Interpretation of Blood Tests for Tuberculosis

Interferon-Gamma Release Assays (IGRAs): Mechanism and Interpretation

Tuberculosis (TB) remains a global health concern, requiring precise diagnostic tools for early detection. One of the key blood tests for TB is the Interferon-Gamma Release Assay (IGRA). These tests measure the release of interferon-gamma (IFN-γ) from T cells in response to Mycobacterium tuberculosis antigens.

Mechanism of IGRAs

• IGRAs detect latent tuberculosis infection (LTBI) by measuring immune response.
• They rely on T-cell mediated secretion of IFN-γ when exposed to TB-specific antigens.
• Compared to the traditional tuberculin skin test (TST), IGRAs offer higher specificity and do not require multiple patient visits.

Interpretation of IGRA Results

IGRA results are reported as Positive, Negative, or Indeterminate. The interpretation depends on several factors:

• Positive IGRA: Suggests likely TB exposure, warranting further clinical evaluation.
• Negative IGRA: Indicates absence of detectable immune response to TB-specific antigens.
• Indeterminate IGRA: May result from technical issues, low immune response, or improper sample handling.

Advantages of IGRAs

Some notable advantages of IGRAs include:

• No requirement for follow-up visits, unlike TST.
• Elimination of false positives due to Bacillus Calmette-Guérin (BCG) vaccination.
• Higher sensitivity and specificity in diagnosing latent tuberculosis infection.

Conclusion

IGRAs play a critical role in modern TB diagnostics, particularly for high-risk populations and individuals with prior BCG vaccination. Understanding the mechanism and interpretation of IGRA results is essential for clinicians to ensure accurate diagnosis and appropriate patient management.