The purpose of tuberculin (PPD) skin testing is to identify people who have the tuberculosis germ in their bodies. Such persons may feel fine and have no signs of illness with tuberculosis. But these people may have enough germs to cause sickness later on.
BCG vaccination given during childhood makes it harder to interpret PPD tests. BCG is given to children in many countries where tuberculosis is common. BCG immunization helps protect children against severe symptoms and death from tuberculosis. BCG is not as helpful in protecting adults from tuberculosis.

A positive PPD reaction is a swelling in the skin detected two to three days after the test is applied. BCG vaccination may produce a positive reaction but only during a few years after the BCG was given, and the reaction is usually small. Skin reactions smaller than a dime are generally considered normal. Skin reactions a dime or more in size are considered positive and abnormal. A positive skin test means that the person has been exposed to tuberculosis and has this germ in his or her body. If this is the case, then the tuberculosis germ can become active at any time and cause illness.

Most adults who received BCG before age 7 who have a positive PPD skin test after age 21 have the tuberculosis germ in their bodies. This positive reaction is NOT usually because of BCG they received as a child.

If you have a positive PPD skin test, you should get a chest X-ray to look for active tuberculosis, since this germ most commonly affects the lungs. You also should report any symptoms that suggesting active tuberculosis. These symptoms include long standing cough, sweating at night or unexplained weight loss. If there is no cough and the chest xray is normal, then the condition is not likely to be contagious and won't be spread to others.

If you have none of these symptoms and have a normal chest X-ray, then you are not considered contagious and you are able to work or volunteer normally. Your doctor can write a note confirming this for anyone who needs this information.

As people get older, the tuberculosis germ can become active, start to grow and cause illness. Without preventive medication, 15 out of 100 people (15%) with a positive PPD will develop active tuberculosis at some time. With medication, only three in 100 people (3%) with a positive PPD will develop active tuberculosis.

If you have a positive PPD and normal chest xray, your doctor may recommend that you take medicine to prevent the tuberculosis from becoming active and making you sick. Medication, usually isoniazid (INH), is taken daily for nine months to prevent future symptoms of tuberculosis. While INH is generally quite safe, it can cause temporary changes in the liver.

Please talk with your doctor about your risks of developing tuberculosis symptoms and the possible benefits of medication to prevent illness in the future.