At a Glance
Why Get Tested?
To help determine whether or not you may have been exposed to and become infected by the Mycobacterium tuberculosis (TB) bacteria
When to Get Tested?
When you have diseases or conditions that weaken your immune system and put you at a greater risk of developing active tuberculosis. When you have had close contact with someone who has active TB, or work or live in a high risk environment. When you have lived for a period of time in a foreign country where TB may be more common. Sometimes as part of an examination prior to starting a new job (such as a healthcare worker).
No sample is required. A small amount of purified protein derivative (PPD) solution is injected just under the first layer of skin of your inner forearm.
Test Preparation Needed?
The Test Sample
What is being tested?
Tuberculosis (TB) is an infectious disease caused by a bacterium named Mycobacterium tuberculosis. It may affect many body organs, but primarily affects the lungs (pulmonary). Tuberculosis, (once called consumption), has been causing death for thousands of years.
TB may cause a latent or inactive infection or an active, progressive disease. About 90% of the people who become infected with TB manage to control its growth and confine the TB to a few cells in the body. The bacteria in these cells are inactive but still alive. The patient is not sick, they do not have any symptoms, and they are not infectious, but they do have a "latent TB infection." If the patient's immune status is later compromised, the latent TB bacteria may begin to grow again, leading to an active case of tuberculosis disease. This active TB does cause illness in the patient and it can be passed to others through respiratory secretions such as sputum (spit or phlegm) or aerosols released by coughing, sneezing, laughing, or breathing.
If the test is positive, further testing will be done to look for signs of active tuberculosis.
How is the sample collected for testing?
No sample is required. The test is performed on the patient's skin. A purified protein derivative (PPD) solution that contains TB antigens, but not live bacteria, is used to provoke a hypersensitivity skin reaction (a red raised bump) in those who may have been infected by TB.
A healthcare worker will wipe your inner forearm with alcohol and let the skin dry. Using a syringe and a tiny needle, they will inject a small amount (0.1mL) of PPD solution just under the first layer of your skin. When done correctly, the injection forms a small bubble of fluid that looks like a blister. The site should be left uncovered and undisturbed; and perfumes and other cosmetics should not be applied. It will then need to be examined by a healthcare worker at 48 and/or 72 hours after the injection to see if a local skin reaction has occurred.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
Ask a Laboratory Scientist
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.