This image depicts the hands of a clinician in the process of marking the widest edges of this patient’s induration (a hard, dense, raised formation) with a pen for accurate measurement of the Mantoux tuberculin skin test reaction on this person’s left arm. This skin test is used to evaluate people for latent tuberculosis (TB) infection. After the skin test is administered as an intradermal injection, the patient should return in 48 to 72-hours to have the reaction measured, and interpreted by a health care provider. To locate the skin test site, the arm should be inspected in good light, and on a firm surface. A light, gentle motion is used to sweep the fingertips over the surface of the forearm to locate the margins, or edges of induration. As in this image, the widest edges of the induration are marked with a pen, using the fingertips as a guide. A millimeter ruler is then used to measure the diameter of the induration between the two marks.