Tuberculosis PPD Skin Test

Tuberculosis Test. Image Source; Getty Images

The tuberculosis skin test (also known as PPD, tuberculin, or Mantoux) determines if someone has developed an immune response to the bacterium that causes tuberculosis (TB). This response can occur if someone currently has TB, if they were exposed to it in the past, or if they received the BCG vaccine against TB (which is not administered in the U.S.). Estimates indicate that one-third of the world's population has latent TB, and around 1.3 million people worldwide die of TB each year. The tuberculin test or PPD (purified protein derivative) test are other names for the tuberculosis skin test.

The tuberculin skin test is based on the fact that infection with M. tuberculosis bacterium produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium. Medical professionals extract the components of the organism from TB cultures and are the core elements of the classic tuberculin PPD (also known as purified protein derivative). This PPD material is used for skin testing for tuberculosis. Reaction in the skin to tuberculin PPD begins when specialized immune cells, called T cells, sensitized by prior infection, are attracted by the immune system to the skin site where they release chemical messengers called lymphokines. These lymphokines induce induration (a hard, raised area with clearly defined margins at and around the injection site) through local vasodilation (expansion of the diameter of blood vessels) leading to fluid deposition known as edema, fibrin deposition, and attraction of other types of inflammatory cells to the area.

An incubation period of two to 12 weeks is usually necessary after exposure to the TB bacteria in order for the PPD test to be positive. Anyone can have a TB test, and physicians can perform the test on infants, pregnant women, or HIV-infected people with no danger. It is only contraindicated in people who have had a severe reaction to a previous tuberculin skin test.

The standard recommended tuberculin test, known as the Mantoux test, is administered by injecting a 0.1 mL of a liquid containing 5 TU (tuberculin units) of PPD into the top layers of skin (intradermally, immediately under the surface of the skin) of the forearm. Health care providers should use a skin area that is free of abnormalities and away from veins. Typically, medical professionals make the injection using a 27-gauge needle and a tuberculin syringe. Inject the tuberculin PPD just beneath the surface of the skin. A discrete, pale elevation of the skin (a wheal) 6 mm-10 mm in diameter should be produced when the injection is done correctly. This wheal or "bleb" is generally quickly absorbed. If it becomes apparent that the first test was improperly administered, another test can be given at once, selecting a site several centimeters away from the original injection.

"Reading" the skin test means detecting a raised, thickened local area of skin reaction, referred to as induration. Induration is the key item to detect, not redness or bruising. Read skin tests 48-72 hours after the injection when the size of the induration is maximal. Tests read after 72 hours tend to underestimate the size of the induration and are not accurate.

The basis of the reading of the skin test is the presence or absence and the amount of induration (localized swelling). A physician will measure the diameter of the induration transversely (for example, perpendicular) to the long axis of the forearm and recorded in millimeters. The area of induration (palpable, raised, hardened area) around the site of injection is the reaction to tuberculin. It is important to note that redness is not measured.

A tuberculin reaction is classified as positive based on the diameter of the induration in conjunction with certain patient-specific risk factors. In a healthy person whose immune system is normal, induration greater than or equal to 15 mm is considered a positive skin test. If blisters are present (vesiculation), the test is also considered positive.

In some groups of people, the test is considered positive if induration less than 15 mm is present. For example, an area of induration of 10 mm is considered positive in the following groups:

• Recent immigrants from high-prevalence areas
• Residents and employees of high-risk areas
• IV drug abusers
• Children under 4 years old
• People who work with mycobacteria in laboratories

An induration of 5 mm is considered positive for the following groups:

• People with suppressed immune systems
• HIV-infected people
• People with changes seen on chest X-ray that are consistent with previous TB
• Recent contacts of people with TB
• People who have received organ transplants

On the other hand, a negative test does not always mean that a person is free of tuberculosis. People who have been infected with TB may not have a positive skin test (known as a false negative result) if their immune function is compromised by chronic medical conditions, cancer chemotherapy, or AIDS. Additionally, 10%-25% of people with newly diagnosed tuberculosis of the lungs will also have a negative result, possibly due to poor immune function, poor nutrition, accompanying viral infection, or steroid therapy. Over 50% of patients with widespread, disseminated TB (spread throughout the body, known as miliary TB) will also have a negative TB test.

A person who received a BCG vaccine (administered in some countries but not the U.S.) against tuberculosis may also have a positive skin reaction to the TB test, although this is not always the case. This is an example of a false positive result. The positive reaction that is due to the vaccine may persist for years. Those who were vaccinated after the first year of life or who had more than one dose of the vaccine have the greatest likelihood of having a persistent positive result than those who were vaccinated as infants.

People infected with other types of mycobacteria other than Mycobacterium tuberculosis may also have false-positive TB skin tests.

The test typically does not produce side effects. There is a very slight risk of having a severe reaction to the test, including swelling and redness of the arm, particularly in people who have had tuberculosis or been infected previously and in those who have previously had the BCG vaccine. Allergic reactions are also rare complications.

Since the test does not use live bacteria, so there is no chance of developing tuberculosis from the test.