Medical Edge — TB testing encouraged even after positive result

Tribune Media Services


DEAR MAYO CLINIC: My daughter’s doctor said her PPD was positive and sent her for a chest X-ray to rule out TB. Is it possible to have a false-positive result? Her pediatrician says she should follow up with an Infectious diseases doctor and start on medication, even if her chest film is clear. Should she continue to have PPD tests in the future? Do they affect her chances of activating TB?

The test used to help evaluate for tuberculosis (TB) infection involves injecting a substance called purified protein derivative (PPD) tuberculin within the skin of your inside forearm. Health care providers determine if the tuberculin skin test is positive or negative by the appearance and size of a raised bump (induration) produced 48 to 72 hours after the test. The reaction isn’t an infection; it is the individual’s immune system response to the test.

A positive tuberculin skin test usually means the person either currently or previously was infected with the bacteria that cause tuberculosis — Mycobacterium (M.) tuberculosis. But, the test results don’t separate inactive infection from active tuberculosis disease. Individuals who have a positive result should be evaluated for active tuberculosis. "M. tuberculosis" infection without evidence of the active disease is called latent tuberculosis infection (LTBI). Since individuals with LTBI do not have active disease, they are not contagious to others. LTBI is generally treated with single-drug therapy to destroy dormant or residual bacteria and minimize the risk for developing tuberculosis in the future.


False-positive tuberculin skin test results are possible. Potential sources for false-positive results include previous administration of bacille Calmette-Guirin (BCG), a tuberculosis vaccine, especially if it’s been given recently. BCG vaccine is common in countries with high rates of active tuberculosis, but it’s not routinely used in the United States. Other reasons for a false-positive include infection with other mycobacteria that don’t cause TB, hypersensitivity reactions to the tuberculin skin test protein, and errors in interpreting the test result.

For individuals who suspect a false-positive tuberculin skin test result, two options exist. The test can be repeated and read by a health care provider specifically trained in its interpretation. The second option is a relatively new blood test, known as an interferon gamma release assay. This test uses a blood draw to identify and distinguish infection with "M. tuberculosis" from most non-tuberculosis mycobacteria and BCG vaccination. It is more expensive, however, than the standard skin test.

Once a person has a true-positive tuberculin skin test, the immune system remains sensitized and future results will generally remain positive. Thus, retesting in the future is not necessary. Having a tuberculin skin test does not increase the risk of activating tuberculosis. — John Wilson, M.D., Infectious Diseases, Mayo Clinic, Rochester, Minn.

Readers: Here’s yet another item to add to the long list of health problems that can be caused by smoking cigarettes: Among those with degenerative or wear-and-tear knee arthritis (osteoarthritis), smokers have greater cartilage loss and more severe knee pain than do nonsmokers. That’s according to a study led by Mayo Clinic researchers and published in a recent issue of "Annals of the Rheumatic Diseases."

The study involved 159 men who were at least 45 years old and had knee pain from osteoarthritis. Nineteen men, or 12 percent of the total, were still smoking. At the start of the study, magnetic resonance imaging (MRI) scans were taken of each participant’s more painful knee. In addition, the men rated their pain on a pain scale. Researchers followed up with additional MRIs on each of the men at 15 months and 30 months. Pain ratings were also noted at these intervals.

After adjusting for factors that may have influenced cartilage damage, such as weight and age, researchers found that smokers had about two-and-a-half times greater risk of cartilage loss, compared with those who had never smoked or stopped smoking. Smokers also reported higher pain scores. — Adapted from "Mayo Clinic Health Letter"

E-mail a question to , or write: Medical Edge from Mayo Clinic, c/o TMS, 2225 Kenmore Ave., Suite 114, Buffalo, N.Y., 14207.

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