What is a two-step PPD test?

A two-step PPD test involves taking two separate PPD tests at least a week apart, explains The Vaccine Center and Travel Medicine Clinic. Each test requires two doctor visits: one to receive the test injection and another two to three days later to read the test results.

The Vaccine Center and Travel Medicine Clinic explains that using a two-step PPD process with two separate tests is more likely to give an accurate result than a single test. The PPD, or purified protein derivative, test identifies tuberculosis, explains Healthline. The test involves receiving a small injection of proteins that react to tuberculosis. After two to three days, the doctor examines the injection site. No swelling or minor swelling is a negative result. Interpretation of a positive result depends on both the patient's medical history and the size of the swelling.