Induration is a physical sign in medicine referring to a localized area of hard, firm tissue. This finding is detected by touch and indicates a change in the underlying structure of the affected area, often the skin and the tissue just beneath it. Induration provides clues about the nature and extent of an underlying biological process. Unlike simple swelling, this distinct type of tissue change is a significant diagnostic indicator in various medical contexts.
Defining Induration
Induration is characterized by tissue that feels abnormally dense, firm, and resistant to compression when palpated. This sensation is distinctly different from simple swelling, or edema, which is caused by an accumulation of thin, watery fluid. Edematous tissue is typically soft and may exhibit “pitting,” where a temporary depression remains after pressure is applied. Indurated tissue maintains its rigid, compressed structure because the material causing the hardening is complex and not easily displaced.
The physical observation of induration signifies a structural transformation within the tissue layers. While it may appear raised, thick, or shiny, its defining feature is the palpable firmness. This alteration reflects a change in the cellular or protein composition, suggesting an active process like inflammation, infection, or the deposition of abnormal material.
Biological Mechanisms of Tissue Hardening
Induration is the macroscopic result of specific microscopic changes that alter the tissue’s physical properties. One primary mechanism involves dense cellular infiltration, where immune cells, such as lymphocytes and macrophages, migrate into the affected area as part of an inflammatory response. The sheer volume and tightly packed arrangement of these cells create a firm mass that displaces normal, softer tissue components. This reaction is a hallmark of delayed-type hypersensitivity responses.
Another factor contributing to the firmness is the exudation of protein-rich fluid from blood vessels into the surrounding tissue. Unlike the low-protein fluid of simple edema, this exudate contains fibrin and other large molecules that coagulate within the tissue matrix. This coagulation acts like a biological cement, forming a dense, non-pitting substance that resists palpation.
Chronic hardening is often caused by fibrosis, which involves the excessive deposition of connective tissue, primarily collagen. This collagen overgrowth leads to a scar-like texture that is unyielding and inelastic, representing a permanent structural change. These three processes—cellular infiltration, protein exudation, and collagen deposition—frequently occur together, resulting in the characteristic firmness of induration.
Clinical Contexts Where Induration Matters
The most common application of induration measurement is in the Tuberculin Skin Test (TST), also known as the PPD test, used for screening tuberculosis infection. In this test, a small amount of purified protein derivative (PPD) is injected into the forearm, and the size of the resulting induration is measured 48 to 72 hours later. Only the diameter of the firm, raised area is measured, not the surrounding redness, which is called erythema.
The measurement of induration in the PPD test directly reflects the magnitude of the immune system’s delayed hypersensitivity reaction. A positive result is defined by specific induration sizes depending on the patient’s risk factors, indicating a previous exposure to the tuberculosis bacteria. For example, the threshold for a positive result is lower (e.g., 5 millimeters) for high-risk individuals compared to those with no known risk factors.
Induration is also an important sign in the assessment of localized infections, such as cellulitis. While the skin becomes visibly red and swollen in this condition, the presence of underlying induration suggests a deeper, more severe inflammatory process or an abscess formation. Furthermore, a temporary, localized induration can form after certain vaccinations or injections due to the body’s reaction to the injected substance. Assessing the degree of induration helps clinicians differentiate between routine post-injection inflammation and a more concerning complication.