TB (tuberculosis) skin test

What is the test?

Tuberculosis is a bacterial infection that most often involves the lungs, but can involve many other organs. Although antibiotics can treat most cases, TB remains a significant cause of death worldwide. The TB skin test, also called the purified protein derivative (PPD) test or Mantoux test, shows if you've ever been infected with the bacteria that cause tuberculosis.

Infections with these bacteria can be active or inactive. In active infections, there usually is a high bacterial load, and the person is contagious when he or she coughs. In people with inactive infections, the bacteria are alive deep within the lungs, but "asleep." Because inactive infections can later "wake up" and become active, it is important to recognize and treat both types of TB infections.

How do I prepare for the test?

Vaccinations, corticosteroids such as prednisone and other drugs that suppress the immune system such as biologic agents can affect the results of the test. So, tell your doctor if you've recently been vaccinated for an infectious disease or if you're taking a corticosteroid or other immune suppressant.

What happens when the test is performed?

A clinician injects a small amount of liquid containing protein extracted from killed TB bacteria just under the surface of the skin of your forearm, through a tiny needle. You might feel a slight pinch from the injection, and see a small swollen area form where the liquid has been injected. The clinician often draws a circle around the injection spot and asks you to avoid washing it away.

Then, 48-72 hours later, you return to have the injection area examined. If the skin is firm and raised where the injection was given, the clinician measures the size of the affected area. The larger it is, the greater the likelihood that you have been infected with the TB bacteria at some time in the past, and that you have a current infection. The test cannot distinguish between an inactive and active infection.

What risks are there from the test?

There are no risks.

Must I do anything special after the test is over?

Remember to return to have the injection site examined.

How long is it before the result of the test is known?

The result is known two to three days later when the skin is examined. If the test is positive, your doctor will ask if you have had a positive TB skin test in the past and if you have any symptoms that might indicate active TB. Your doctor will likely order a chest x-ray, even if you feel fine.