What is induration and how is it treated in a patient with unknown demographic and medical history?

What is Induration?

Induration is a hard, raised, palpable swelling that develops at the site of tuberculin injection, representing a delayed-type hypersensitivity (DTH) reaction involving T-cells and macrophages. 1

Key Characteristics of Induration

• Induration is fundamentally different from erythema (redness), which should never be measured or recorded when reading a tuberculin skin test (TST). 1, 2

• The cellular immune response creates a firm, palpable area that can be felt and measured, unlike simple redness which is not clinically significant for TST interpretation. 1

• Induration represents the accumulation of immune cells at the injection site and indicates prior exposure to Mycobacterium tuberculosis or related mycobacteria. 1

Proper Measurement Technique

• The transverse diameter of induration (perpendicular to the long axis of the forearm) should be measured 48-72 hours after tuberculin injection and recorded in millimeters. 3, 1, 2

• Measurement must be performed in good light with the forearm slightly flexed at the elbow, using both inspection and palpation to determine the boundaries. 1, 2

• The ball-point pen method can decrease interobserver variability when measuring induration. 1, 2

• Trained personnel must perform the reading—patient self-reading is never acceptable. 2

• Tests read after 72 hours tend to underestimate the true size and should be avoided. 2

• The absence of induration should be recorded as "0 mm," not simply "negative." 1

Risk-Stratified Interpretation of Induration Size

The interpretation of induration depends entirely on the patient's risk factors for tuberculosis:

≥5 mm Induration is Positive in:

• HIV-positive persons 3, 1, 2
• Recent contacts of tuberculosis case patients 3, 1, 2
• Persons with fibrotic changes on chest radiograph consistent with prior TB 3, 1, 2
• Patients with organ transplants or receiving immunosuppressive therapy (equivalent of >15 mg/day prednisone for ≥1 month) 3, 1
• Children younger than 4 years of age or those exposed to high-risk adults 3, 1

≥10 mm Induration is Positive in:

• Recent immigrants (within 5 years) from high-prevalence countries 3, 1, 2
• Injection drug users 3, 1, 2
• Residents and employees of high-risk congregate settings (prisons, nursing homes, hospitals, homeless shelters) 3, 1, 2
• Mycobacteriology laboratory personnel 3, 1, 2
• Persons with clinical conditions increasing TB risk: silicosis, diabetes mellitus, chronic renal failure, hematologic disorders, head/neck/lung malignancies, >10% weight loss, gastrectomy, jejunoileal bypass 3, 1, 2

≥15 mm Induration is Positive in:

• Persons with no known risk factors for TB 3, 1, 2

Skin Test Conversion

• An increase in induration of ≥10 mm within a 2-year period is considered a skin test conversion, indicating recent infection with M. tuberculosis. 3, 1, 2

• This definition applies to persons with initially negative tuberculin skin tests who undergo repeat testing (such as healthcare workers). 3

Special Considerations

BCG Vaccination

• Tuberculin skin testing is not contraindicated in BCG-vaccinated persons, and positive reactions should be interpreted as M. tuberculosis infection when risk factors are present. 3, 2

• No method can reliably distinguish tuberculin reactions caused by BCG vaccination from those caused by natural mycobacterial infection. 3

• BCG-induced reactivity typically wanes over time and is unlikely to persist >10 years after vaccination. 1

Immunocompromised Patients

• HIV infection and other immunosuppressive conditions can cause anergy (inability to mount a DTH response), making the lower ≥5 mm cutoff necessary to increase test sensitivity. 1, 4

• Negative TST does not exclude TB infection in immunocompromised patients due to anergy. 4

• Anergy testing is not recommended for routine use as it is controversial and unreliable. 3, 4

Clinical Management After Positive Induration

• All persons with positive tuberculin skin test results require chest radiography to exclude active tuberculosis disease. 3, 1

• A positive TST indicates M. tuberculosis infection but does not differentiate between latent TB infection and active TB disease. 1

• In immunocompromised patients, CT chest is mandatory even with normal chest radiography, as chest X-rays are frequently deceptively normal in this population. 4

• Sputum examinations are indicated in those with suspicious signs/symptoms or abnormal chest radiographs. 3

Treatment for Latent TB Infection (Positive Induration Without Active Disease)

• Single drug therapy with isoniazid for 9 months or rifampin for 4 months is standard treatment. 3

• Two to four drug therapy for 2-3 months should be used if suspicion of resistant organisms is high (areas where resistance is >10%). 3

• Liver function tests should be monitored every 2-4 weeks during antituberculous treatment. 3

Critical Pitfalls to Avoid

• Never measure or record erythema alone—only induration matters. 1, 2

• Never accept patient self-reading of tuberculin skin tests. 2

• Never interpret a positive test as "negative" in BCG-vaccinated individuals when risk factors are present. 2

• Never delay reading beyond 72 hours as this underestimates true induration size. 2

• Never rely on chest radiography alone in immunocompromised patients—proceed directly to CT chest. 4

• Never assume negative TST/IGRA excludes active TB in immunocompromised patients. 4