induration

Induration is the process or result of becoming hard, or the state of hardness in a material, tissue, or object. The term is used across disciplines, including medicine, geology, and materials science, and it often denotes hardening caused by inflammation, fibrous tissue deposition, mineralization, or cementation.

In medicine, induration describes a firm, palpable area of tissue that feels hard on examination. It can result from chronic inflammation, fibrosis, infiltration by cells, calcification, or scarring. Indurated lesions may occur in the skin, breast, lymph nodes, or subcutaneous tissues and can indicate a range of causes from benign scar tissue to malignancy. The concept is also central to tuberculin skin testing.

In tuberculin skin testing, induration refers to the raised, firm area at the injection site measured 48 to 72 hours after administration. The size in millimeters determines positivity, with thresholds varying by the individual’s risk factors and history. Erythema (redness) alone is not counted as induration.

In geology and materials science, induration describes the hardening of soft sediments into rock through diagenesis, cementation, or mineral precipitation. In industrial processes, induration refers to a heat-treatment step that hardens shaped particulates, such as iron-ore pellets or ceramic bodies.

Etymology: from Latin indurare, to harden.