Latent Tuberculosis

Posted by Graham McMahon • April 15th, 2011

The latest article in our Clinical Practice review series, Latent Tuberculosis Infection in the United States, comes from Drs. C. Robert Horsburgh, Jr. of Boston University School of Public Health and Eric J. Rubin of Harvard School of Public Health.

There is no way to detect the presence of latent Mycobacterium tuberculosis in an individual patient directly. Instead, the assessment of latent infection relies on measurement of host immune responses as a surrogate for the presence of viable bacteria, an imperfect approach. Until recently, the only test for latent tuberculosis infection was the tuberculin skin test.

Clinical Pearls

What is the prevalence and activation rate of latent tuberculosis?

Data from a representative survey of the U.S. population showed that 4.2% of persons who were screened with this test during 1999 and 2000 had latent tuberculosis infection. The lifetime risk of the development of active disease in an immunocompetent person with latent tuberculosis depends on demographic and clinical characteristics and ranges from 1 to 13%.

How should a patient with latent tuberculosis be managed?

Randomized trials have shown that treatment is highly effective, with approximately 90% protection provided by completion of a 9-month course of isoniazid and 60 to 80% protection provided by completion of a 6-month course. The second recommended regimen — administration of rifampin for 4 months — has not been directly evaluated, but in one randomized trial in which patients received rifampin for 3 months, 60% protection was conferred.

Table 3. Drug Regimens for the Treatment of Latent Tuberculosis Infection.

Morning Report Questions

Q: According to the authors of this paper, interferon-(gamma)-release assays (IGRAs) are recommended for screening for latent tuberculosis for what groups of patients?

A: The authors prefer the use of IGRAs when the prevalence of recent infection is likely to be high: these populations include close contacts of patients with active tuberculosis, recently arrived foreign-born persons, drug users, incarcerated persons, and homeless persons. IGRAs are also of value for screening persons who have received the BCG vaccine.

Q: Tuberculin skin testing is recommended for screening for latent tuberculosis for what groups of patients?

A: For populations in whom prevalence is low or infection is likely to have been remote, the authors prefer the tuberculin skin test because IGRAs have either not been well studied (e.g., among smokers or persons with diabetes mellitus) or have performed poorly (e.g., among health care workers).

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