What is the TB Ferron Gold test and when is it indicated?

QuantiFERON-TB Gold Test: Overview and Indications

The QuantiFERON-TB Gold (QFT-G) test is an in vitro blood test that detects interferon-gamma release from sensitized T cells when exposed to specific Mycobacterium tuberculosis antigens, and it is indicated for diagnosing both latent tuberculosis infection (LTBI) and active tuberculosis disease. 1

What is the QuantiFERON-TB Gold Test?

The QFT-G test is an enzyme-linked immunosorbent assay (ELISA) that:

• Detects the release of interferon-gamma (IFN-γ) in fresh heparinized whole blood from sensitized persons
• Uses synthetic peptides representing two specific M. tuberculosis proteins:
  • Early secretory antigenic target-6 (ESAT-6)
  • Culture filtrate protein-10 (CFP-10)
• Provides results within 24 hours without requiring a follow-up visit
• Was approved by the FDA in 2005 as an aid for diagnosing M. tuberculosis infection 1

Advantages Over Traditional Tuberculin Skin Test (TST)

The QFT-G test offers several advantages over the traditional TST:

• Greater specificity: The test uses M. tuberculosis-specific antigens (ESAT-6 and CFP-10) that are absent from all BCG vaccine strains and most non-tuberculous mycobacteria 1
• Single patient visit: Results can be available in less than 24 hours without requiring a second visit 1
• Objective results: As a laboratory-based assay, it's not subject to reader bias and errors in placement and interpretation 1
• No boosting effect: Unlike TST, it doesn't trigger an anamnestic response that could affect future tests 1

Clinical Indications

The QFT-G test is indicated in the following scenarios:

1. Diagnosis of M. tuberculosis infection:

   • For both latent TB infection (LTBI) and active TB disease 1
   • Particularly useful in BCG-vaccinated individuals due to higher specificity 1

2. Contact investigations:

   • Screening of close contacts of TB cases 1, 2
   • Particularly valuable in healthcare settings after accidental exposure 3

3. Evaluation of recent immigrants from TB-endemic countries 1

4. Sequential-testing surveillance programs:

   • For infection control in healthcare workers 1, 3
   • For monitoring high-risk populations 1

5. Pre-immunosuppressive therapy screening:

   • Before initiating immunotherapy or intensive chemotherapy 4
   • Particularly important in cancer patients 4

Test Performance and Interpretation

The sensitivity of QFT-G is approximately 85% (comparable to TST) for detecting infection in persons with untreated culture-confirmed TB 1, 5. The specificity is around 73-97%, with higher specificity than TST particularly in BCG-vaccinated individuals 1, 5.

Results are interpreted as follows:

• Positive: Indicates M. tuberculosis infection (either LTBI or active TB)
• Negative: Suggests absence of M. tuberculosis infection
• Indeterminate: Provides no useful diagnostic information and may require repeat testing 4

Risk Factors for Positive QFT-G Results

Several factors are independently associated with positive QFT-G results:

• Origin from a TB endemic country (OR 6.82)
• Recent stay in a TB endemic country
• Duration of TB exposure
• Previous TB disease (OR 11.60) 6

Limitations and Considerations

• QFT-G may have reduced sensitivity in immunocompromised patients 4
• Indeterminate results should never be interpreted as absence of infection 4
• Technical factors, including improper handling or processing of blood specimens, can affect results 4
• Blood must be incubated with test antigens within 12 hours of collection 4
• The test alone is insufficient to confirm the diagnosis of active TB disease 5

Special Considerations

• In severely immunocompromised patients or children <5 years with recent TB exposure, treatment for LTBI should be considered regardless of test results 4
• When active TB disease is clinically suspected, additional diagnostic evaluations should not be delayed while awaiting QFT-G results 4
• A dual testing approach (IGRA and TST) may increase detection sensitivity in high-risk patients 4

The QFT-G test represents an important advancement in TB diagnostics, offering improved specificity while maintaining sensitivity comparable to traditional methods. However, results should always be interpreted in the context of clinical and epidemiological factors.