From the Guidelines
No, the Interferon-Gamma Release Assay (IGRA) test alone is not sufficient to differentiate between active and latent tuberculosis (TB). The IGRA test, which includes commercial tests like QuantiFERON-TB Gold and T-SPOT.TB, only detects whether a person has been infected with Mycobacterium tuberculosis by measuring the immune response to TB antigens. It cannot distinguish whether the infection is currently active or latent. To diagnose active TB, additional tests are necessary, including chest X-rays, sputum microscopy, culture, and molecular tests like GeneXpert MTB/RIF. Clinical symptoms such as persistent cough, weight loss, night sweats, and fever must also be evaluated. The distinction between active and latent TB is crucial for treatment decisions, as active TB requires a multi-drug regimen (typically isoniazid, rifampin, ethambutol, and pyrazinamide for 6-9 months), while latent TB may be treated with simpler regimens like isoniazid for 6-9 months or isoniazid plus rifapentine for 3 months. The IGRA works by detecting interferon-gamma released by T-cells in response to TB antigens, indicating exposure to TB bacteria, but this immune response occurs in both active and latent infections 1.
Some key points to consider when interpreting IGRA results include:
- The sensitivity and specificity of IGRA tests can vary, especially in immunocompromised individuals or those with HIV infection 1.
- IGRA results should not be used in isolation to diagnose active TB, but rather as part of a comprehensive diagnostic workup that includes clinical evaluation, radiographic tests, and microbiological tests 1.
- The WHO recommends against the use of IGRAs for active TB in low-income and middle-income countries due to the poor specificity resulting from a high background prevalence of latent TB infection 1.
In terms of treatment decisions, it is crucial to differentiate between active and latent TB to ensure that patients receive the appropriate treatment regimen. Active TB requires prompt treatment with a multi-drug regimen to prevent disease progression and transmission, while latent TB may be treated with simpler regimens to prevent reactivation. The IGRA test can be a useful tool in identifying individuals with latent TB infection, but it should not be relied upon as the sole diagnostic test for active TB. Instead, a comprehensive diagnostic approach that includes clinical evaluation, radiographic tests, and microbiological tests should be used to confirm the diagnosis of active TB 1.
From the Research
IGRA Test Limitations
- The IGRA test is not sufficient to differentiate between active and latent Tuberculosis (TB) 2, 3, 4.
- The test has limitations in terms of sensitivity and specificity, and its accuracy can be affected by various factors such as HIV status and CD4 count 2.
- The IGRA test is designed to diagnose latent TB infection, but it does not distinguish between current or past TB infection, or between latent infection and active disease 3.
Diagnostic Accuracy
- The sensitivity and specificity of IGRA tests vary depending on the specific test used and the population being tested 2, 5, 4.
- A study found that the sensitivity and specificity of the QuantiFERON-TB Gold In-tube (QFT-GIT) test were 76% and 42%, respectively, while the T-SPOT.TB test had a sensitivity and specificity of 84% and 47%, respectively 2.
- Another study found that combining the IGRA test with a TNF-α-release assay (TARA) increased the specificity for active TB to 93%, without compromising sensitivity 5.
Clinical Utility
- The IGRA test is not recommended as a rule-in or rule-out test for active TB in a high-burden setting 2.
- Chest radiography had a better negative predictive value than IGRA tests in smear-negative patients, even in HIV-infected patients 2.
- The IGRA test may be useful as an adjunct to diagnosis of active TB disease, but it should not be relied upon as the sole diagnostic tool 3.