Tuberculin skin test
Tuberculosis (TB) is an airborne infectious disease caused by the bacteria Mycobacterium tuberculosis. The two most common types of tests to screenfor this disease are the Mantoux PPD tuberculin skin test, which is generally considered the most reliable, and the TB tine test. 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.
Because TB is spread through the air, especially in poorly ventilated areas,it is more commonly found among people living in crowded conditions, such asjails, nursing homes, and homeless shelters. Often, a TB skin test will be given as part of a physical examination when a person 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. Anyone who has had close contact with someone who hasactive tuberculosis (such as a friend or family member); has been around someone with active TB; has a weakened immune system (immunocompromised), eitherfrom a chronic disease, such as HIV infection, or as a result of a tissue ororgan transplant or other medical treatment designed to suppress the immune system; or displays symptoms of the disease should be tested. Symptoms includea persistent cough, fever, weight loss, night sweats, fatigue, and loss of appetite.
Although generally considered safe, it is important to inform the person conducting the test if you may be pregnant, have had a positive TB test in the past, or have had tuberculosis in the past. People who have had a positive TB test in the past 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 testedbecause the local reaction to the test may be so severe that it requires surgical care.
TB skin tests are usually given at a clinic, hospital, or doctor'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 a very small amount of a substance called PPD tuberculin justunder the top layer of the skin (intracutaneously). Tuberculin is a mixture of antigens obtained from the culture of M. tuberculosis. Antigens areforeign particles or proteins that stimulate the immune system to produce antibodies. The test is usually given on the inside of the forearm about halfwaybetween the wrist and the elbow, where a small bubble will form as the tuberculin is injected. The skin test takes just a minute to administer and feelsmore like a pinprick than a shot.
After 48-72 hours, the test site will be examined by a trained person for evidence of swelling. People who have been exposed to tuberculosis will developan immune response, causing a slight redness or swelling at the injection site. 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, the 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 tinetest because the small test instrument has several small tines that lightlyprick 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 notpossible 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 tobe less reliable.
After having a TB skin test, it is extremely important to make sure that thepatient keeps the appointment to have the test reaction read. The patient isinstructed to keep the test site clean, uncovered, and to not scratch or rubthe area. Should severe swelling, itching, or pain occur, or if the patient has trouble breathing, the clinic or health care provider should be contactedimmediately.
In people who have not been exposed to TB, there will be little or no swelling at the test site after 48-72 hours. This is a negative test. Negative testscan be interpreted to mean that the person has not been infected with the tuberculosis bacteria or that the person has been infected recently and not enough time has elapsed for the body to react to the skin test. Persons become sensitive between two and ten weeks after the initial infection. As a result,if the person has been in contact with someone with tuberculosis, the test should be repeated in three months. Also, because it may take longer than 72 hours for an elderly individual to develop a reaction, it may be useful to repeat the TB skin test after one week to adequately screen these individuals. Immunocompromised persons may be unable to react sufficiently to the Mantoux test, and either a chest x ray or sputum sample may be required.