From the Guidelines
Testing for active tuberculosis (TB) should involve a combination of sputum smear microscopy, sputum culture, and molecular tests like GeneXpert MTB/RIF, as well as chest X-rays and, in some cases, bronchoscopy or biopsies, to confirm infection and guide treatment. The primary tests for active TB include:
- Sputum smear microscopy to examine for TB bacteria
- Sputum culture to grow the bacteria in a lab and confirm infection
- Molecular tests like GeneXpert MTB/RIF to detect TB DNA and rifampin resistance
- Chest X-rays to identify lung abnormalities characteristic of TB In some cases, additional tests may be necessary, such as:
- Bronchoscopy to collect samples from the lungs
- Biopsies of affected tissues for extrapulmonary TB
- Blood tests like interferon-gamma release assays (IGRAs) or tuberculin skin tests (TST) to indicate TB infection, although these do not distinguish between active and latent TB 1. According to the most recent guidelines, a high level of suspicion should be maintained in immunocompromised hosts, particularly those with AIDS, as imaging manifestations may not fit a classic primary or reactivation pattern 1. It's also important to note that a negative culture result does not exclude the diagnosis of active TB, and treatment should be initiated based on clinical, pathologic, and radiographic features of the patient, as well as epidemiologic information and results of initial diagnostic tests 1. The use of IGRAs is increasingly recommended, but most current guidelines do not use objective, transparent methods to grade evidence and recommendations, and do not disclose conflicts of interests 1. CT scans may be helpful in cases where chest radiography does not show classic findings of TB, and can increase the specificity of the diagnosis of TB 1. Overall, the goal of testing for active TB is to confirm infection and guide treatment, which typically includes a multi-drug regimen for at least 6 months, to reduce transmission risk and improve patient outcomes.
From the Research
Testing Methods for Active TB
There are several methods to test for active TB, including:
- Sputum smear microscopy: This method involves examining a sputum sample under a microscope to detect the presence of Mycobacterium tuberculosis bacteria 2, 3.
- Sputum culture: This method involves growing the bacteria in a laboratory to detect its presence 2, 3.
- Xpert-MTB/RIF: This is a rapid diagnostic test that can detect the presence of Mycobacterium tuberculosis bacteria and resistance to rifampicin 2, 4.
- Interferon-gamma release assay (IGRA): This method involves measuring the immune response to Mycobacterium tuberculosis bacteria in the blood or bronchoalveolar lavage fluid 5, 6, 4.
- CapitalBio TB/NTM kit detection test: This is a diagnostic test that can detect the presence of Mycobacterium tuberculosis and non-tuberculous mycobacteria 2.
Sensitivity and Specificity of Testing Methods
The sensitivity and specificity of these testing methods vary:
- Xpert-MTB/RIF: 79.6% sensitive and 97.4% specific 2, 4.
- Sputum smear microscopy: 41% sensitive and 99% specific 5.
- Sputum culture: 85.7% sensitive for non-tuberculous mycobacteria 2.
- IGRA: 92% sensitive and 87% specific for bronchoalveolar lavage fluid 5.
- CapitalBio TB/NTM kit detection test: 77.1% sensitive and 45.7% sensitive for non-tuberculous mycobacteria 2.
Combination of Testing Methods
Combining multiple testing methods can increase the sensitivity and specificity of active TB diagnosis:
- Combination of Xpert-MTB/RIF and CapitalBio TB/NTM kit detection test: increased sensitivity to 88.4% 2.
- Combination of sputum smear microscopy and culture: increased sensitivity to 84.2% 2.
- Combination of Xpert-MTB/RIF and IGRA: may increase diagnostic accuracy in sputum smear-negative TB suspects 4.