Risks
There is a very slight risk of having a severe reaction to the tuberculin skin test, especially if you have had tuberculosis (TB). An allergic reaction can cause a lot of swelling and pain at the site. A sore may be present.
You cannot get a TB infection from the tuberculin skin test, because no live bacteria are used for the test.
What Affects the Test
The following may stop you from having the test or may change your results.

• A BCG (bacille Calmette-Guérin) vaccination. If you have had a BCG vaccination, you may have a positive PPD skin test.
• Medications that suppress the immune system, such as corticosteroids.
• Conditions that weaken the immune system, such as an 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.
• You have a recent TB infection. It takes 2 to 10 weeks for the immune system to react to TB bacteria.
• Your baby is younger than 3 months old. A baby's immune system is not fully developed at this age.

After the test
Expect some redness at the skin test site. The site may itch, but it is important that you do not scratch it, since this may cause redness or swelling that could make it hard to read the skin test. If itching is a problem, put a cold washcloth on the site and then dry it.
A positive reaction usually remains visible for about 1 week.
A strong positive reaction may cause mild pain. Talk to your health professional if you have:

• A fever
• Swelling in your arm
• Swollen lymph nodes in your armpit

You must see your health professional 2 to 3 days after the test to have the skin test checked.

The reaction is read by measuring the diameter of the bump

A positive tuberculin skin test does not mean you have a contagious (active) infection. The test cannot tell if the infection is active or inactive (latent TB). It also cannot tell the difference between a TB infection and a TB vaccination.
The test cannot tell how long you have been infected with TB or if the infection can be passed to others (active TB).
More tests—such as a chest X-ray, a sputum culture, or both—are usually done to see if you have an active TB infection.