T-cell assays for tuberculosis infection: deriving cut-offs for conversions using reproducibility data

Area of Science:

• Immunology
• Infectious Diseases
• Diagnostic Assays

Background:

• Interferon-gamma release assays (IGRA) are alternatives to tuberculin skin tests for tuberculosis screening.
• Interpreting serial IGRA results is challenging due to limited evidence on conversion and reversion cut-offs.
• Understanding within-person variability is crucial for serial IGRA testing.

Purpose of the Study:

• To evaluate the short-term reproducibility of the QuantiFERON-TB Gold In Tube (QFT) assay.
• To assess test-retest and within-person variability of QFT results.
• To inform optimal cut-offs for QFT conversions and reversions.

Main Methods:

• Pilot study conducted in India with 14 healthcare workers.
• Four serial QFT tests performed over 12 days (Day 0, 3, 9, 12).
• QFT ELISA repeated twice on the same specimens to assess test-retest reproducibility; continuous IFN-gamma levels analyzed for within-person variability.

Main Results:

• Extremely high test-retest concordance (kappa = 0.94) for dichotomous QFT results.
• Discordance observed around the cut-off point, with both increases and decreases in IFN-gamma.
• Moderate to high within-person reproducibility; negative results generally remained negative, while positive results varied.
• Increases of more than 16% in IFN-gamma levels are statistically improbable in the short-term.

Conclusions:

• Hypothesize that QFT conversion requires a negative-to-positive result change and at least a 30% increase in baseline IFN-gamma.
• Suggests that long-term variability may be at least double short-term variability.
• Larger studies are needed to confirm findings and establish definitive IGRA conversion thresholds.