The Mantoux skin test is given by using a needle and syringe to inject 0.1 ml of 5 tuberculin units of liquid tuberculin between the layers of the skin (intradermally), usually on the forearm (Figure 1). A tuberculin unit is a standard strength of tuberculin.
The tuberculin used in the Mantoux skin test is also known as purified protein derivative, or PPD. For this reason, the tuberculin skin test is sometimes called a PPD skin test.
With the Mantoux skin test, the patient's arm is examined 48 to 72 hours after the tuberculin is injected. Most people with TB infection have a positive reaction to the tuberculin. The reaction is an area of induration (swelling that can be felt) around the site of the injection. The diameter of the indurated area is measured across the forearm (Figure 2); erythema (redness) around the indurated area is not measured, because the presence of erythema does not indicate that a person has TB infection (Figure 3).
Classifying the Reaction
An induration of 10 or more millimeters is considered a positive reaction for the following people:
An induration of 15 or more millimeters is considered a positive reaction for people with no risk factors for TB. In most cases, people who have a very small reaction or no reaction probably do not have TB infection.
For people who may be exposed to TB on the job (such as health care workers and staff of nursing homes or correctional facilities), the classification of the skin test reaction as positive or negative depends on
Therefore, in facilities where the risk of exposure to TB is very low, 15 or more millimeters of induration may be considered a positive reaction for employees with no other risk factors for TB. In facilities where TB patients receive care, 10 or more millimeters of induration may be considered a positive reaction for employees with no other risk factors for TB.
Most people who have a positive skin test reaction will have a positive reaction if they are skin tested later in their lives, regardless of whether they receive treatment. This is because the tuberculin skin test detects the immune response to tuberculin, not the presence of tubercle bacilli in the body
False-Positive PPD Reactions
Sometimes people who are not infected with M. tuberculosis will have a positive reaction to the PPD tuberculin skin test.
This is called a false-positive reaction. The two most common reasons for false positive PPD reactions are infection with nontuberculous mycobacteria (mycobacteria other than M. tuberculosis) and vaccination with BCG (bacillus Calmette-Gurin).
BCG is a vaccine for TB disease that is used in many countries. However, studies have not proven its effectiveness. People who are infected with nontuberculous mycobacteria or who have been vaccinated with BCG may have a positive reaction to the tuberculin skin test even if they do not have TB infection.
There is NO RELIABLE WAY to distinguish between a positive PPD reaction caused by true TB infection and a reaction caused by other mycobacteria or by vaccination with BCG.
However, the reaction is more likely to be truly caused by TB infection if any of the following are true:
False-negative reactions may be caused by
Anergy is the inability to react to skin tests because of a weakened immune system. Many conditions, such as HIV infection, cancer, or severe TB disease itself, can weaken the immune system and cause anergy. HIV infection is a main cause of anergy.
Because of their risk for anergy and their risk for TB, in selected situations HIV-infected people may be tested for anergy if they have a negative reaction to the tuberculin skin test. However, anergy testing is not recommended as a routine component of TB screening among HIV-infected persons.
Another cause of false-negative reactions is recent TB infection (infection within the past 10 weeks). It takes 2 to 10 weeks after TB infection for the body's immune system to be able to react to tuberculin. Therefore, after TB has been transmitted, it takes 2 to 10 weeks before TB infection can be detected by the tuberculin skin test. For this reason, close contacts of someone with infectious TB disease who did not react to the PPD tuberculin skin test should be retested 10 weeks after the last time they were in contact with the person who has TB disease.
A third cause of false-negative reactions is very young age. Because their immune systems are not yet fully developed, children younger than 6 months old may have a false-negative reaction to the tuberculin skin test.
A false-positive reaction or a false-negative reaction may occur when the tuberculin skin test is given incorrectly or the results are not measured properly. False-positive and false-negative reactions to the tuberculin skin test are summarized in Table 2.
Any patient with symptoms of TB should be evaluated for TB disease, regardless of his or her skin test reaction. In fact, people with symptoms of TB should be evaluated for TB disease right away, at the same time that the tuberculin skin test is given.
The symptoms of pulmonary TB disease include coughing, pain in the chest when breathing or sputum (The general symptoms of TB disease (extrapulmonary) include weight loss, fatigue, malaise, fever, and night sweats.
* Any patient with symptoms of TB should be evaluated for TB disease, regardless of his or her skin test reaction.