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What is the method of reading the tuberculosis skin test?
"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. Skin tests should be read 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.
How are skin test results interpreted?
The basis of the reading of the skin test is the presence or absence and the amount of induration (localized swelling). The diameter of the induration should be measured 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
- Pediatric patients exposed to high-risk adults
- People who work with mycobacteria in laboratories
An induration of 5 mm is considered positive for the following groups:
- People whose immune system is suppressed
- 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 who are infected with other types of mycobacteria other than Mycobacterium tuberculosis may also have false-positive TB skin tests.