An QuantiFERON-TB Gold Plus (QFT-Plus) assay is performed on a healthcare worker for latent TB screening. The result shows: TB1 tube IFN-γ = 0.60 IU/mL, TB2 tube IFN-γ = 0.55 IU/mL, Nil = 0.20 IU/mL, Mitogen = 8.0 IU/mL. How should this result be interpreted?

• A Negative (TB1 and TB2 values are below absolute cutoff of 1.0 IU/mL)
• B Positive for LTBI (TB1 minus Nil = 0.40, above 0.35 IU/mL threshold) ✓
• C Indeterminate (Mitogen response inadequate)
• D Invalid (Nil value exceeds 8.0 IU/mL)

Explanation

QFT-Plus result is positive when [TB1 − Nil] ≥0.35 IU/mL AND [TB1 − Nil] ≥25% of Nil value (when Nil is 0.20–8.0 IU/mL). Here TB1 − Nil = 0.60 − 0.20 = 0.40 IU/mL (≥0.35), and 0.40 is ≥25% of 0.20 (0.20 × 25% = 0.05); therefore the result is positive. The mitogen tube ≥0.5 IU/mL confirms validity. A Nil >8 IU/mL would make the test indeterminate.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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