Can you be infected with TB by taking the skin test? My family recently had TB skin tests. My 17-year-old fainted when we left the doctor's office, which is highly unusual for her. We returned to the office, and she was checked out and said to be fine. She did not feel well the rest of the day. Bruising occurred on her arm and not on anyone else's. I'm concerned that a vein was hit, and also about possible long-term consequences from the test.

J.B.F.

Tuberculosis, or TB, is a chronic infection that usually involves the lungs. It is caused by the bacterium Mycobacterium tuberculosis. At the time of first infection, there may be a mild flu-like illness. In most cases, the bacterium spreads to the upper parts of the lungs and to other parts of the body; however, the immune system keeps the infection in check. Later in life -- sometimes years later -- the infection can reactivate, most commonly causing typical pulmonary tuberculosis. In this illness, the patient has cough, fever and weight loss.

At the time of first infection, the immune system is able to control the infection after learning to recognize the bacterium. We test for this immune recognition with the PPD skin test. (Some people refer to this as the TB skin test or just the TB test.) A small amount of a protein derived from Mycobacterium tuberculosis is injected into the skin. After two or three days, a health professional examines the skin. A positive result occurs when there is a certain degree of induration (hardening) of the skin around the injection site. Most people with a positive test are indeed infected with Mycobacterium tuberculosis. This means that they are at risk for reactivation of disease at a later time.

Your daughter's reaction to her PPD may have no relation whatsoever to tuberculosis. I suspect your daughter had a "vagal" episode. These occur when pain or fear cause the blood pressure and heart rate to fall temporarily, causing a fainting spell. This is what happens when someone faints after seeing something terrifying. (Yes, sometimes this really does happen. It's not just in the movies.) The bruising could be due to improper injection of the PPD. The needle may have struck a vein and that may have been the reason she had more pain than is usual.

I would not expect your daughter to have any long-term consequences from this episode at all, and it certainly does not mean that she has TB. Remember, there are no active bacteria in the PPD, just a small amount of protein.