Tuberculin Skin Test
The primary test for tuberculosis (TB), a
serious, potentially life-threatening, bacterial infection of the lungs, is a simple skin test known as a Mantoux tuberculin test, or purified protein derivative (PPD) test. This test evaluates whether the body reacts to the existence of the bacteria that cause tuberculosis. Although TB infections are now rare in the United States, certain people are susceptible to them, such as those with weakened immune systems, travelers who come from or visit countries with high rates of the disease, and those who work in the health care industry.
Reasons for a Tuberculin Skin Test
In certain states, school-aged children must pass a tuberculosis screening in order to be permitted to enter the school system. Other individuals may be required to have a tuberculosis skin test in order to meet the requirements of a particular college of job opportunity. Other reasons people may be tested for TB involve the fact that they are considered at heightened risk for contracting the disease because they have:
- Spent time with someone with tuberculosis
- HIV/AIDS
- A weakened immune system or autoimmune disorder
- Had an organ transplant
- Had an abnormal chest X-ray
- Come form a region where TB is prevalent
- Symptoms of TB, such as fever, cough, weight loss or cough
- Used illegal drugs
In addition, people who have lived in, or worked at, institutions where TB is more common, may also be at increased risk for the illness and should be periodically tested. This includes individuals who have resided in, or been employed by, prisons, homeless shelters, or certain healthcare facilities.
The Tuberculin Skin Test Procedure
In order to administer the tuberculin test, a small amount of TB protein in placed under the top layer of skin on the individual's inner forearm. If the individual has been exposed to TB ((Mycobacterium tuberculosis), the skin reacts to the antigens in the sample by developing swelling and a firm red bump at the site. This reaction occurs within 48 hours and usually lasts for approximately a week. While the tuberculin skin test does indicated whether the individual has been infected with the tuberculosis bacteria, it does not indicate whether the infection is latent or active, how long the patient has been infected, or whether the individual is contagious.
Since the tuberculin test does not always yield accurate results, a positive skin test is followed up by tests of the patient's blood and sputum and by a chest X-ray in order to confirm or disprove the preliminary diagnosis.
Contraindications for a Tuberculin Skin Test
There are times when a tuberculin skin test in not appropriate or when its results may not be accurate. Tuberculin skin tests should not be administered to patients who:
- Are known to have a TB infection
- Have had a severe reaction to TB antigens in the past
- Have been vaccinated against TB with bacille Calmette-Guérin (BC-G)
- Have been treated with corticosteroids
- Have HIV or cancer
- Have a skin condition that will make results difficult to assess
- Are severely underweight malnourished
- Are younger than 3 months old
The tuberculin skin test should not be administered if the patient has received a vaccination against measles, mumps, rubella, polio or chickenpox within the previous 6 weeks.
Risks of a Tuberculin Skin Test
Because no live bacteria are used in a tuberculin skin test, individuals being tested are not at risk of developing tuberculosis. There is, however, a very slight risk of an allergic reaction, involving severe swelling, itching and pain at the site, especially if the individual has had TB in the past. Patients who have allergic reactions to the tuberculin skin test, or who experience a swollen arm, a fever, or enlarged lymph nodes require immediate medical attention.