Currently, most cases of latent TB infection are diagnosed by a tuberculin skin test (“TST” or also known as “PPD” which stands for Purified Protein Derivative), which produces a delayed hypersensitivity type response to an extract made from M. tuberculosis (the causative germ for TB). TST is the most commonly used method to test for latent TB infection (LTBI). This test has been available for around 100 years, is easily administered, and has relatively low cost. Numerous medical studies have shown that those with a positive TST had an increased future risk of active TB, whereas negative test subjects had low risk. Furthermore, studies confirm that treatment with anti-tuberculous medication in patients with positive skin testing reduces their risk of active TB, but treating those with negative TST provides no benefit, even in immunocompromised hosts.
Your doctor will administer the test by cleaning the forearm and then injecting a small amount of the extract under the top layer of skin.
There is no special preparation for this test. The reaction will take 48-72 hours to develop. You should return to your doctor during this time to have the area checked to determine if you had a significant reaction to the PPD. The reaction is measured in millimeters of hard swelling (induration) at the site (not erythema or redness).
A positive test indicates Tuberculosis infection some time in the past, but not necessarily from a recent exposure. Once infected, an individual usually remains reactive to the skin test forever. Therefore, once you test positively, there is generally no reason to repeat skin testing in later years. There is also a risk of a vigorous reaction (anamnestic immune recall or “boosting”) in the subsequent test.
People can have a false positive reaction because of previous bacillus Calmette-Guérin (BCG) vaccination (an immunization given to children in certain countries) or infection from non-tuberculous mycobacterial germs. Most experts agree that after about 15 years, the BCG vaccine no longer produces a positive PPD reaction. Tuberculin tests have the disadvantage in that they may produce false negative tests because of other underlying disease, malnutrition, or overwhelming TB infection.
A negative reaction (no induration) or size of induration that falls below the cutoff for each risk group may mean that you have not been infected with the bacteria that cause TB. In a healthy person with no known risk factors for Tuberculosis and whose immune system is normal, induration greater than or equal to 15 mm is considered a positive skin test. There are different cutoffs for other risk groups. An induration of 5 mm or more is positive in patients with HIV/AIDS, organ transplant recipients, recent close contacts of those with active TB, patients with evidence of old TB on x-ray, and immunosuppressed patients. A reaction of 10 mm or more is positive for recent arrivals (<5 years) from TB endemic areas, residents and employees of high risk settings (such as prisons), and those with certain medical conditions (such as silicosis, diabetes mellitus, history of intravenous drug abuse, chronic renal failure, malnutrition, head & neck or lung cancer, or prior gastrectomy or jejunoileal bypass).
Scientists are developing new TB tests that offer the hope of faster and more accurate TB testing at a lesser expense. These include polymerase chain reaction (PCR) detection of bacterial DNA, and assays to detect the release of interferon gamma in response to mycobacterial proteins such as ESAT-6. Ideally, such tests would not be affected by BCG immunization or environmental mycobacteria, and so generate fewer false positives, while at the same time producing fewer false negative results.