Patient Basics: TB (Tuberculosis) Skin Test

Originally published by Harvard Health.

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 one of the most common causes 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, the bacteria are reproducing rapidly, 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 doctor 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 doctor 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 doctor 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 likely order a chest x-ray. You may also do blood and urine tests. If you do not have an active infection, your doctor will prescribe an antibiotic given over several months, to help prevent you from developing active tuberculosis. If you do have an active infection, a much more intensive treatment involving multiple antibiotics is required.