From the Guidelines
Blood tests alone cannot definitively diagnose active tuberculosis, but the interferon-gamma release assay (IGRA) is used as a supportive test, despite its limitations in distinguishing between active and latent TB infection. When diagnosing active tuberculosis, several blood tests can be utilized to support the diagnostic process, although they cannot provide a definitive diagnosis on their own. The most commonly used blood tests include:
- Interferon-gamma release assays (IGRAs), such as QuantiFERON-TB Gold and T-SPOT.TB, which measure the immune system's response to TB bacteria by detecting the release of interferon-gamma from white blood cells 1.
- Complete blood count (CBC), which may show anemia or an elevated white blood cell count during infection.
- Erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP), which indicate inflammation.
- Liver function tests, which are important when monitoring patients on TB treatment.
However, as noted in a recent study, IGRAs have limitations, including low specificity in individuals with suspected TB, and should not be used as the sole diagnostic method for active TB 1. For a definitive diagnosis of active TB, microbiological tests like sputum culture, nucleic acid amplification tests, or histopathological examination of affected tissue are necessary. It's also worth noting that some guidelines recommend against the use of IGRAs in the diagnosis of active TB in adults, but suggest they could be used as an adjunct test in certain clinical situations, such as in patients with extrapulmonary TB or in children 1.
From the Research
Blood Tests for Active Tuberculosis
The following blood tests are used to diagnose active tuberculosis:
- Interferon gamma release assays (IGRAs) 2, 3, 4, 5, 6
- T-SPOT.TB 2, 3, 4, 5, 6
- QuantiFERON-TB Gold In-Tube (QFT-GIT) 2, 3, 4, 5, 6
Characteristics of Blood Tests
The characteristics of these blood tests are:
- T-SPOT.TB: more sensitive than QFT-GIT for diagnosing active TB 2
- QFT-GIT: less sensitive than T-SPOT.TB but still effective in diagnosing active TB 2, 3, 4
- IGRAs: can distinguish between latent TB infection and active TB, but may have cross-reactivity with non-tuberculous mycobacteria (NTM) 5, 6
Clinical Utility
The clinical utility of these blood tests is:
- IGRAs: useful for diagnosing latent TB infection and monitoring treatment response 4
- T-SPOT.TB and QFT-GIT: can be used to rule out active TB, but may not be cost-effective in all cases 2
- Novel assays: may offer improved sensitivity and specificity for diagnosing active TB, but require further evaluation 2