An IGRA (QuantiFERON-TB Gold Plus) result shows: TB1 (CD4 peptides) = 1.2 IU/mL, TB2 (CD4+CD8 peptides) = 1.8 IU/mL, Nil = 0.1 IU/mL, Mitogen = 8.5 IU/mL. This IGRA result is interpreted as:

• A Positive — TB1 (or TB2) minus Nil ≥0.35 IU/mL AND ≥25% of Nil value ✓
• B Indeterminate — mitogen control <0.5 IU/mL indicates technical failure
• C Negative — absolute values below 2.0 IU/mL are always classified as negative
• D Indeterminate — Nil value exceeds 8.0 IU/mL indicating background inflammation

Explanation

QuantiFERON-TB Gold Plus (QFT-Plus) interpretation uses the following algorithm: Result is POSITIVE if TB1 or TB2 (minus Nil) ≥0.35 IU/mL AND ≥25% of Nil control value. In this case: TB1 − Nil = 1.2 − 0.1 = 1.1 IU/mL (≥0.35) AND 1.1/0.1 = 11× Nil (≥25%), confirming a positive result. The Mitogen control (8.5 IU/mL) confirms the test is technically valid (mitogen response ≥0.5 IU/mL ensures T cells are functional). QFT-Plus adds a TB2 tube with peptides stimulating both CD4+ and CD8+ T cells, potentially improving sensitivity in immunocompromised patients compared to QFT-GIT (which only measures CD4+ responses).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.