Time interval for QuantiFERON-TB Gold Plus conversion after last exposure with TB

Who

• Study population: Close contacts of notified tuberculosis (TB) cases

• Inclusion criteria:

  • Aged ≥15 years

  • Initial negative QFT within 56 days of last exposure date (LED)

  • Follow-up QFT within 180 days of LED

• Sample size: 23,236 contacts

• Demographics:

  • Largest age groups: 30–39 years (23.9%), 20–29 years (20.0%)

  • QFT conversion increased with age, highest in those aged 70–79 years (5.8%)

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What

• Primary finding:

  • 3.5% (804/23,236) of contacts experienced QFT conversion on follow-up

  • Median time to QFT conversion was 10 weeks post-LED (IQR 9–11 weeks)

• Timing of conversion:

  • 73% converted within ≤10 weeks

  • 27% converted after 10 weeks, up to 25 weeks

• Clinical outcomes:

  • 45 contacts were diagnosed with active TB disease

  • 69% (31/45) had QFT conversion

  • Some active TB cases were identified only because repeat QFT occurred after 10 weeks

• Authors’ conclusion:

  • A longer window period (≥10 weeks) is more effective for detecting later QFT conversions

  • Repeating QFT too early may miss TB infection and early active disease

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When

• Study period: 1 January 2018 – 31 December 2022

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Where

• Setting: National TB contact investigation program in Singapore

• Data source: National TB Registry

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Why

• To determine the optimal timing for repeat QFT testing after TB exposure

• To address uncertainty around the QFT window period, particularly regarding later conversions that may be clinically significant

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How

• Study design: Retrospective cohort study

• Exposure reference: Last exposure date (LED) to an infectious TB case

• Outcome definition:

  • QFT conversion = negative initial QFT → positive follow-up QFT

• Analysis:

  • Timing of QFT conversion relative to LED

  • Stratification by age and follow-up interval

  • Identification of active TB disease following QFT results

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Summary implication:
This large national cohort study demonstrates that while most QFT conversions occur within 10 weeks of TB exposure, over one-quarter occur later. Extending the repeat QFT window to at least 10 weeks post-exposure may improve detection of TB infection and prevent missed diagnoses of active TB, especially in programmatic contact investigations.

Source: Kyaw, W.M., Tay, J.Y., Lim, L.K.Y. and Ng, D.H.L., 2025. Time interval for QuantiFERON-TB Gold Plus conversion after last exposure with tuberculosis. ERJ Open Research, 11(3).