A purified protein derivative (PPD) skin test is a test that determines if you suffer from tuberculosis (TB).
Tuberculosis is a serious infection, usually of the lungs, caused by the bacteria Mycobacterium tuberculosis. This bacteria spreads by breathing in the air exhaled by an infected person. The bacteria can remain inactive in your body for years. When your immune system becomes weakened, TB can become active and produce symptoms such as fever, weight loss, coughing, and night sweats. If tuberculosis doesn’t respond to antibiotics, it’s referred to as drug-resistant TB. This is a serious public health problem in many regions of the world, including Southeast Asia and Africa.
When TB infects your body, it becomes extra sensitive to certain elements of the bacteria, such as the purified protein derivative. A PPD test checks your body’s current sensitivity, which will tell doctors whether or not you have TB.
Tuberculosis is a highly contagious disease. The World Health Organization (WHO) estimates that TB is second only to HIV and AIDS as the greatest global killer. However, the disease is relatively rare in the United States and most people in the United States infected with it don’t show symptoms.
You also should get a PPD skin test if you work in the healthcare field. All healthcare workers must be routinely screened for TB.
You also need a PPD skin test if:
- you’ve been around someone with TB
- you have a weakened immune system due to certain medicines such as steroids or certain diseases such as cancer, HIV, or AIDS
The doctor or nurse will swab the skin of your inner forearm with alcohol. You will then get a small shot that contains PPD under the top layer of your skin. You may feel a slight 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’s a very small risk of severe redness and swelling on your arm, especially if you’ve had a previous positive PPD test and you’re having the test again.
If the area of skin where you received the PPD injection isn’t swollen or is only slightly swollen 48 to 72 hours after the injection, the test results are negative. A negative result means that you most likely haven't 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 to 9 mm of firm swelling) is a positive result in people who:
- take steroids
- have HIV
- have received an organ transplant
- have a weakened immune system
- 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, but a positive result doesn’t always mean that they have active tuberculosis. More tests are necessary to confirm the diagnosis.
Larger reactions (10 mm of swelling or more) are 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 come from a country that’s had 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 people without a 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 bacillus Calmette-Guerin (BCG) vaccine against tuberculosis may have a false-positive reaction to the PPD test. Some countries outside the United States that have a high prevalence of TB give the BCG vaccine. Many people born outside of the United States have had the BCG vaccine, but it’s not given in the United States due to its questionable effectiveness.
Your doctor will follow up on positive results with a chest X-ray, a CT scan, and a sputum test that looks for active TB in the lungs.
The PPD skin test isn’t foolproof. 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 result.