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
Conditions that weaken the immune system, such as
HIV infection or cancer. The result also may be affected if a person is
Some vaccinations for infections, such as
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
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
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
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.