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Today, the incidence of tuberculosis in the United States is the lowest ever recorded. This downward trend has been driven largely by steady decreases in incidence among persons born in this country. However, case rates have not decreased among foreign-born persons living in the United States, and tuberculosis in the foreign-born now accounts for most of the reported domestic cases. Latent tuberculosis infection is also more common among foreign-born persons: one third of the world's population, but less than 10 percent of the U.S. population, has latent infection.
In recent decades, the proportion of foreign-born health care workers in this country has increased. One fourth of all practicing physicians in the United States graduated from foreign medical schools, and the number of foreign-born nurses employed here has increased in response to a shortage of registered nurses. Although these health care workers play a crucial role in health care delivery, one consequence is that an increasing proportion of U.S. health care providers are infected with M. tuberculosis.
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The New York City case highlights these challenges. In 2003, a nurse from the Philippines who worked in a newborn nursery and maternity ward had pulmonary tuberculosis that went undiagnosed for two months. An opportunity to avert this situation had been missed 11 years earlier, when a routine preemployment tuberculin skin test was positive, but she declined treatment for latent tuberculosis infection. She reasoned that most adults from the Philippines who had positive tuberculin skin tests were not treated and that BCG vaccination at birth could explain the positive result.
Approximately 1500 persons had had contact with the nurse while she was considered infectious, but only one third of them could be located for follow-up. Among those located, at least four infants were found to be infected. A similar episode occurred recently in Boston, where the epidemiologic investigation involved more than 5000 potential contacts of a health care worker with pulmonary tuberculosis.
Driver et al. recently reported that in 2002, the incidence of tuberculosis among foreign-born health care workers in New York State was nearly 10 times that among U.S.-born health care workers in that state.2 In their study, most health care workers with tuberculosis had previously been known to have a positive tuberculin skin test, and one half had previously met the criteria for treatment with isoniazid, yet few had been treated. Clearly, treatment of latent tuberculosis infection should be provided to all health care workers who meet the established criteria (see table), though public health laws do not mandate such treatment in persons who decline it. After BCG vaccination at birth, tuberculin reactivity wanes within 6 to 12 months. Among persons who are vaccinated after 1 year of age, tuberculin reactivity may persist longer, but it rarely lasts more than 10 years if there is no subsequent infection with M. tuberculosis. Thus, previous BCG vaccination should generally not influence the interpretation of a tuberculin skin test in persons who were vaccinated more than 10 years earlier. However, foreign-born health care workers frequently attribute positive skin tests to BCG vaccination and are less likely to recommend isoniazid for themselves or for members of their family than for others.4
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The CDC recently established guidelines for use of the new assay.5 Previous BCG vaccination is less likely to lead to a false positive result on this test than it is on the tuberculin skin test. The new test may have lower sensitivity than the older test, but it is difficult to make comparisons because there is no gold standard for the detection of latent infection. There are limited data regarding the use of the QFT-G test in persons who have recently been exposed to tuberculosis, immunocompromised persons (including those infected with the human immunodeficiency virus), children younger than 17 years of age, populations with a high likelihood of harboring latent tuberculosis infection, and persons undergoing periodic screening. Nonetheless, the CDC indicates that the QFT-G test can be used in place of the tuberculin skin test in any circumstances, including contact investigations, evaluation of recent immigrants who previously received BCG vaccination, and screening of health care workers.5 Additional studies of the new assay are needed to assess its value for predicting the development of active tuberculosis.
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The recent contact investigations in New York City and Boston revealed that relatively few persons had become infected through exposure to the index patient, and to date, no secondary cases of active tuberculosis have occurred. A low frequency of adverse health outcomes often results in a perceived lack of urgency about treatment and makes it difficult to persuade people with latent tuberculosis infection to complete a course of treatment. However, both episodes demonstrate the tremendous potential for a bad outcome when infection is left untreated.
It is important that patients be able to trust their health care providers to "do no harm." All health care workers in the United States, regardless of their country of birth, must earn that trust by doing everything possible to minimize risk to patients. In this regard, assiduous adherence to the available guidelines for the prevention of transmission of M. tuberculosis in health care settings, including aggressive management of latent tuberculosis infection, is a crucial step in the right direction.
Dr. Sterling reports that he will receive research funding from Oxford Immunotec. Dr. Haas reports having received grant support from Schering-Plough. No other potential conflict of interest relevant to this article was reported.
Source Information
From the Division of Infectious Diseases, Department of Medicine, Vanderbilt University School of Medicine, Nashville.
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