Tuberculin Skin Test Health Article

Definition

Tuberculosis (TB) is an airborne infectious disease caused by the bacteria Mycobacterium tuberculosis. The two most common types of tests that are used to screen for this disease are the Mantoux PPD tuberculin skin test, which is generally considered the most reliable, and the TB tine test. These tests are sensitive screening tools that are designed to help identify individuals who may have been infected by tuberculosis bacteria. A diagnosis of tuberculosis is never made based on the results of a TB skin test, but requires further testing including a sputum culture and a chest x ray.

Purpose

Because TB is spread through the air, especially in poorly ventilated areas, it is more commonly found among people living in crowded conditions, such as jails, nursing homes, and homeless shelters. It is estimated that between 10 and 15 million people in the United States have latent tuberculosis. Many new cases of tuberculosis are multi-drug resistant making early detection of exposure a high public health priority. Often, a TB skin test will be given as part of a physical examination when an organization is hiring a new employee, particularly for those individuals seeking employment in the healthcare or food service professions.

People can be exposed to TB without showing any symptoms or necessarily developing the disease. Individuals with normally functioning immune systems generally prevent the spread of the bacteria by "walling off" or encysting the bacteria within the body. Such a structure in the lungs is called a "Ghon" body. Anyone who has had close contact with another person who has tuberculosis (such as a friend or family member); has been around someone with active TB; has a weakened immune system (immunocompromised), either from a chronic disease, such as HIV infection, or as a result of a tissue or organ transplant or other medical treatment designed to suppress the immune system; or displays symptoms of the disease should be tested. Symptoms of TB include a persistent cough, fever, weight loss, night sweats, fatigue, and loss of appetite. Often, individuals must receive the test in order to enter school or begin work.

Precautions

Although generally considered safe, it is important for the person being tested to inform a tester about any possibility of pregnancy, any previous positive TB test, or of any active tuberculosis in the past. People who previously have had a positive TB test will probably always have a positive test and should not be tested again. Also, anyone who is known to have active TB should not be tested because the local reaction to the test may be so severe that it requires surgical care.

There are several situations when TB test results might not be accurate. These includes situations involving people who:

• Have had vaccinations (such as those for measles, polio, rubella or mumps) within the last four weeks.
• Currently have, or recently recovered from a viral infection.
• Are taking steroids.
• Have severe malnutrition.

Description

TB skin tests are usually given at a clinic, hospital, or physician's office. Sometimes the tests are given at schools or workplaces. Many cities provide free TB skin tests and follow-up care. The Mantoux PPD tuberculin skin test involves injecting 0.1 mL of PPD tuberculin standardized to a dose of five units just under the top layer of the skin (intracutaneously). Tuberculin is a mixture of antigens obtained from the culture of M. tuberculosis. Antigens are foreign particles or proteins that stimulate the immune system to produce antibodies. Two different tuberculin preparations are available, Old Tuberculin (OT) and Purified Protein Derivative (PPD). The test is usually given on the inside of the forearm about halfway between the wrist and the elbow, where a small bubble (wheal) will form as the tuberculin is injected. The skin test takes just a minute to administer and feels more like a pinprick than a shot.

After 48-72 hours, the test site must be examined by a trained person for evidence of swelling. People who have been exposed to tuberculosis will develop an immune response, causing a slight redness and swelling at the injection site. This is called a delayed hypersensitivity reaction, and it is mediated by immune T lymphocytes and macrophages rather than antibody. Immune lymphocytes enter the site and release products that stimulate inflammation and the migration of macrophages into the area. This results in erthyma and accumulating cells, and cause the lesion to become hard (induration). Reactions may not peak until after 72 hours in elderly individuals or those who are being tested for the first time. If there is a lump or swelling, a health care provider will use a ruler to measure the size of the reaction.

The other method of TB skin test is called the multiple puncture test or tine test because the small test instrument has several small tines that lightly prick the skin. The small points of the instrument are either coated with dried tuberculin or are used to puncture through a film of liquid tuberculin. The test is read by measuring the size of the largest papule. Because it is not possible to precisely control the amount of tuberculin used in the tine test, a positive test should be verified using the Mantoux test. For this reason, the tine test is not as widely used as the Mantoux test and is considered to be less reliable.

It is possible that a person who has TB may receive a negative test result (called a "false negative") or a person who does not have TB may receive a positive test result (called a "false positive"). If there is some doubt, the test may be repeated or the person may be given a diagnostic test using a chest x ray or have sputum cultured to determine whether TB is present or active in the lungs. It is often recommended that a two-step PPD test be given to health care workers and persons whose response to PPD may be diminished. The test is given in the usual manner and if negative, repeated within one to three weeks. A positive reaction on the second test is considered an indication of exposure to TB even if the first result is negative.