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Tuberculosis

LTBI in Latin American countries

02/15/2007

Question:

Which is the value of mantoux > 15 mm in people living in countries where the TB infection is considered endemic? It´s valid to treat whith prophylactic INH, but people in my country said that the rise of resistance to drugs is posible if you treat this people. Is recomended to treat with prophylactic INH acodring the recomendations of CDC in US or is diferent for countries like India or Central America where the disease is considered endemic?

Answer:

The purified protein derivative (PPD) tuberculin skin test is a screening test used to detect exposure to tuberculosis. Like any screening test, the values used to determine a positive or negative result are based on (1) the prevalence of tuberculosis in the region where patients are screened, and (2) the sensitivity and specificity of the test, in this case PPD. In the United States, the prevalence of M. tuberculosis infection is low (5 - 10%).

As a result, the PPD has a low positive predictive value (The number of true positives among those who test positive). To address this high false positive rate (and increased potential for adverse effects from treatment), the current 3-tiered PPD cutoff values were introduced with lower cutoffs for persons with a greater likelihood of being truly positive: > = 5mm for HIV+ persons, recent contacts of persons with known TB, fibrotic changes on CXR consistent with old TB, and patients with organ transplants and other immunosuppressed patients > = 10mm for recent arrivals into the US (< 5 years) from high prevalence countries, injection drug users, residents and employees of high-risk congregate settings, mycobacteriology laboratory personnel, persons with clinical conditions that make them high-risk, and children < 4 years of age or exposed to adults in high-risk categories >= 15mm for persons with no risk for Tb.

In addition, the Centers for Disease Control and Prevention, US Public Health Service and the American Thoracic Society do not recommend placing a PPD on persons who are not already suspected of being exposed. If you are in a country where tuberculosis is endemic, the 15mm cutoff is less helpful because false positives are significantly less. 5mm should be the cutoff if the patient has been exposed to recent contacts with TB.

Most importantly, the reason for screening for latent tuberculosis is to lessen the burden of disease among the community and to decrease the chances of an epidemic. The risk of drug resistance is greatest when patients with tuberculosis do not complete their full course of therapy. This risk drops significantly in the setting of direct observed therapy where patients are provided with their medications daily and observed while they ingest the pills.

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Response by:

J Daryl   Thornton, MD, MPH J Daryl Thornton, MD, MPH
Assistant Professor of Medicine
School of Medicine
Case Western Reserve University