TB Quantiferon / IGRA (Interferon-Gamma Release Assay for Latent Tuberculosis)
Also known as: QuantiFERON-TB Gold Plus, T-SPOT.TB, IGRA Blood Test, Latent TB Blood Test, TB Gold Test
Parameters Measured in the Report
A breakdown of individual biomarkers and biological reference intervals typically found in your report:
| Biomarker | Standard Range | Unit | What It Means |
|---|---|---|---|
TB Antigen minus Nil (IU/mL) (Quantiferon TB Response) | Negative: < 0.35 (Positive: ≥ 0.35 IU/mL and ≥ 25% of Nil Value) | IU/mL | Quantitative interferon-gamma release by CD4+ and CD8+ T-cells in response to specific M. tuberculosis antigens; unaffected by BCG vaccination. |
Mitogen minus Nil (Positive Control) (Mitogen Control) | Valid Response: ≥ 0.50 | IU/mL | Positive immune control verifying that patient T-cells are immunocompetent and capable of producing interferon. |
Nil Control (Negative Background) (Background IFN-g) | Valid: ≤ 8.0 | IU/mL | Background non-specific interferon present in serum; values >8.0 invalidate the assay. |
* Note: Reference ranges may vary slightly between testing instruments and reagent kits. Always refer to the reference interval printed on your specific pathology report.
Clinical Focus
In vitro production of Interferon-Gamma (IFN-gamma) by memory T-lymphocytes upon incubation with specific Mycobacterium tuberculosis peptide antigens (ESAT-6, CFP-10).
Sample & Specimen
Specialized four-tube blood collection set (Nil, TB1, TB2, Mitogen)
Fasting Protocol
No fasting required. Maintain regular water intake.
Turnaround Time
24–48 hours
What Is This Test?
Why Did Your Doctor Order This Test?
Physicians routinely prescribe the TB Quantiferon (IGRA) in the following clinical scenarios:
How to Prepare Before Your Test
- No fasting or special preparation required.
- Blood must be collected into four specific manufacturer tubes filled precisely to the indicator line (1 mL per tube), mixed by inversion 10 times, and incubated at 37°C within 16 hours of collection.
Understanding Your Results
What higher or lower readings generally mean in clinical practice:
If Values Are Higher Than Normal
A Positive Quantiferon TB test (≥0.35 IU/mL) confirms infection with *Mycobacterium tuberculosis*. It does not distinguish between latent infection and active clinical disease. A positive result mandates a chest X-ray and symptom review; if active disease is ruled out, preventive latent TB therapy (e.g., 3 months of once-weekly rifapentine + isoniazid) is prescribed to eliminate future reactivation.
If Values Are Lower Than Normal
A Negative result confirms that *M. tuberculosis* infection is unlikely. A report of "Indeterminate" occurs if the patient is severely immunocompromised (failed mitogen response) or background interferon was elevated, requiring a repeat test.
Frequently Asked Questions
Why is the Quantiferon blood test better than the traditional Mantoux skin prick test?
The Mantoux skin test gives false-positive readings in almost everyone who received the childhood BCG vaccine. Quantiferon tests specific proteins that do NOT exist in the BCG vaccine, eliminating false alarms. Furthermore, Quantiferon requires only a single blood draw rather than a return visit 48 hours later.
Does a positive Quantiferon test mean I am contagious and spreading TB?
No! Most positive Quantiferon tests represent "Latent TB." The bacteria are dormant, you feel completely fine, and you cannot spread TB to anyone. Doctors prescribe preventive medication to stop the dormant bacteria from waking up years later.
Related Laboratory Tests
Tests frequently prescribed together or used as follow-up investigations:
Sputum AFB & CBNAAT (Tuberculosis / GeneXpert MTB/RIF)
Microscopic Acid-Fast Bacilli (Ziehl-Neelsen / Auramine O fluorescent stain) and automated real-time PCR detection of Mycobacterium tuberculosis DNA plus Rifampicin resistance gene (rpoB).
Complete Blood Count (CBC / Hemogram)
Counts and evaluates red blood cells, white blood cells, hemoglobin, hematocrit, and platelets.
Inflammatory Markers (ESR & C-Reactive Protein / hs-CRP)
Rate of red blood cell settling (ESR) and hepatic acute-phase reactant protein concentration (CRP / hs-CRP).
Medical & Diagnostic Disclaimer
This guide is published for educational and patient informational purposes only. It is not a substitute for professional medical evaluation, diagnosis, or clinical advice.
A laboratory result that falls slightly outside standard statistical reference intervals does not automatically indicate disease; conversely, a normal result does not guarantee the absence of medical conditions. Only your attending physician can interpret lab findings in the context of your medical history, physical exam, and symptoms.