What is the specific test used to detect prior exposure to tuberculosis?

Tests for Prior Tuberculosis Exposure

Two tests are used to detect prior exposure to tuberculosis: the Tuberculin Skin Test (TST), also called the Mantoux test, and Interferon-Gamma Release Assays (IGRAs), which include QuantiFERON-TB Gold (QFT-G) and T-SPOT.TB. 1

Primary Testing Options

Tuberculin Skin Test (TST/Mantoux Test)

• The TST involves intradermal injection of purified protein derivative (PPD) tuberculin, with measurement of induration (not erythema) after 48-72 hours 1, 2
• An induration diameter ≥5 mm is considered positive for any contact of an infectious TB case 1
• The test detects a delayed-type hypersensitivity response indicating prior exposure to Mycobacterium tuberculosis or BCG vaccination 3, 4

Interferon-Gamma Release Assays (IGRAs)

• QuantiFERON-TB Gold (QFT-G) measures interferon-gamma release from lymphocytes when blood is incubated with M. tuberculosis-specific proteins ESAT-6 and CFP-10 1
• T-SPOT.TB (ELISPOT test) uses a similar principle but is formatted differently 1
• IGRAs are unaffected by BCG vaccination and most non-tuberculous mycobacteria, providing greater specificity than TST 1, 5

When to Use Each Test

IGRA is Preferred Over TST in:

• Patients with prior BCG vaccination, as TST produces false-positives in this population 1, 6
• Situations where patient return for TST reading (48-72 hours later) is unlikely 7
• Patients already on immunosuppressive therapy 7
• Foreign-born individuals from countries where BCG vaccination is routine 1

Either Test Can Be Used:

• The CDC states that QFT-G can be used in all circumstances where TST is currently used, including contact investigations 1
• QFT-G should be used in place of, not in addition to, TST 1
• A positive result from either test should prompt the same evaluation and management 1

Critical Testing Timing

Initial Testing

• Test all contacts without documented prior TB or latent TB infection (LTBI) at initial encounter, ideally within 7 days for high-priority contacts 1

Follow-Up Testing (The 8-10 Week Window)

• If the initial test is negative, repeat testing must occur 8-10 weeks after the last exposure using the same test type 1, 8
• This timing reflects the window period required for the immune system to mount a detectable response after M. tuberculosis exposure 1, 8
• A negative test obtained <8 weeks after exposure is unreliable for excluding infection 1, 8

Important Clinical Caveats

Do Not Retest These Groups:

• Persons with documented prior LTBI or TB disease do not need repeat testing after exposure 1, 7
• Instead, evaluate them clinically if TB disease is suspected 1

Test Limitations in Immunocompromised Patients:

• Both QFT-G and TST may be less sensitive in severely immunosuppressed adults, children <5 years, or patients about to receive TNF-α inhibitors 1
• A negative result alone should not exclude M. tuberculosis infection in these high-risk groups 1

BCG Vaccination Considerations:

• A positive TST in a BCG-vaccinated contact of an infectious TB case should be interpreted as evidence of recent M. tuberculosis infection, not just vaccine effect 1
• The distortion from BCG is almost insignificant in adults >30 years of age 1
• TST may be false-negative in patients on corticosteroids >1 month or immunomodulators >3 months 1

Two-Step Testing:

• The routine two-step TST procedure (used to detect boosting in serial testing programs) should typically NOT be used for contact investigations 1
• A contact whose second test is positive after an initial negative result should be classified as recently infected, not as a booster phenomenon 1

Confirmatory Testing Strategy

• For asymptomatic persons at low risk with a positive initial test, perform a second confirmatory test (either IGRA or TST) 7
• Consider the person infected only if both tests are positive 7
• After any positive test, obtain chest radiography to distinguish latent TB from active disease 7