From the Guidelines
Latent tuberculosis (TB) should be diagnosed using either commercial interferon-gamma release assays or Mantoux tuberculin skin testing, as recommended by the World Health Organization guidelines for low TB burden countries 1. To diagnose latent TB, healthcare providers can use two types of tests:
- The tuberculin skin test (TST), which involves injecting a small amount of purified protein derivative (PPD) under the skin of the forearm and measuring the size of the skin reaction 48-72 hours later.
- Interferon-gamma release assays (IGRAs), such as QuantiFERON-TB Gold or T-SPOT.TB, which are blood tests that measure the immune response to TB proteins. These tests are used to identify individuals who have been exposed to TB, but it's essential to consider risk factors, including recent exposure to active TB, origin from high-prevalence countries, immunosuppression, or certain medical conditions. It's crucial to note that these tests cannot distinguish between latent and active TB, so additional evaluation, including chest X-rays and sputum tests, may be needed to rule out active disease, as suggested by the guidelines 1. The diagnosis of latent TB is critical to prevent progression to active disease, especially in high-risk individuals, such as those living with HIV, adult and child contacts of pulmonary TB cases, and patients with certain medical conditions, as highlighted in the guidelines 1. Key considerations for diagnosis include:
- Using either TST or IGRA to test for LTBI, as both are acceptable methods 1.
- Performing chest radiography before LTBI treatment to rule out active TB disease, as recommended by the guidelines 1.
- Considering risk factors, such as recent exposure to active TB, to determine the need for testing and treatment, as emphasized in the guidelines 1.
From the Research
Diagnostic Tests for Latent Tuberculosis Infection
- The diagnosis of latent tuberculosis (TB) infection requires a positive test for infection and negative evaluation for active disease 2.
- Current tests measure an immunologic response and include the tuberculin skin test (TST) and interferon-gamma release assays (IGRAs), such as T-SPOT.TB and QuantiFERON 2.
- IGRAs are preferred in bacille Calmette-Guérin-vaccinated populations, while the TST is still used when cost or logistical advantages over the IGRAs exist 2, 3.
Interferon-Gamma Release Assays
- IGRAs perform as well or better than the TST in most targeted populations, especially in bacille Calmette-Guérin (BCG)-vaccinated populations 3.
- A positive IGRA in a person at low risk for TB exposure should be confirmed with a repeat test or another method before recommending LTBI treatment 3.
- IGRAs have a better predictive ability than TST, and individuals who are positive by IGRA might benefit from preventive treatment 4, 5.
Comparison of Diagnostic Tests
- A systematic review and meta-analysis found that IGRAs have a higher pooled sensitivity and specificity than TST for detecting TB infection 5.
- Another study found that IGRAs, especially T-SPOT.TB, are more effective at detecting TB infection than TST, despite their higher cost 5.
- A comparison of IGRAs with TST for identifying LTBI that progresses to active TB found inconsistent results, with some studies showing that IGRAs outperform TST, while others found no significant difference 6.
Targeted Populations
- Diagnosis of latent TB infection should be targeted toward individuals and groups with high risk of progression to active TB, such as immunocompromised people and those recently arrived from countries with high TB burden 3, 6.
- Low-risk populations should not be screened for LTBI 3.
- Population-based studies with adequate sample size and follow-up are required to adequately compare the performance of IGRA with TST in people at high risk of TB 6.