By Martha A. Simpson, D.O., M.B.A.
Associate Professor of Family Medicine
Ohio University College of Osteopathic Medicine

SOME PEOPLE WITH NON-ACTIVE TB STILL NEED TO TAKE MEDICATION

Question: I recently had a tuberculosis skin test for work and it was positive. I had a chest X-ray done and it was negative. My doctor said I didn’t have tuberculosis, but I needed to take medicine for about 6 to 9 months. This doesn’t make sense to me. If I don’t have TB, why do I need to take medicine?

Answer: This is an excellent question about a positive tuberculin skin test (TST). First of all, the abbreviation of TST is new. We used to refer to this skin test as a PPD, which stands for purified protein derivative. This makes sense because TST is usually performed by injecting a small amount of PPD antigen into the skin. I suspect that we will use both names for a few years.

A TST is positive in an otherwise healthy person if within 48 to 72 hours you develop a lump at the test site that measures at least 15 millimeters. If you have certain chronic medical conditions like diabetes, AIDS or cancer, to name a few, the test is positive even if the lump is as small as 5 to 10 millimeters.

A positive result means that you have been exposed to tuberculosis and have been infected with the tuberculosis bacillus (a type of bacterium). The negative chest X-ray means you do not have active TB and are not contagious. You have what is sometimes called “latent TB.” Most people in this category do not go on to develop active TB. Only about 10 percent of those with latent TB ever develop an active infection. It is this 10 percent risk that warrants treatment.

If an individual like yourself is treated for six to nine months with a medication called isoniazid (INH), the lifetime risk for developing active TB drops to about 3 percent. INH is a strong medication that can have some side effects and requires regular blood tests to check your liver function. Generally, INH is a pill that is taken once a day.

After this treatment is completed, it is important to get regular checkups and chest X-rays to be sure there is no active disease. You can no longer be screened with the TST since it will always be positive. Active TB has non-specific symptoms such as fatigue, general malaise, weight loss, night sweats, and sometimes, a cough that won’t go away. If active TB develops, a person commonly needs four medications to cure the infection, so treating the positive TST to prevent full-blown TB is the best thing to do.

Finally, not everyone is a candidate for the INH treatment for a positive TST. Care is individualized based on age, underlying physical status and likelihood of completing the course of treatment. Stopping the INH too soon can increase your odds of getting active TB.

I should add that a new test that involves taking a blood sample has been approved for TB screening and may become more widely used in the near future. It’s called the QuantiFERON TB Gold test. It will likely be more expensive than the TST, but may be beneficial in cases where it is difficult for the person being tested to return for a reading within the necessary time frame.

Family Medicine® is a weekly column. To submit questions, write to Martha A. Simpson, D.O., M.B.A., Ohio University College of Osteopathic Medicine, P.O. Box 110, Athens, Ohio 45701, or via e-mail to readerquestions@familymedicinenews.org. Medical information in this column is provided as an educational service only. It does not replace the judgment of your personal physician, who should be relied on to diagnose and recommend treatment for any medical conditions. Past columns are available online at www.familymedicinenews.org.