What constitutes a 1-step tst?

PPD: Ppd test one time, as oppose to a 2-step tst where the PPD test is repeated within 1 month of the first ppd.

TB test: Tuberculin skin test consists of injecting a small amount of bacterial protein into the skin. The site is checked 3 days later for redness and swelling. Sometimes the test is repeated a second time after 1-3 weeks if it is negative, because the first test can serve as reminder to the immune system and can make the second test positive. When it's done twice, it's a two-step test, otherwise 1 step.