This figure shows the correct method for measuring a reaction to the tuberculin skin test. The size of the reaction is measured by the width of induration, not erythema. In the example shown, the reaction measures 10 mm.
Redrawn from: Testing for Tuberculosis Infection and Disease. In: Core Curriculum on Tuberculosis: What the Clinician Should Know, Sixth Edition, Centers for Disease Control and Prevention, 2013. Available at:
https://www.cdc.gov/tb/education/corecurr/index.htm.