Medical Edge Newspaper Column

TB Testing After Positive Result

October 1, 2007
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?

Answer:
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-Guérin (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.