What is being tested?
Tuberculosis (TB) is an infectious disease caused by a bacterial infection. It may affect many body organs, but primarily targets the lungs. Tuberculosis, caused by Mycobacterium tuberculosis and once called consumption, has been causing illness for thousands of years.

TB may cause an inactive (latent) infection or an active, progressive disease. The immune systems of about 90% of the people who become infected with TB manage to control its growth and confine the TB infection to a few cells in the body. The bacteria in these cells are inactive but still alive. The person does not have any symptoms, and they are not infectious, but they do have a "latent TB infection." If the person's immune status is later compromised, the dormant mycobacteria may begin to grow again, leading to an active case of tuberculosis disease. This active TB does cause illness in the person, and it can be passed to others through respiratory secretions such as sputum or aerosols released by coughing, sneezing, laughing, or breathing.

If the TB skin test is positive, the person may have an active case of TB or the latent form of infection. Further testing will be done to diagnose the presence of an active tuberculosis infection.

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 Mycobacterium tuberculosis 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 1cc syringe and a tiny needle, he will inject a small amount 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. The site must be examined by a healthcare worker at 48 and/or 72 hours to see if a local skin reaction has occurred.