Why is Interferon-Gamma Release Assay (IGRA) preferred over Tuberculin Skin Test (TST) in individuals exposed to tuberculosis (TB)?

From the Guidelines

Interferon-Gamma Release Assay (IGRA) is preferred over Tuberculin Skin Test (TST) in individuals exposed to tuberculosis due to its superior specificity, particularly in BCG-vaccinated populations. The most recent and highest quality study 1 recommends IGRA over TST, especially in patients with cancer, due to its higher sensitivity in immunosuppressed individuals and higher specificity in BCG-vaccinated populations.

Key Points

• IGRA tests measure the immune response to specific TB antigens in a blood sample, eliminating false positives that commonly occur with TST in people who received the BCG vaccine 1.
• IGRA requires only a single patient visit for blood collection, while TST necessitates a follow-up visit 48-72 hours later for result interpretation.
• IGRA results are more objective, as they provide quantitative laboratory measurements rather than relying on visual skin reaction assessment, which can be subjective.
• Both tests detect TB infection but cannot distinguish between latent TB infection and active TB disease, so additional evaluation including chest X-rays and sputum tests are needed for diagnosis of active disease 1.

Considerations

• IGRA tests are more expensive and require laboratory infrastructure, which may limit their availability in resource-constrained settings.
• The choice between IGRA and TST should be based on the individual's specific situation, including their vaccination history and immune status 1.

Recommendations

• Use IGRA as the preferred test for detecting TB infection in individuals exposed to tuberculosis, especially in BCG-vaccinated populations 1.
• Consider the individual's specific situation and medical history when choosing between IGRA and TST 1.
• Always perform additional evaluation, including chest X-rays and sputum tests, to diagnose active TB disease in individuals with positive IGRA or TST results 1.

From the Research

Advantages of IGRA over TST

• IGRA is preferred over TST in individuals exposed to tuberculosis (TB) due to its higher specificity, as it uses antigens specific for M tuberculosis, resulting in fewer false-positive results 2.
• IGRA yields fewer false-positive results than the TST, especially in individuals who have received the Mycobacterium bovis-bacille Calmette-Guerin vaccine or have been exposed to nontuberculous mycobacteria 2, 3.
• The advantages of using IGRAs, including their logistical advantages and enhanced specificity over TST, favor their expanded use in individuals exposed to TB 3, 4.

Comparison of IGRA and TST

• A study comparing TST and IGRA in diagnosing latent tuberculosis infection (LTBI) in a high TB-burden setting found that neither test predicted subsequent development of active TB among household contacts of pulmonary TB patients 5.
• Another study found that IGRA and TST yielded discordant results, and the value of IGRA in addition to TST remains undetermined, suggesting that the two tests should be jointly interpreted in decision-making 4.
• IGRA has been shown to have higher conversion rates than TST, especially in individuals with baseline IGRA positive quantitative values <1 IU/mL 4.

Clinical Application and Limitations

• IGRAs have logistical advantages and enhanced specificity over TST, but they lack the sensitivity and reproducibility normally expected from diagnostic tests in clinical practice 3.
• The clinical application and limitations of IGRAs, including their use in epidemiologic studies and areas of uncertainty related to discordant IGRA results, have been reviewed and discussed 3.