I am a registered nurse who has had eight negative PPDs in the past three years. My current employer has issued mandatory testing for all employees only three months after my last PPD. I am concerned about the safety of so many tests. Do you have any information about repeated PPDs?

F.

For such a commonly administered test, the PPD seems to be frequently misunderstood. The PPD, which stands for purified protein derivative, is designed to detect infection with Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). When people are infected with Mycobacterium tuberculosis, they are usually asymptomatic. At some later time, the infection may reactivate, causing typical pulmonary tuberculosis, a chronic, progressive pneumonia.

Experts estimate that the lifetime risk of developing pulmonary TB after infection is about 10 percent. Half of this risk is in the first two years after infection. This is the main reason we screen certain people yearly with the PPD -- to detect recent infection. People who have been infected recently can take isoniazid (trade name INH), an antibiotic, to greatly decrease the risk of developing disease.

The PPD contains no live organisms, just purified proteins. Therefore, it is impossible to develop tuberculosis from the test. In addition, repeated testing does not "sensitize" the recipient and cause later tests to be positive. I have personally heard nurses and physicians mistake this point. Repeated testing does not cause a future test to be positive.

However, repeated testing can cause confusion about whether a person was infected recently or in the more distant past. As time passes after infection with Mycobacterium tuberculosis, the size of the PPD response, meaning the zone of induration (thickening) of the skin, wanes. In the first few years after infection, the zone of induration around the site of the PPD is usually more than 15 millimeters (0.6 inch) across. After many years without testing and without reinfection, the zone of induration will be smaller or even nonexistent. If the patient is tested again soon after the first test, the repeat test may be positive and the zone much larger. This is called the booster phenomenon. However, this does not occur unless the person was already infected withMycobacterium tuberculosis in the first place.

If your last negative test was your first test ever, and if you had a PPD three months later that was positive, you would not know for sure whether the second positive was due to recent infection or due to the booster phenomenon. In either case, you would truly be positive for Mycobacterium tuberculosis, but if the booster phenomenon is at work, then you would not be a "converter" or a new positive. If a physician assumed that you were a converter, he or she would also assume that you were at the highest risk of disease and might want to treat you with isoniazid unnecessarily.

We can avoid this confusion by performing a two-step test. The first time a person, say a health-care worker, is tested, he or she can be retested one week later. If both tests are negative, then the person is very likely uninfected. Any future positive test is likely to represent conversion and a new infection.