I. Introduction

While 95% of cases of tuberculosis are in developing countries, it’s still a concern in the United States, with cases reported in all 50 states and the District of Columbia. There were 9,025 incidents of tuberculosis reported in the United States in 2018, and 515 deaths were caused by the disease in the previous year. Approximately 13 million Americans have a latent TB infection. Up to 10% of these individuals will develop tuberculosis in their lifetime if the infection is left untreated.

This guide provides information about how tuberculosis is spread, who’s at risk, and what a TB infection looks like. We also discuss how a TB skin test screens for Mycobacterium tuberculosis bacteria, the difference between latent and active TB, the importance of treatment, and typical recovery time.

II. Overview of Tuberculosis Skin Testing

Why should I be tested?

A TB skin test is used to screen for the Mycobacterium tuberculosis bacteria, which causes latent TB infection or active TB disease. With appropriate treatment, a latent infection can be prevented from progressing into TB disease. TB disease is contagious and can be fatal if it’s not diagnosed and treated properly, but it is curable.

When should I be tested?

You should ask a doctor about tuberculosis testing if you’re in a high-risk group, even if you’re not currently experiencing symptoms. According to the Centers for Disease Control and Prevention, you should be tested if:

  • You’ve been in close contact with someone who has TB disease
  • You’re from a part of the world where TB disease is prevalent
  • You live or work in a nursing home, long-term care residence, homeless shelter, or correctional institution where you’re exposed to people at high risk for TB
  • You’re a healthcare worker who cares for patients at increased risk for TB disease — some healthcare workers may require testing every 6 to 12 months

Additionally, babies, children, and adolescents who spend time with adults at risk should be tested.
People with weakened immune systems are most at risk of having a latent TB infection develop into TB disease. High-risk groups include people with:

  • HIV
  • Diabetes
  • Severe kidney disease
  • Organ transplants
  • A TB infection within the last two years or TB that has not been properly treated
  • History of drug use, both injection and non-injection

TB tests are generally not required if you have a low risk of infection.

What is required for the test?

A TB skin test is done in two parts over a 48 to 72 hour period. If you’re unable to return to the clinic or health department on time to complete the test, you may be asked to take the test at another time.

What do I need to do to prepare for the test?

Preparation is not required for a TB skin test.

III. The Basics of Tuberculosis

Tuberculosis is spread when an infectious person speaks, coughs, or sneezes. Tiny droplets of Mycobacterium tuberculosis bacteria are expelled into the air and breathed in by other people, eventually settling into their lungs.

If a person’s immune system is able to fight the bacteria, the TB may remain latent or inactive. At this point, the bacteria aren’t contagious and can’t be spread. However, the infection can develop into TB disease in the future, so it’s important for an infected person to receive treatment to prevent the bacteria from becoming active.

A latent TB infection develops into TB disease in about 5 to 10% of cases. When an infection progresses, the bacteria are active in the body and growing, causing a person to become ill. This can occur within days or weeks of infection or years later if a person’s immune system eventually becomes weakened.

TB generally affects the lungs (pulmonary tuberculosis), but it can also move into the bloodstream and impact other parts of the body, such as the brain, spine, kidneys, glands, and joints (extrapulmonary tuberculosis).

Symptoms of TB include:

  • Persistent coughing that lasts more than 21 days
  • Chest pain when breathing or coughing
  • Cough accompanied by blood or phlegm
  • Lack of appetite and/or weight loss
  • Fatigue or weakness
  • Fever or chills
  • Sweating when sleeping at night

While these symptoms may occur with colds, flu, and other conditions, it’s important to see a physician to determine the cause or to request a TB test, particularly if you suspect you’ve been exposed to a person with tuberculosis.

IV. How a Tuberculosis Skin Test Works

A TB skin test is also called a tuberculin skin test, PPD (purified protein derivative) test, or Mantoux test. It screens for the presence of Mycobacterium tuberculosis bacteria that cause TB and must be administered by a trained healthcare professional.

Tuberculin Skin Test

A TB skin test takes place in two parts. During your first visit, a small amount of fluid called tuberculin purified protein derivative (PPD) is injected into your forearm using a syringe needle. This antigen is made from the bacteria that causes TB, but it isn’t infectious and won’t make you ill. You must wait two to three days after the injection to see how your body reacts to the presence of the protein.

When given correctly, the injection results in a small bump or wheal on the surface of your skin that should disappear in 10 to 15 minutes. If this bump doesn’t appear, the test could have been incorrectly administered and may need to be repeated on your other arm.

You must return within 48 to 72 hours to have the test interpreted. A healthcare worker visually examines your skin’s reaction where the protein was injected and measures the swelling or thickening, which is called an induration. The test results are based on the size of the induration in relation to your TB risk factors and should only be interpreted by a trained professional. For this reason, the test must be performed by qualified staff at a clinic or public health department.

V. Treatment for Tuberculosis

Tuberculosis is treatable if diagnosed early and properly treated. Your physician can recommend an appropriate care plan. It’s critical that you take any medication as prescribed and until you’ve finished it to make sure all of the TB bacteria is eliminated.

Latent Tuberculosis Infection

If you test positive for the presence of the bacteria but it’s not active, the most common treatment is an antibiotic that’s taken daily for six to nine months to kill the bacteria and prevent it from developing into TB disease in the future. A commonly prescribed medication for treating latent TB is isoniazid.

Active Tuberculosis Disease

The treatment for active tuberculosis depends on a number of factors, such as your overall physical health, your age, and where the bacteria is growing in your body. You may be hospitalized for a short period of time before you can resume normal activities without concerns of spreading the bacteria.

Your doctor will recommend a course of treatment typically involving several medications taken for six months to a year. The most commonly prescribed medications are isoniazid, rifampin, pyrazinamide, and ethambutol. Your symptoms may begin clearing up within a few weeks of taking the medications.

It’s essential to take any medication exactly as prescribed by your doctor until the entire course of treatment is complete. Even though your symptoms may improve, it’s critical to ensure all of the bacteria is eliminated. Any bacteria that hasn’t been eliminated can resurface and become drug-resistant, making TB much more difficult to treat.

VI. FAQs

Are there risks associated with this test?

You might experience mild bruising, itching, or irritation in the area where the antigen has been injected. Avoid scratching or rubbing the injection site, and do not apply lotions, ointments, or bandages. Keep the area as clean and dry as possible. A cold compress may be applied if you’re feeling discomfort. Severe reactions such as anaphylaxis, blistering, or significant swelling are rare.

What does a negative TB skin test result mean?

A negative TB skin test result means there was no reaction to the antigen, and a TB infection is unlikely.

Is it possible to have a false negative test result?

Yes. The skin test may not detect a recent infection to which the body has yet to respond or an infection that’s many years old. Some people with weakened immune systems may not react to skin tests. Results can also be affected by a recent viral illness or vaccination for measles or smallpox or an improperly administered or interpreted test.

What does a positive TB skin test result mean?

A positive result means the test has detected the Mycobacterium tuberculosis bacteria. The test result cannot tell when you were infected or if the infection is latent or active — only that the bacteria is present in your body.

Is it possible to have a false positive test result?

Yes. If you have previously received a bacille Calmette-Guerin (BCG) vaccination for TB, you may have a positive reaction to a skin test. BCG is not commonly given in the United States but is widely used in countries where TB is prevalent. A false positive result can also occur if the skin test is not properly administered or interpreted.

Why do I need further tests if my TB skin test result is positive?

Your physician may order other tests to determine whether you have a latent or active infection, as the type of TB affects the course of treatment. Imaging tests, such as a chest X-ray or CT scan, can detect signs of TB in your lungs. Your doctor can also take samples of mucus or phlegm and send it for testing to see if the TB bacteria is active.

How long does it take to develop TB disease?

Some people develop TB within weeks of coming in contact with the bacteria, but others do not become sick for years. Many people have a latent infection and never become ill. You’re most likely to develop TB when your immune system is weak and can’t fight the bacteria.

Should I have a TB blood test instead of a skin test?

A tuberculin blood test, also known as Interferon-Gamma Release Assay or IGRA, measures the reaction of your immune system to TB bacteria. A healthcare practitioner draws a sample of blood to be analyzed at a laboratory.

An IGRA is more costly than a skin test but isn’t affected by the BCG vaccination, so it may be appropriate if you have been immunized for TB. Your physician may also recommend a blood test if you have a skin rash that makes it difficult to read the results of the test or if you’re unable to return to the clinic or health department for the second part of the skin test. Similar to a TB skin test, a blood test can’t tell if an infection is latent or active.

VII. Additional Resources

To learn more about tuberculosis and testing for TB, use the following resources.

NameWebsiteSummary
American Lung Associationwww.lung.orgInformation about TB symptoms, causes, diagnosis, and treatment
Centers for Disease Control and Preventionwww.cdc.govFact sheets about tuberculosis; personal stories of people diagnosed with and/or treated for TB
National Organization for Rare Disorderswww.rarediseases.orgTB causes, symptoms, and associated disorders
World Health Organizationwww.who.intFact sheets about tuberculosis

VIII. Sources Used in This Article