A tuberculosis (TB) skin test is the first method a doctor may use to try to determine if a person has TB. A TB skin test involves injecting a person with a small amount of tuberculin to see how the body responds to it.

Tuberculosis is an airborne disease caused by an infection of the Mycobacterium tuberculosis bacteria.

This article explores what happens during a TB skin test, what the results might mean, and what to do once a person has them.

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The first part of the TB skin test will be an injection into the forearm with a small amount of tuberculin.
Image credit: Greg Knobloch, Centers for Disease Control and Prevention’s Public Health Image Library, 2004

The skin test for TB, otherwise known as a Mantoux tuberculin test, can seem a bit intimidating, but it is quite straightforward.

The TB skin test has two parts. In the first, a doctor will inject someone with a small amount of a sterile solution, containing tuberculin.

Tuberculin is a fraction of purified protein derived from Mycobacterium tuberculosis. If an individual is infected with TB, their immune system will react to the tuberculin given in the TB skin test.

The injection is usually done on the inside of the forearm. When it is done correctly, the injection will create a small, pale-colored bump on the skin called a wheal.

The second stage of diagnosis must take place between 48-72 hours after the tuberculin injection.

At this appointment, the doctor will check to see what has happened to the wheal on the skin. If a person does not attend this appointment, they will have to start the process over again.

During the second appointment, a doctor will look to see how the body has responded to the injected tuberculin.

To do this, a doctor will measure the diameter of the wheal on the forearm and ask questions about the individual’s medical history and their environment.

A doctor will need to consider several things when interpreting the results of a TB test. The main consideration is the size of the bump on the arm:

  • test bump smaller than 5 millimeters (mm), test result negative
  • test bump larger than 5 mm, test result in the positive range

If the results are in the positive range, the doctor will investigate further by finding out about other factors in a person’s life.

Factors that can affect the results of a TB skin test include:

  • having recent contact with another person with TB
  • working at a medical facility, such as a hospital, care center, or medical lab
  • having TB in the past
  • receiving an organ transplant
  • taking immunosuppressant drugs
  • being HIV positive
  • coming recently from a country where TB is common
  • using injected drugs

Very young children or children exposed to adults with TB are also at a higher risk for TB.

In some cases, the body has a dramatic response to the skin test. This may cause the wheal to grow to over 15 mm in diameter. This indicates a positive outcome no matter what other circumstances there may be.

The outcomes for TB skin tests are not always clear-cut, as explained here:

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The TB skin test can have a number of factors that may affect the results.
  • Testing positive: This indicates that the body has been infected with the TB bacteria. An infection makes an individual extra sensitive to the tuberculin injection, which causes the test site to grow in diameter.
  • Testing negative: This means the body is unlikely to be infected with the bacteria. It is not sensitive to the tuberculin, and any symptoms are likely to be from something else.
  • False positive: There is the chance for results to show a false positive. People who have been vaccinated against TB, using the bacillus Calmette-Guérin (BCG) vaccine, can sometimes show a positive result, even if they are not infected with the bacteria. This is less common for vaccines given in the United States. It is also possible for the test to read positive falsely if it is not administered correctly, or if the person is infected with bacteria similar to TB.
  • False negative: This can happen when a person is infected with the bacteria. Examples of this are when someone has a weak immune system or has been exposed to pathogens, such as measles and smallpox. People with recent TB infections and very old TB infections can also show false negative test results. If the test is done incorrectly, a false negative might occur.

In many cases, doctors will use additional methods to be sure the results are as accurate as possible.

There is room for error with TB skin tests. Doctors use them as one part of a more detailed diagnosis, as opposed to stand-alone tests.

The results of a TB skin test help determine the next steps in a person’s treatment. If someone has TB, they may be started on medication immediately. If the diagnosis is unclear, the doctor will use other methods to reach a correct diagnosis.

X-rays and CT scans

One of the next steps will involve looking for signs of TB in the lungs, by using either an X-ray or CT scan.

TB causes changes in the lungs. Most commonly, little white spots will be visible, which means the body is fighting bacteria.

X-rays are usually accurate enough, but a CT scan may also be used to give a closer look. CT scans provide a more detailed picture, which helps a doctor decide what action to take.

Sputum test

If the X-ray or CT scan images show evidence of TB, a doctor will usually test the person’s sputum. This is the mixture of saliva and mucus that is coughed up due to an infection.

A sputum test is used to determine what type of TB bacteria is attacking the body. This also helps decide how best to treat it.

Blood tests

Some people may have a bad reaction to the TB skin test. In these cases, they may be given a blood test called the Interferon Gamma Release Assay (IGRA).

Although this test suits some people, it is not right for everyone. As such, people should always talk to a doctor to determine which test is best for them.

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Symptoms of TB may include fever, fatigue, and night sweats.

It is common for a person with the TB infection to experience no symptoms initially, and to only develop these once the bacteria has become active in the body.

When the bacterial infection is active, a person may notice many symptoms, including:

  • night sweats
  • persistent cough
  • loss of appetite
  • unusual weight loss
  • fever
  • general fatigue

If TB progresses, the cough may get worse, and a person may begin coughing up blood.

A positive TB skin test is an indicator that the body is infected with TB. It may be latent and show no symptoms, or it may already be active and causing symptoms.

However, it is important to remember that TB is treatable. Following a doctor’s recommendations can help ensure the most effective treatment and the best outcome.