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Tests & Procedures : Lab Tests

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TB Screening (Skin) 

Does this test have other names?

Tuberculin test, TST, Mantoux, PPD (purified protein derivative)

What is this test?

This test determines whether you have been infected with tuberculosis (TB), a highly contagious bacterial infection. It's possible to have latent or inactive TB and not feel sick, or you can have active TB disease with symptoms.

Why do I need this test?

You might need this test if you have recently been exposed to someone who has TB or if your doctor suspects you may have a TB infection.

Symptoms of TB include:

  • Cough

  • Fever

  • Night sweats

  • Weight loss

TB usually affects the lungs, but it can spread to other parts of your body, including your joints, spine, brain, and kidneys, and cause additional symptoms.

You also might have this test if you:

  • Have HIV or another disease that weakens your immune system

  • Use illegal drugs

  • Recently emigrated from areas where TB is more common, such as some Eastern European or Latin America countries

If you are a health care worker, you might have this test periodically as part of your facility's infection control program. Testing for TB is often part of routine prenatal care.

What other tests might I have along with this test?

If you test positive on a TB skin test, your doctor will probably order a chest X-ray, sputum smear (a test on mucus you cough up), and TB culture to confirm the results. 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

The test may be interpreted as positive if the skin where you were injected is hard, raised, red, and swollen. However, redness alone is not considered a positive reaction.

A positive skin test means it's likely that you were infected with TB bacteria at some point in time, but this does not necessarily mean that you have an active TB infection.

In many cases, if you have a healthy immune system, your body will "wall off" the TB bacteria soon after you are infected, and you will not go on to develop an active TB infection. Further tests are necessary to see if you have active or inactive TB.

If you have no reaction, the skin test is considered negative, and you are not likely to have inactive TB or TB disease.

How is this test done?

Your inner forearm is disinfected and a small amount of fluid called tuberculin is injected into your skin. A circle is drawn around the injection site with long-lasting ink. You need to return to the testing site after 48 to 72 hours to be examined by a trained health care worker.

Does this test pose any risks?

When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

You cannot get TB from the skin test.

What might affect my test results?

If you have been vaccinated against tuberculosis with bacilli Calmette-Guerin (BCG), you can have a positive reaction to the skin test even if you don't have, or never had, a TB infection.

You might have a false negative test if you are:

  • Malnourished

  • On steroid therapy

  • Taking drugs that can affect your immune system, such as medication for AIDS or cancer

How do I get ready for this test?

You don't need to prepare for this test.  Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.