From the Guidelines
The best method to diagnose Tuberculosis (TB) involves a combination of clinical assessment, radiographic imaging, and microbiological testing.
Diagnosis Methods
- The diagnosis of TB is typically confirmed by detecting Mycobacterium tuberculosis in sputum or other bodily fluids using smear microscopy or nucleic acid amplification tests, such as the Xpert MTB/RIF assay 1.
- A tuberculin skin test (TST) using 5 units of purified protein derivative (PPD) or an interferon-gamma release assay (IGRA) may be used to detect latent TB infection, with a positive result defined as an induration of 10mm or greater for the TST or a value above the manufacturer's cutoff for the IGRA 1.
Imaging Techniques
- CT scans can be used to increase the specificity of the diagnosis of TB and may be helpful in cases where chest radiography does not show "classic" findings of TB 1.
- MRI has not been specifically evaluated as a primary imaging modality for patients with suspected or proven TB, but may be technically feasible and described in the literature 1.
Guidelines
- The European Union standards for tuberculosis care recommend seeking bacteriological confirmation through examination of appropriate biological samples and using a combination of clinical, radiographic, and microbiological findings to diagnose TB in children 1.
- The use of IGRAs has increased substantially since 2005, mostly in low-incidence countries, and many studies have evaluated their role in the diagnosis of active and latent TB 1.
Key Points
- A culture of sputum or other clinical specimen that contains M. tuberculosis provides a definitive diagnosis of TB disease 1.
- Nucleic acid amplification tests can facilitate the rapid detection and identification of M. tuberculosis, but should not replace culture and drug-susceptibility testing in patients with suspected TB disease 1.
From the Research
Diagnosis of Tuberculosis (TB)
The diagnosis of TB can be achieved through various methods, including:
- Tuberculin skin test (TST)
- Interferon-gamma release assays (IGRAs)
- Chest x-ray
- Tissue culture
- Acid-fast staining
- Serological tests
- Nucleic acid amplification technology
Comparison of TST and IGRA
Studies have compared the effectiveness of TST and IGRA in diagnosing TB, with results indicating that:
- IGRAs have higher specificity than TST 2, 3, 4
- IGRAs have better correlation with surrogate markers of exposure to M. tuberculosis in low-incidence settings 2
- IGRAs have less cross-reactivity due to BCG vaccination than TST 2, 3
- T-SPOT.TB is more sensitive than TST 3
- QFT-GIT is more sensitive than TST, but similarly specific 3
Advantages and Disadvantages of IGRAs
The advantages of IGRAs include:
- High specificity
- Logistical convenience
- Avoidance of poorly reproducible measurements
- Ability to perform serial testing without inducing the boosting phenomenon However, IGRAs also have some disadvantages, including:
- Higher cost
- Limited availability in some settings
Recommendations for Diagnosis
Based on the available evidence, IGRAs may be useful in low-endemic, high-income settings where cross-reactivity due to BCG might adversely impact the utility of TST 2. Additionally, IGRAs may be more effective at detecting TB infection than TST, especially in high-risk populations 3, 4. However, further research is needed to fully understand the role of IGRAs in TB diagnosis and to address the limitations of current diagnostic methods 5, 6, 4.
Key Findings
- IGRAs have a better predictive ability than TST for the progression of latent infection to active tuberculosis disease 4
- Individuals who are positive by IGRA might benefit from preventive treatment, but those who are positive by TST probably will not 4
- Dual testing might improve detection, but further confirmation is needed 4