Step 1 of a TB skin test involves injecting a small amount of purified protein derivative (PPD) under the skin of your forearm.
This injection is crucial for the test to work correctly. Here's a more detailed breakdown:
- The Injection: The PPD contains proteins derived from Mycobacterium tuberculosis, the bacteria that causes TB. A trained healthcare professional must administer the injection.
- Intradermal Injection: The injection is intradermal, meaning it's placed between the layers of your skin, not into the muscle. When done correctly, a small, pale fluid-filled bump (a wheal) appears on the skin.
- Why Inject? Injecting the PPD allows the body's immune system to react if you have been previously exposed to TB. If you have, your immune cells will recognize the TB proteins and cause inflammation at the injection site.
- What Happens Next? After the injection, you must return to the healthcare provider in 48-72 hours to have the injection site examined for a reaction. This reading is a critical step in determining whether or not you have been exposed to tuberculosis.
It's also important to note that a single TB skin test, as described here, is sometimes used for a routine screening. However, a two-step TB test is often performed to establish a baseline, especially for individuals who get tested regularly, such as healthcare workers. The two-step process helps to differentiate between a new infection and a boosted reaction from a past infection.
