Reasons you may not be able to have the test or why the results may not be helpful include:
A BCG (bacille Calmette-Guérin) vaccination. If you have had a BCG vaccination, you may have a positive PPD skin test even though you don't have TB.
Taking medicines that suppress the immune system, such as corticosteroids.
Conditions that weaken the immune system, such as HIV infection or cancer. The result also may be affected if a person is severely malnourished.
Some vaccinations for infections, such as measles, mumps, rubella, polio, or chickenpox, given within 6 weeks before the tuberculin test. A recent infection with one of these viruses can also interfere with test results for a short period of time. The skin test also may be positive if the person has an infection caused by a mycobacterium other than the one that causes TB.
A very recent TB infection. It takes 2 to 10 weeks for the immune system to react to TB bacteria.
Age younger than 3 months old. A baby's immune system is not fully developed at this age.
A "booster effect." This tends to occur in people who get regular TB skin tests, such as health care workers. The booster effect is a weak or no reaction to one TB skin test followed by a strong reaction from a second test. But the strong reaction to the second test does not mean that the person has just become infected with TB. Instead, the reaction may be because of:
A TB infection that occurred long ago. Over time, a person's immune system tends to stop reacting strongly to the TB bacteria. So the first TB skin test stimulates (boosts) the immune system, which then reacts strongly to the second skin test.
The BCG vaccine, which contains bacteria similar to the bacteria that cause TB. A person's immune system does not react as strongly to the vaccine bacteria, and this tends to diminish over time. So the person's immune system is boosted by the first skin test and reacts strongly to the second test.
Infection with bacteria similar to Mycobacterium tuberculosis. This may also cause a first TB skin test to boost the immune system, allowing it to react strongly to a second test, even though a new TB infection has not occurred.