populations as the denominator, we calculated the case rates of active TB per 100,000 persons with LTBI stratified by age for both United States– and foreign-born populations (see Table 1). The annual incidence rate was 1.9-fold higher among the United States–born than foreign-born persons, and higher rates were observed for every age group except those 65 years and older, a strata with wide confidence intervals for the LTBI prevalence estimate. We hope the authors could address whether a twofold error in the LTBI prevalence as estimated by NHANES could account for these observations.
We recognize that not all TB cases in the United States arise from prevalent LTBI infection, but we believe that the small proportion of cases from recent infection or occurring in foreign-born persons who arrive with active TB is unlikely to account for the difference. One possible explanation is that the TST may be overestimating the LTBI prevalence in the foreign born, as suggested by studies using interferon-  release assays (IGRAs) (3, 4). We would not suggest that these data mean that the United States–born population is at greater risk of TB than the foreign-born population or that the TST should not be used in the foreign-born. These observations do support the recognized need for a newer diagnostic test that better predicts who is at risk for active TB. IGRAs offer that potential (5), but until resources are available to conduct large prospective studies, the true accuracy of IGRAs remains unknown, thus delaying wide-scale implementation and potentially progress toward TB elimination.
Denver Public Health FOOTNOTES Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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