IGRA Testing Timeline After TB Exposure
Following known exposure to Mycobacterium tuberculosis, an IGRA should be performed at initial contact and then repeated at 8-10 weeks after the last exposure if initially negative, as the test may not become reliably positive until this window period has elapsed. 1, 2
Critical Timing Considerations
The 8-10 Week Window Period
The CDC explicitly recommends repeat testing at 8-10 weeks after the end of exposure for contacts with an initial negative IGRA result, acknowledging that data on the precise timing of IGRA conversion after new infection are not currently available. 1
A negative test obtained before 8 weeks after exposure is unreliable for excluding infection, as this reflects the window period required for the immune system to mount a detectable response after M. tuberculosis exposure. 2
The 8-10 week recommendation for IGRAs mirrors the established protocol for tuberculin skin testing, applied due to lack of specific IGRA conversion data at the time these guidelines were developed. 1
Research Evidence on Actual Conversion Times
While guidelines recommend the 8-10 week window, the highest quality research study tracking actual IGRA conversion times found that conversion generally occurred 4-7 weeks after exposure, though it could occur as late as 14-22 weeks in some contacts. 3
In this military outbreak study of 27 contacts with serial IGRA testing, among 10 soldiers with initially negative results: 3 converted at 2 weeks, 3 at 4 weeks, and 3 at 14 weeks after exposure. 3
One contact did not convert during the entire 30-week observation period despite documented exposure. 3
Practical Testing Algorithm
Initial Testing
Perform IGRA (or TST) at the time exposure is recognized, ideally within 7 days for high-priority contacts. 2
Document the timing of last exposure to the infectious TB case. 1, 2
Follow-Up Testing Strategy
If the initial test is negative and performed less than 8 weeks after last exposure, repeat testing at 8-10 weeks is mandatory. 1, 2
Use the same test format (IGRA type) for repeat testing to minimize conversions that occur due to test differences rather than true infection. 1
Important Caveats
Persons with documented prior LTBI or TB disease do not require repeat testing after exposure, but should undergo clinical evaluation if TB disease is suspected. 1, 2
In severely immunocompromised contacts (HIV-infected, children <5 years, or those starting TNF-α antagonists), a negative IGRA result—even at 8-10 weeks—should not definitively exclude M. tuberculosis infection, as these tests may have reduced sensitivity in these populations. 1, 2
The guideline-recommended 8-10 week window represents a conservative approach based on TST data, while emerging research suggests many conversions occur earlier (4-7 weeks), though late conversions up to 14-22 weeks can occur. 1, 3 The 8-10 week timeframe remains the standard of care for repeat testing to balance sensitivity with practical implementation. 1, 2