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What Can the NHANES Data Tell Us about the Tuberculin Skin Test and the Risk for Active Tuberculosis?

Drs. Belknap, Wall, and Reves have presented in their letter an intriguing comparison of active tuberculosis (TB) cases reported in the United States in 2000 with the number of persons with latent TB infection (LTBI) that we estimated in the United States in our analysis of the NHANES data, collected in 1999–2000 (1). They point out that the ratio of TB:LTBI numbers for United States–born persons was almost twice that for foreign-born persons. We agree that one possible explanation of this observed difference between United States– and foreign-born persons might be that the tuberculin skin test (TST), used in the NHANES survey, overestimates LTBI prevalence among persons born outside the United States. In a meta-analysis of studies comparing TST with the newly developed interferon- release assay (IGRA), Menzies and coworkers concluded that IGRA is more specific than TST in studies of persons vaccinated with bacille Calmette-Guérin (BCG) (2).

Caution is advisable, however, when comparing data collected with two different approaches and subject to different potential sources of bias. The NHANES LTBI data come from a survey conducted among the civilian, noninstitutionalized population in the United States. Populations at high risk for LTBI, such as incarcerated individuals, the homeless, and persons in nursing homes, were not included in NHANES, and therefore the contributions of these populations to estimates of total numbers with LTBI are excluded. The U.S. TB incidence data include all cases reported during 2000. Thus, the numerator of the ratio calculated by Belknap and colleagues in their letter includes populations that are not included in the denominator. In addition, foreign-born persons are more likely to have benefited from LTBI screening and treatment programs than United States–born persons, which would reduce the likelihood of progression to active TB among persons with LTBI in the foreign born more than in those born in the United States. In the calculations by Belknap and colleagues, persons previously treated for LTBI are included in the denominator but, because of their treatment, have a reduced risk of being in the numerator. Nevertheless, we agree with Belknap and colleagues that to determine which of these possibilities is contributing most to the ratio they report, large prospective studies are required to determine the accuracy of the various tests for LTBI in predicting which persons will progress to active TB disease.

Thomas R. Navin

National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Centers for Disease Control and Prevention
Atlanta, Georgia

Jeanne M. Courval

National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Centers for Disease Control and Prevention
Atlanta, Georgia
and
Public Health Foundation Enterprises
City of Industry, California

José E. Becerra, Diane E. Bennett and Kenneth G. Castro

National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Centers for Disease Control and Prevention
Atlanta, Georgia

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

1. Bennett DE, Courval JM, Onorato I, Agerton T, Gibson JD, Lambert L, McQuillan GM, Lewis B, Navin TR, Castro KG. Prevalence of tuberculosis infection in the United States population: the National Health and Nutrition Examination survey, 1999–2000. Am J Respir Crit Care Med 2008;177:348–355.[Abstract/Free Full Text]
2. Menzies D, Pai M, Comstock G. Meta-analysis: new tests for the diagnosis of latent tuberculosis infection: areas of uncertainty and recommendations for research. Ann Intern Med 2007;146:340–354.[Abstract/Free Full Text]

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