Normal Range for QuantiFERON-TB Gold Test
A QuantiFERON-TB Gold test is interpreted as negative (normal) when the TB Response is <0.35 IU/ml, provided the Nil control is ≤0.7 IU/ml and the Mitogen control is ≥0.5 IU/ml. 1
Interpretation Criteria
The FDA-approved interpretation for QuantiFERON-TB Gold In-Tube (QFT-GIT), which is the current standard version, uses the following thresholds 1, 2:
Negative (Normal) Result:
- TB Response <0.35 IU/ml
- This indicates no evidence of Mycobacterium tuberculosis infection
Positive Result:
- TB Response ≥0.35 IU/ml AND ≥25% of the Nil control value
- When Nil is between 0.7-8.0 IU/ml, the TB Response must be ≥25% of the Nil value to be considered positive
Indeterminate Result:
- Mitogen control <0.5 IU/ml (inadequate immune response), OR
- Nil control >8.0 IU/ml (excessive background)
Important Clinical Context
Borderline Zone Considerations
While the FDA cutoff is 0.35 IU/ml, recent evidence suggests a "borderline range" of 0.20-0.99 IU/ml warrants careful interpretation 3. In this borderline zone:
- 42-66% of patients retest as convincingly negative (<0.20 IU/ml) when a second sample is drawn 4, 3
- Test variability is highest near the cutoff, with normal within-subject variation of ±0.24 IU/ml for results between 0.25-0.80 IU/ml 5
- Consider repeat testing for results between 0.20-0.99 IU/ml to improve diagnostic reliability 3
Expected Values for Controls
Normal Nil control values in most U.S. populations are typically **<0.7 IU/ml** 1. Values >0.7 IU/ml suggest elevated background interferon-gamma and may indicate:
- Inflammatory conditions
- Recent infections
- Need for repeat testing
Normal Mitogen control values should be ≥0.5 IU/ml 1, demonstrating adequate T-cell function. Values <0.5 IU/ml may occur with:
- Immunosuppression
- HIV infection
- Corticosteroid use
- Technical issues with specimen handling
Critical Caveats
Test Limitations
- A negative result does NOT exclude TB infection in high-risk individuals (recent contacts, immunocompromised patients, those with clinical/radiographic TB findings) 1, 6
- Cannot distinguish latent TB infection from active TB disease - clinical evaluation with chest X-ray and symptom assessment is mandatory 1
- Sensitivity is only 67-82% for active TB disease 2, 7, meaning false negatives occur
When to Repeat Testing
Repeat testing 8-10 weeks after exposure is recommended for 6:
- Initial negative results in recent TB contacts
- Indeterminate results when clinical suspicion persists
- Results in the borderline range (0.20-0.99 IU/ml)
Special Populations
Lower sensitivity occurs in 1, 7:
- HIV-positive patients
- Children <5 years old
- Severely immunocompromised individuals
In these populations, negative results require particularly cautious interpretation and may warrant additional testing or empiric treatment based on exposure risk.