What is induration?

Definition of Induration

Induration is a hard, raised, palpable swelling that develops at the site of inflammation or injection as part of a delayed-type hypersensitivity reaction. 1

Characteristics of Induration

• Induration is characterized by hardness or firmness of tissue that can be felt upon palpation 2, 1
• It differs from erythema (redness) alone, which should not be confused with induration 2, 1
• Induration is caused by a cellular immune response involving T-cells and macrophages that accumulate at the site of inflammation or injection 1
• The reaction typically begins 5-6 hours after injection (in the case of tuberculin skin testing), reaches maximum induration at 48-72 hours, and then gradually subsides 2

Clinical Assessment of Induration

• Induration is assessed through visual inspection and palpation of the affected area 2, 1
• When measuring induration (such as in tuberculin skin testing), it should be done in good light with proper positioning of the affected area 2
• The diameter of induration should be measured transversely (perpendicular to the long axis in case of forearm) 2, 1
• Induration can be objectively measured using specialized devices like the Indentometer, which measures the stiffness of skin by determining how deeply a probe can penetrate 3

Clinical Significance of Induration

• Induration is a key diagnostic finding in several clinical contexts:

  • In diabetic foot infections, local swelling or induration is one of the clinical signs used to diagnose infection 2
  • In tuberculin skin testing, the size of induration (not erythema) is measured to determine if a person has been infected with Mycobacterium tuberculosis 2, 1
  • In intravascular catheter-related infections, induration around the catheter exit site is a sign of infection 2
  • In radiation therapy, induration can be a late effect in irradiated tissue 4

• The interpretation of induration size in tuberculin skin testing varies based on risk factors:

  • ≥5 mm is considered positive in high-risk individuals (HIV-positive, recent TB contacts, etc.) 2, 1
  • ≥10 mm is considered positive in persons with moderate risk factors 2, 1
  • ≥15 mm is considered positive in persons with no known risk factors for TB 2, 1

Pathophysiology of Induration

• Induration results from local vasodilatation, edema, fibrin deposition, and recruitment of inflammatory cells to the area 2
• In the context of tuberculin skin testing, induration represents a delayed (cellular) hypersensitivity reaction 2
• MRI studies of induration in irradiated breast tissue have shown that increased fluid content in tissues and skin edema are important contributors to palpable induration 4
• Induration may persist for extended periods after initial injury or inflammation, as seen in post-radiation changes 4, 5

Clinical Applications

• Proper assessment of induration is crucial for:
  • Diagnosing and classifying severity of infections 2
  • Interpreting tuberculin skin test results 2, 1
  • Monitoring treatment response in conditions like psoriasis and scleroderma 3
  • Assessing complications of medical procedures such as chemotherapy administration 6

Understanding induration and distinguishing it from simple erythema is essential for accurate clinical assessment and diagnosis across multiple medical specialties.