Tuberculosis plays with us,
slips into the lungs, and rests quietly for most of those exposed. Eventually, some of those exposed develop active disease, leading to
death if not properly treated.
Tuberculin skin testing (TST--also called the Mantoux test) remains the only practical method for screening asymptomatic people for tuberculosis exposure. The test, however, has significant limitations.
The specifics of the test can be found in the writeup above; interpretation of the test, however, is confounded by the presence of atypical mycobacteria, bacteria that do not produce tuberculosis yet produce a reaction to the tuberculin. Even more confusing, the test is often negative--up to 10% of children infected with tuberculosis will test negative because of anergy.
People who have received the BCG vaccination will, in general, have some sort of reaction to a TST. The BCG vaccine, however, is not 100% effective in preventing tuberculosis. While the vaccine may help prevent miliary and meningeal tuberculosis, it is less clear how effective it is in preventing pulmonary tuberculosis. In areas where the prevalence of tuberculosis is high, getting the vaccine makes sense. Once one is vaccinated, however, the TST interpretation becomes problematic: is one reacting to the TST because of the BCG or because of actual exposure?
Why is it used? It's cheap, relatively easy to do, and (if one pays attention) not difficult to interpret, as long as one knows the limits of the test.
48-72 hours after the test is administered, one measures the width of induration, or hardness, transversely to the long axis of the arm. The area of redness means nothing. Induration by itself does not indicate a positive reaction.
If you are in an area with a low prevalence of atypical bacteria (Alaska, US, for instance), any reaction likely reflects real exposure to tuberculosis. In the southeastern United States, however, most (95%) of the children with an induration of 5-9 mm are not infected with tuberculosis.
The percentage of people with a given reaction (say 10 mm) who actually have tuberculosis depends on the prevalence of both the tuberculosis bacilli as well as the atypical mycobacteria in a community. What we want to know is the positive predictive value of the test, and the only way to know that is to know the rate of tuberculosis in the population being tested.
As a result, the Centers for Disease Control and Prevention in the United States has developed different definitions of a positive TST in different patient populations. If you were born in the States, have no known exposure to tuberculosis, and are older than 4 years old, any reaction less than 15 mm is negative. If that same child lives in a household where another family member has active tuberculosis, any induration of 5 mm or more is considered positive. If a child is at risk for the disease, a negative test does not exclude exposure. Confusing? You bet!
2000 Red Book: Report of the Committee of Infectious Diseases, American Academy of Pediatrics, Elk Grove Village, IL 2000.
Tuberculosis and Nontuberculous Mycobacterial Infections, 4th Edition,David Schlossberg, MD, FACP, W.B. Saunders Company, Philadelphia, PA , 1999.