Understanding the PPD Skin Test and Tuberculosis
A PPD (purified protein derivative) skin test is a test used to determine if you suffer from tuberculosis (TB).
Tuberculosis is a serious infection, usually of the lungs, caused by bacteria called mycobacterium tuberculosi that are spread by breathing air exhaled by an infected person. The TB bacteria can remain inactive in your body for years. When your immune system is weakened, TB can become active and produce symptoms such as fever, weight loss, coughing, and night sweats. Drug-resistant TB (tuberculosis that doesn’t respond to antibiotics) is a serious public health problem in many parts of the world, including Southeast Asia and Africa.
When your body is infected with TB, it becomes extra sensitive to certain elements of the bacteria, such as the purified protein derivative. PPD. A PPD test checks your body’s current sensitivity, which will inform doctors whether or not you have TB.
Who Should Get a PPD Skin Test?
Tuberculosis is a highly contagious (spreadable) disease. The World Health Organization estimates that 2 billion people worldwide have inactive TB and about 3 million people worldwide die of TB every year. (WHO) However, the disease is relatively rare in the United States and most people in the U.S. who are infected with the bacteria do not show symptoms.
You need a PPD skin test if:
- you have been around someone with TB
- you work in the healthcare field (all healthcare workers must be routinely screened)
- you have a weakened immune system due to certain medicines like steroids, or diseases such as cancer, HIV, or AIDS
How is the PPD Skin Test Performed?
The skin of your inner forearm will be swabbed with alcohol. You will then get a small shot that contains PPD. The needle is gently placed under the top layer of your skin. You may feel a small sting. A bump or small welt will form, which usually goes away in a few hours.
After 48 to 72 hours, you must return to your doctor’s office. A nurse or other medical professional will check the area where you received the shot to see if you’ve had any reaction to the PPD. There is a very small risk of severe redness and swelling in your arm, especially if you have had a previous positive PPD test and are having the test again.
Understanding Your PPD Skin Test Results
If the area of skin where you received the PPD injection is not swollen or only slightly swollen 48 to 72 hours after the injection, the test results are negative. A negative result means that you most likely have not been infected with the bacteria that cause tuberculosis. The amount of swelling may be different for children, people with HIV, the elderly, and others at high risk.
A small reaction, called an induration, at the site of the test (5 mm of firm swelling) is considered a positive result in people who:
- have HIV
- have received an organ transplant
- have a weakened immune system or take steroids
- have been in close contact with someone who has active TB
- have changes on a chest X-ray that appear to be the result of a previous TB infection
Members of this high-risk group may require treatment, though a positive result does not always mean that they have active tuberculosis. More tests are necessary to confirm the diagnosis.
Larger reactions (10 mm of swelling or more) are considered a positive result in people who:
- have had a negative PPD skin test in the past two years
- have diabetes, kidney failure, or other conditions that increase their TB risk
- are healthcare workers
- are intravenous drug users
- are immigrants who have immigrated from a country with a high TB rate in the past five years
- are under age 4
- are infants, children, or adolescents who have been exposed to high-risk adults
- live in certain group settings, such as prisons, nursing homes, and homeless shelters
For everyone else without a previous known risk factor for TB, a 15 mm or larger firm swelling at the injection site indicates a positive reaction.
False Positive and False Negative Results
People who received a BCG (bacille Calmette-Guerin) vaccine against tuberculosis may have a false-positive reaction to the PPD test. The BCG vaccine is given in some countries outside the U.S. that have a high prevalence of TB. Many foreign-born people have had the BCG vaccine, though it is not given in the U.S. due to its questionable effectiveness.
Positive results should be followed up with a chest X-ray, a CT (computed tomography) scan, and a sputum test, which looks for active TB in the lungs.
The PPD skin test is not foolproof. It’s important to note that one in five people infected with the bacteria that cause TB may not have any reaction to the test. Diseases such as cancer and medicines like steroids and chemotherapy that weaken your immune system may also cause a false-negative test result.