TUBERCULOSIS (TB) PPD Skin Test:
The PPD stands for purified protein derivative, and is a method used to diagnose silent or latent tuberculosis infection. The injection itself is rather minor and goes just beneath the skin, ideally creating a small welt under the skin. This welt goes away in a few hours.
The injection site needs to be seen by a physician or nurse between 48-72 hours to read a result. Any longer than 72 hours, and the test must be repeated. If the test is positive (see information below for more about positive results), the physician will advise a Chest X-Ray be taken at the office.
-Ocean Front Urgent Care
Why the Test is Performed
This test is done to find out if you have ever come in contact with the bacteria that causes TB.
TB is an easily spread (contagious) disease. It most often affects the lungs. The bacteria can remain inactive (dormant) in the lungs for many years. This situation is called latent TB.
Most people in the United States who are infected with the bacteria do not have signs or symptoms of active TB.
You are most likely to need this test if you:
- May have been around someone with TB
- Work in health care
- Have a weakened immune system, due to certain medicines or disease (such as cancer or HIV and AIDS)
A negative reaction usually means you have never been infected with the bacteria that cause TB.
A negative reaction means the skin where you received the PPD test is not swollen, or the swelling is very small. This measurement is different for children, people with HIV, and other high risk groups.
The PPD skin test is not perfect. Up to 1 in 5 people infected with the bacteria that cause TB may not have a reaction. Also, diseases or medicines that weaken the immune system may cause a false-negative result.
What Abnormal Results Mean
An abnormal (positive) result means you have been infected with the bacteria that cause TB. You may need treatment to lower the risk of the disease coming back (reactivation of the disease).
It is important to note that test results depend on the person being tested.
A small reaction (5 mm of firm swelling at the site) is considered to be positive in people:
- Who have HIV
- Who have received an organ transplant
- Who have a suppressed immune system or are taking steroid therapy (about 15 mg of prednisone per day for 1 month)
- Who have been in close contact with a person who has active TB
- Who have changes on a chest x-ray that look like past TB
Larger reactions (greater than or equal to 10 mm) are considered positive in:
- People with a known negative test in the past 2 years
- People with diabetes, kidney failure, or other conditions that increase their chance of getting active TB
- Health care workers
- Injection drug users
- Immigrants who have moved from a country with a high TB rate in the past 5 years
- Children under age 4
- Infants, children, or adolescents who are exposed to high-risk adults
- Students and employees of certain group living settings, such as prisons, nursing homes, and homeless shelters
In people with no known risks of TB, 15 mm or more of firm swelling at the site indicates a positive reaction.
There is a very small risk of severe redness and swelling of the arm in people who have had a previous positive PPD test and who have the test again. There have also been a few cases of this reaction in people who have not been tested before.
A positive skin test does not always mean that a person has active TB. More tests must be done to check whether there is active disease.
Many people who were born outside the United States may have had a vaccine called BCG, which can lead to a false-positive test result. Most experts say that a past BCG vaccination should not change the PPD result when the test is done in people with an increased risk of TB infection or disease.
Purified protein derivative standard; TB skin test; Tuberculin skin test; Mantoux test
Ellner JJ. Tuberculosis. In: Goldman L, Schafer AI, eds. Goldman’s Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 332.
Fitzgerald DW, Sterling TR, Haas DW. Mycobacterium tuberculosis. In: Mandell GL, Bennett JE, Dolan R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Elsevier Churchill-Livingstone; 2009:chap 250.