How is a tuberculin skin test (TST) performed, interpreted, and managed for someone exposed to active tuberculosis?

Tuberculin Skin Test (TST) for TB Exposure

For someone exposed to active tuberculosis, administer 0.1 ml of 5 TU PPD intradermally using the Mantoux method, read at 48-72 hours measuring transverse induration diameter, interpret as positive if ≥5 mm for any TB contact, and repeat testing 8-10 weeks post-exposure if initial test is negative. 1

Administration Technique

The TST must be performed using the Mantoux method only - multiple puncture tests (Tine, Heaf) are insufficiently accurate and should never be used 2. Inject 0.1 ml of 5 tuberculin units (TU) PPD intradermally into the dorsal or volar forearm surface 2. You must produce a visible "wheal" at injection; if significant leakage occurs or the injection goes subcutaneous, repeat immediately at another site 3.

Reading and Interpretation

Timing and Measurement

• Read the test 48-72 hours after administration 2
• Measure only the transverse diameter of induration (not erythema) in millimeters 2
• If erythema >10 mm exists without induration, the injection was likely too deep and requires retesting 3

Interpretation for TB Contacts

All contacts of active TB cases use a ≥5 mm cutoff for positivity 1. This differs from general population screening and reflects the high-risk status of exposed individuals.

The standard three-tier interpretation system applies to different populations 2:

• ≥5 mm positive: HIV-positive persons, recent TB contacts, fibrotic chest radiograph changes, immunosuppressed patients (including >15 mg/day prednisone for ≥1 month), organ transplant recipients
• ≥10 mm positive: Recent immigrants from high-prevalence countries (within 5 years), injection drug users, residents/employees of high-risk congregate settings, mycobacteriology lab personnel, patients with diabetes/silicosis/chronic conditions
• ≥15 mm positive: Persons with no TB risk factors

Post-Exposure Testing Protocol

The Window Period

The critical window period is 8-10 weeks after exposure ends - not the previously cited 2-12 weeks 1. Any negative test obtained <8 weeks post-exposure is unreliable for excluding infection and mandates repeat testing.

Testing Algorithm by Priority

High-priority contacts (children <5 years, immunocompromised persons):

• Test at initial encounter, ideally within 7 working days 1
• If initial TST <5 mm and exposure was <8 weeks ago: start window prophylaxis after excluding active TB disease
• Repeat TST at 8-10 weeks post-exposure
• If second test remains negative: discontinue prophylaxis (unless immunocompromised - see below)
• If second test ≥5 mm: complete full LTBI treatment course 1

Medium-priority contacts:

• Test within 14 working days 1
• Follow same two-step protocol if initial test negative and within window period

Low-priority contacts:

• Single TST at end of window period (8-10 weeks) preferred to avoid booster phenomenon misinterpretation 1

Critical Caveat: Immunocompromised Contacts

Even if TST administered >8 weeks post-exposure is negative, immunocompromised contacts should receive full LTBI treatment after excluding active TB disease 1. This reflects their substantially elevated risk of progression regardless of test results.

Management After Positive TST

All contacts with ≥5 mm induration or any TB symptoms require further evaluation 1:

1. Chest radiograph (starting point for all)
2. Sputum collection only if clinically indicated - not routine for healthy contacts with normal radiographs 1
3. High-priority contacts (children <5 years, immunocompromised) require full diagnostic evaluation regardless of TST result or symptoms 1

Important Pitfalls

Do not use two-step testing for contacts - this is for serial testing programs to detect boosting 1. A contact whose second test becomes positive after initial negative should be classified as recently infected, not as a booster phenomenon (though boosting is more common in foreign-born individuals) 1.

BCG vaccination history is not a contraindication to TST 3. The larger the reaction, the greater the probability of true TB infection. For contacts, use the ≥5 mm cutoff regardless of BCG status.

Previous positive TST: Contacts with documented previous positive TST but no prior LTBI treatment can be considered for treatment as part of contact investigation 1. Immunocompromised contacts with previous positive TST should receive full LTBI treatment after excluding active disease, regardless of prior treatment history 1.