Understanding Induration in Tuberculin Skin Testing

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 2
The induration is caused by a cellular immune response involving T-cells and macrophages that accumulate at the site of tuberculin injection 1

TST should be read 48-72 hours after administration, when induration is at its maximum 3, 2
The transverse diameter of induration (perpendicular to the long axis of the forearm) should be measured and recorded in millimeters 2, 4
Measurement should be performed in good light with the forearm slightly flexed at the elbow 2
The ball-point pen method of Sokal can help decrease interobserver variability when measuring induration 2
The absence of induration should be recorded as "0 mm," not "negative" 2

The American Thoracic Society recommends three different cut-off points for interpreting induration size: ≥5 mm, ≥10 mm, and ≥15 mm 3, 5, 7
≥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, and children younger than 4 years or those exposed to high-risk adults 5, 3, 7
≥10 mm is considered positive in recent immigrants from high-prevalence countries, injection drug users, residents and employees of high-risk congregate settings, mycobacteriology laboratory personnel, and persons with clinical conditions that place them at high risk 3, 5, 7
≥15 mm is considered positive in persons with no known risk factors for TB 3, 7

BCG-induced tuberculin reactivity typically wanes over time and is unlikely to persist >10 years after vaccination 8
A positive TST in BCG-vaccinated persons should still be considered indicative of TB infection, especially in persons from high-prevalence countries 8
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 8
HIV infection can cause anergy (inability to mount a DTH response) even in persons infected with M. tuberculosis, and a lower cut-off (≥5 mm) is used for HIV-positive individuals to increase test sensitivity 3
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 5

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 6