Could You Tell Me More About TB Testing Results?

I had a tuberculin test done two years ago and it came back positive. My doctor requested an x-ray which I had done two years in a row and they both came back negative. My doctor is telling me to exercise and watch my diet, but what I really want to know is: Is there a chance that I might not have it? Your answer will help ease my concern. I used to work in health care.

The diagnosis of tuberculosis (TB) has always been complicated, and the interpretation of the tuberculin test, also known as the PPD test, is equally complicated. The American Thoracic Society published a new official position on this last year.

The Society coined a new diagnostic term, latent tuberculosis infection (LTBI) which is applied to someone who has a positive tuberculin test with no evidence of active tuberculosis. A chest x-ray is required. At the same time they pulled together many previous recommendations regarding PPD testing into one policy position defining high, medium and low risk groups, as follows:

* people at high risk of developing active tuberculosis if exposed to the causative bacteria. This group includes those with HIV infection, people on immunosuppressive medications, people who have been in close contact with someone known to have active TB, and those with a chest x-ray suggestive of TB. People in this group are considered positive if their PPD test measures 5mm.

* people who are in groups more likely to have active TB. This includes recent immigrants from poor countries, IV drug users, residents and employees of prisons, nursing homes and hospital employees, and people with a variety of other conditions making active TB more likely, such as silicosis, diabetes, leukemia and others. For people in this group, the test is considered positive if it measures 10mm.

* everyone else is considered to be at low risk, and routine PPD testing is not recommended. If such a person is tested, the test is only considered positive if it measures 15mm.

What does it mean to have LTBI? TB is almost always caught by breathing in droplets containing the TB germs that are floating in the air, coughed out by someone with active disease. These bacteria are trapped in the lungs where they grow and divide and stimulate a response by the immune system. Immune system cells literally eat the TB bacteria, but can’t kill them. Instead they are locked up in tiny nodules called granulomas, both in the lungs and in the regional lymph nodes. This is called a primary infection. The vast majority of people are not sick and experience no symptoms when this is happening, but the stimulation of the immune system means that they will test positive on the PPD test. After this primary infection, a person is considered to have the latent infection and has about a 10 percent chance of developing active disease during the rest of their lifetime. Anything that reduces a person’s immune response, including HIV infection, long-term cortisone treatment, chemotherapy and older age will increase the likelihood of the latent infection becoming active. For this reason treatment is now recommended for all persons diagnosed with LTBI.

We have no information to allow us to place you in one of the risk categories except that you used to work in a health setting. For you then, the PPD would be considered positive if it measured 10mm. You would be considered to have LTBI, and should be evaluated for treatment of that condition in order to reduce the possibility of your latent infection ever becoming active. Treatment with the antibiotic isoniazid (INH) for 9 months is now considered the standard of care, and will reduce the chance of you developing active disease by 60 to 80 percent.

You also ask if perhaps you are not really infected after all. Like all tests, the PPD has false positives and false negatives. It must be carefully applied and correctly read. Certain infections by bacteria related to the TB bacteria but which do not cause disease can make it positive. Such bacteria are common in rural areas of the U.S., so previous infection with one of those could have produced your positive test. Finally, vaccination with the TB vaccine known as BCG may produce a false positive test. This vaccine is not used in the U.S. Unfortunately there is no way to distinguish between a real positive test indicating LTBI and a false positive test from infection with another bacteria or prior BCG so I can’t suggest anything to help you answer the question you ask.

The information provided on Health Search Online is for educational purposes only and is not a substitute for medical advice, diagnosis or treatment.