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Lung diseases






Tuberculosis (TB) is caused by a specific germ that usually enters the body through the lungs but can spread just about anywhere. When a person is infected with the TB germ, a specific type of reaction normally occurs. This reaction can be seen in TB skin tests. Not all of these reactions are due to TB, however; for example, other germs related to TB can cause a similar, but usually smaller, reaction to the skin test. In the US, most healthy individuals will have a skin test of greater than 12 mm if they have actually been infected with TB. Certain persons, particularly those with immunity problems like AIDS will have a smaller test even if they are infected with TB, so the `cutoff` value is set lower, usually 5 mm. If your test was really 2.5 mm, this suggests a negative test and no treatment is indicated at all, assuming you are otherwise healthy. If it is very important to exclude TB (for example, in some work screening programs), a second TB test will be placed soon after the first one; this second test will pick up a few cases that are missed by the first one.

If a test is positive but there is no evidence of acute illness due to TB, some people may warrant prophylactic anti-TB therapy. Prophylactic means that the goal of treatment is preventing future disease, not the current infection. Usually, TB prophylaxis requires taking a medicine called isoniazid (often abbreviated INH) daily for 1 year. INH can cause many side effects, the most significant of which is liver problems. The risk of getting TB from an exposure (as indicated by a skin test) increases as the years go by; if you are older when you are exposed, chances are you won`t have as many years to develop the disease, so the benefit of taking prophylaxis drops compared to the risk of getting side effects. Most experts recommend that if you are younger than about 35, you should probably take prophylaxis. If you are older, other factors (such as your current health status) may be the deciding factor(s) regarding whether prophylactic therapy is a good idea or not.

All of this changes if there is evidence of active TB, such as an abnormal chest xray or some other health problem attributable to the germ. In these cases, treatment of active TB involves taking multiple medications for several months to a year or more. The drugs can have various side effects, including liver and blood abnormalities, nerve toxicities, damage to the eyes, etc., but it is important to remember that TB is a serious illness to yourself as well as those around you. By aggressively identifying those with TB infections, starting prophylaxis when appropriate and treating those with active tuberculosis, it is hoped that we will soon eliminate one of history`s most significant infections. The Center for Disease Control has an excellent website where you can find out more about TB, its causes and treatments, by following the link below.

Related Resources:

CDC TB Facts

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Response by:

Stuart Green, MD
Assistant Professor of Medicine and Pathobiology
College of Medicine
University of Cincinnati