I tested positive for TB. My doctor says it's latent and that I should take medication for the next 12 months. The medication is very toxic to the liver, so I would have to have X-rays once a month for the next year. My mother said that I had this at birth and that all my family have it. None of them have been told to take anything, and it has not affected their lives. Should I take something that could be harmful to my body?

D.M.J.

There are few routine medical tests as poorly understood as the skin test for tuberculosis. This test, called the tuberculin test or PPD (for purified protein derivative) is designed to determine which people are infected with Mycobacterium tuberculosis (MTB), the organism that causes tuberculosis. (This point is frequently misunderstood by laypeople and even health professionals. It is often stated that a positive tuberculin test means that the person was "exposed" to MTB, but it really means they are infected with MTB.)

The tuberculin test is used as follows: A protein derivative (PPD) is injected into the skin. Forty-eight to 72 hours later, a health professional examines the skin. Generally, if there is more than 10 millimeters (about 0.4 inch) of induration (hardened skin), the test is positive. Over 90 percent of people with 10 mm of induration and virtually all with more than 15 mm (0.6 inch) of induration are indeed infected with MTB. There are, as in any test, some falsely positive tests and some falsely negative ones. If your tuberculin test was positive, you are probably infected with MTB.

Over two billion people in the world today are infected with MTB. Only a minority of these people will ever develop active tuberculosis. After infection in most people, the immune system adequately controls the organism. Later in life, up to 10 percent of people will develop active tuberculosis, usually in the form of chronic pneumonia. The highest risk of disease is in the first two years of infection.

The antibiotic isoniazid (INH) is given to some people with a positive tuberculin test. In most studies, INH has been shown to decrease the risk of developing active tuberculosis by up to 70 percent. As you mentioned, this drug can be toxic to the liver. This toxicity by no means happens in every patient. It occurs more frequently in older people. The risk of toxicity in those under 20 is very, very low, while in those over 65 it may be as high as 3 or 4 percent.

Guidelines for the treatment of people with a positive tuberculin test weigh a patient's risk of developing tuberculosis against the risk of liver toxicity from treatment. In general, those with the highest risk of disease (AIDS patients, those on certain drugs, malnourished people and people who have recently developed a positive skin test) should receive INH. People with a positive test who are younger than 35 (and therefore have a minimal chance of toxicity) should also be treated.

These are just guidelines, and each case should be handled individually. Most clinicians will monitor patients taking INH periodically. It is not generally recommended that every single patient receiving INH have liver testing every month, but it is often done. The thought is that cases of liver toxicity will be discovered early, before serious damage is done.

To address another part of your question, it is not possible to be born with a positive tuberculin test. However, you may have been infected with MTB at a very young age, even as a newborn. In undeveloped countries, most people are infected with MTB at a young age. Early infection is still prevalent enough in the United States that some pediatricians routinely perform the tuberculin test on all youngsters.