Two-step testing is necessary to obtain an accurate baseline when using the TST. The reason this is necessary is because of a condition referred to as Booster Phenomenon. Booster Phenomenon can occur when someone who was infected with TB in the past has a TST placed years after being infected. Because their body is so used to living with TB infection, when an initial TST is placed the persons immune system does not react. However, if a second TST is placed it “boosts” the persons immune system and a positive reaction will result. This positive reaction does not represent a recent infection, rather it is an old infection.