I had a positive TB skin test and negative chest X-ray almost six months ago, and have been taking medication (Isoniazid), one tablet a day. This month is my last month to take this medication. Am I totally cured of TB? Or will I need follow-up care?

Mary

Tuberculosis, or TB, is caused by the bacterium Mycobacterium tuberculosis (MTB). As I have written in this column in the past, TB infection takes several forms that affect the body in different ways. The most common form, which affects more than two billion people worldwide, is having MTB infection without any symptoms. Once TB has invaded the body, the immune system is "activated" by the organism, and the skin test for TB, called the PPD (for purified protein derivative), is positive.

Although the infection is usually controlled by the immune system, it can reactivate at a later time and cause disease, usually a chronic infection of the lung. This generally happens in about 10 percent of cases, with half of this risk occurring in the first two years after infection. Some people, however, such as those with AIDS or other illnesses that weaken the immune system, are at higher risk of developing disease symptoms -- active TB -- after they've become infected with the tuberculosis bacterium.

A positive skin test mandates getting a chest X-ray to rule out the possibility that active TB is already present. If the X-ray is negative, then the doctor will consider prescribing isoniazid (also known as INH). Taking INH for six to 12 months lowers the risk of developing active TB in most patients. However, the drug itself is not without risk. The main concern is that INH may cause liver damage, a risk that increases dramatically in older patients. INH is routinely given to people with a positive skin test who have a high risk of developing active TB. It is also given to people with a positive skin test who have a very low risk of INH-related toxic effects, such as children. Doctors sometimes elect not to prescribe INH to people who have a high risk of INH toxicity and a fairly low risk of developing active TB.

Taking INH does not totally eliminate the risk of developing active TB. Having taken INH for six months has lowered your risk substantially, but not to zero. There are a few things you should know. Since your risk of developing active TB is low, it is not cost-effective to screen you with chest X-rays for the rest of your life. However, if you develop a persistent respiratory infection, you should seek medical care -- perhaps sooner than you would otherwise -- to check for active TB. You should also know that future skin tests are of little value to you. A skin test only indicates that a person has been infected with MTB, and you already know that you are infected.