What is the meaning of induration size in a tuberculin skin test (TST)?

Understanding Induration in Tuberculin Skin Testing

Induration in a tuberculin skin test (TST) refers to the firm, raised area that develops at the injection site, which is measured in millimeters to determine if a person has been infected with Mycobacterium tuberculosis.

What is Induration?

• Induration is the hard, raised, palpable swelling that develops at the site of tuberculin injection as part of a delayed-type hypersensitivity (DTH) reaction 1
• It is different from redness (erythema) alone, which should not be measured when reading a TST 1
• The induration is caused by a cellular immune response involving T-cells and macrophages that accumulate at the site of tuberculin injection 1

Proper Measurement Technique

• TST should be read 48-72 hours after administration, when induration is at its maximum 1
• The transverse diameter of induration (perpendicular to the long axis of the forearm) should be measured and recorded in millimeters 1
• Measurement should be performed in good light with the forearm slightly flexed at the elbow 1
• The ball-point pen method of Sokal can help decrease interobserver variability when measuring induration 1
• The absence of induration should be recorded as "0 mm," not "negative" 1

Interpretation of Induration Size

The interpretation of induration size depends on the patient's risk factors. The American Thoracic Society and CDC recommend three different cut-off points:

≥5 mm is considered positive in:

• HIV-positive persons
• Recent contacts of TB case patients
• Persons with fibrotic changes on chest radiograph consistent with prior TB
• Patients with organ transplants or other immunosuppressed patients
• Children younger than 4 years or those exposed to high-risk adults 1, 2

≥10 mm is considered positive in:

• Recent immigrants (within 5 years) from high-prevalence countries
• Injection drug users
• Residents and employees of high-risk congregate settings
• Mycobacteriology laboratory personnel
• Persons with clinical conditions that place them at high risk (diabetes, chronic renal failure, leukemias, etc.) 1, 2

≥15 mm is considered positive in:

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

Special Considerations

BCG Vaccination

• BCG-induced tuberculin reactivity typically wanes over time and is unlikely to persist >10 years after vaccination 1
• A positive TST in BCG-vaccinated persons should still be considered indicative of TB infection, especially in persons from high-prevalence countries 1, 2
• The size of a tuberculin reaction in a BCG-vaccinated person does not reliably distinguish between reaction caused by M. tuberculosis infection or prior BCG vaccination 1

HIV Infection

• HIV infection can cause anergy (inability to mount a DTH response) even in persons infected with M. tuberculosis 1, 3
• A lower cut-off (≥5 mm) is used for HIV-positive individuals to increase test sensitivity 1, 3
• However, reducing the cut-off from 10 mm to 5 mm provides only limited benefit as the loss of TST sensitivity in HIV-infected patients is predominantly due to anergy (an all-or-nothing phenomenon) 4

Skin Test Conversion

• For individuals with negative tuberculin skin test reactions who undergo repeat testing, an increase in reaction size of ≥10 mm within a 2-year period is considered a skin test conversion, indicating recent infection with M. tuberculosis 1, 2

Clinical Implications

• A positive TST indicates that a person has been infected with M. tuberculosis but does not differentiate between latent TB infection and active TB disease 1
• All persons with positive tuberculin skin test results should undergo further examination, typically starting with a chest radiograph 2
• The size of induration correlates with the risk of developing active TB disease, with larger reactions generally indicating a higher likelihood of true infection 5
• The probability of M. tuberculosis infection at each induration size varies widely depending on the prevalence of TB in the population 5

Understanding induration measurement and interpretation is crucial for accurate diagnosis of TB infection and appropriate clinical decision-making regarding preventive therapy.