Tuberculin skin test

Encyclopedia of Medicine by Altha Roberts Edgren

Definition

Tuberculosis (TB) is an airborne infectious disease caused by the bacteria Mycobacterium tuberculosis. The two most common types of tests 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 screening tools that are designed to help identify individuals who may have been infected by the 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. 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 has active 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 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 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 tested because the local reaction to the test may be so severe that it requires surgical care.

There are several situations when the 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 just recently got over, a viral infection
• Are taking steroids
• Have severe malnutrition.

Description

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 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 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 will 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 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 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.

Preparation

There is no special preparation needed before a TB skin test. A brief personal history will be taken to determine whether the person has had tuberculosis or a TB test before, has been in close contact with anyone with TB, or has any significant risk factors. Directly before the test, the skin on the arm at the injection site is usually cleaned with an alcohol swab and allowed to air dry.

Aftercare

After having a TB skin test, it is extremely important to make sure that the patient keeps the appointment to have the test reaction read. The patient is instructed to keep the test site clean, uncovered, and to not scratch or rub the area. Should severe swelling, itching, or pain occur, or if the patient has trouble breathing, the clinic or health care provider should be contacted immediately.

Risks

The risk of an adverse reaction is very low. Occasionally, an individual who has been exposed to the TB bacteria will develop a large reaction in which the arm swells and is uncomfortable. This reaction should disappear in two weeks. A sore might develop where the injection was given, or a fever could occur, but these are extremely rare reactions.

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 and/or sputum sample culture test to determine whether the disease is present and/or active in the lungs.

Normal results

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 tests can 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.