The Mantoux test a method of detecting whether a person is infected with Mycobacterium tuberculosis. It is widely used test for latent TB. It involves injecting a small amount of a substance called PPD tuberculin into the skin of your forearm. If you have a latent TB infection, your skin will be sensitive to PPD tuberculin and a small, hard red bump will develop at the site of the injection, usually within 48 to 72 hours of having the test. If you have a very strong skin reaction, you may need a chest X-ray to confirm whether you have active TB disease.