When should tuberculosis infection testing be performed after a known exposure, and when is repeat testing indicated?

When to Test for TB After Exposure

Test immediately when exposure is recognized, then repeat testing at 8–10 weeks after the last exposure if the initial test is negative. 1

Initial Testing Protocol

Perform a tuberculin skin test (TST) or interferon-gamma release assay (IGRA) as soon as the exposure is identified—ideally within the first week for high-priority contacts—regardless of any prior baseline results. 1, 2 This immediate test serves as a reference point, though it cannot reliably exclude infection if performed during the window period. 2, 3

• All exposed persons require a symptom evaluation at the time exposure is recognized, assessing for cough, fever, night sweats, weight loss, and hemoptysis. 1, 4
• Document the date of last exposure to calculate the proper timing for repeat testing. 2

The Critical 8–10 Week Repeat Test

The immune system requires 8–10 weeks after M. tuberculosis exposure to mount a detectable response on TST or IGRA testing. 2, 3 This is the most important concept in TB exposure management.

• All contacts with an initial negative test must undergo repeat testing 8–10 weeks after the last exposure, using the same test type (TST or IGRA) as the initial test to avoid false conversions from methodological differences. 1, 2
• A negative test obtained less than 8 weeks after exposure is unreliable for excluding infection—this represents the window period before immune response becomes detectable. 2, 3
• A positive result on the 8–10 week repeat test indicates infection occurred during the exposure period and warrants full evaluation and treatment for latent TB infection. 3

The CDC adopted this 8–10 week interval based on established TST protocols, though emerging research suggests most IGRA conversions occur 4–7 weeks after exposure, with some as late as 14–22 weeks. 2, 5 Despite this variability, the 8–10 week window remains the accepted standard of care. 2

Who Does NOT Need Repeat Testing

Persons with documented prior latent TB infection (LTBI) or TB disease do not require repeat testing after a new exposure. 1 These individuals should instead receive clinical evaluation if active TB disease is suspected based on symptoms. 1, 3

High-Risk Contacts Requiring Immediate Action

Certain populations cannot wait for the 8–10 week repeat test and require immediate intervention:

• Children under 5 years must begin treatment for presumptive infection (window prophylaxis) immediately after excluding active disease, even with negative initial testing, due to their vulnerability to severe disseminated TB and TB meningitis. 3
• HIV-infected and other severely immunocompromised contacts should receive immediate prophylactic treatment after active disease is excluded, regardless of initial test results. 3
• Close contacts with intense exposure (prolonged contact in poorly ventilated settings) should be offered immediate prophylaxis after ruling out active disease. 3

Critical Pitfalls to Avoid

Never assume a negative test during the first 8 weeks post-exposure rules out infection—this is the most common and dangerous error in TB contact management. 2, 3 The window period is real and clinically significant.

• Do not delay prophylactic treatment in high-risk contacts (especially young children and immunocompromised persons) while waiting for the 8–10 week repeat test. 3
• Both TST and IGRA may be less sensitive in severely immunosuppressed adults, children <5 years, or patients on TNF-α inhibitors—a negative result at 8–10 weeks should not be considered definitive for ruling out infection in these groups. 2
• Use the same test platform (TST or IGRA) for repeat testing to avoid apparent conversions caused by methodological differences rather than true infection. 2

Test Selection Considerations

IGRA is preferred over TST in patients with prior BCG vaccination (which causes false-positive TST results), in situations where patient return for TST reading is unlikely, and in patients already on immunosuppressive therapy. 2, 4 Either test is acceptable for contact investigations, and a positive result from either should prompt the same evaluation and management. 2

Follow-Up After Negative Repeat Testing

If the 8–10 week repeat test remains negative and the individual is asymptomatic, no further testing is required. 3 However, all contacts should be educated to monitor for TB symptoms (persistent cough, hemoptysis, night sweats, weight loss, fever) and seek immediate medical evaluation if any develop, even years later. 3