From the Guidelines
The most common tuberculosis (TB) diagnostic markers include the tuberculin skin test (TST), interferon-gamma release assays (IGRAs) like QuantiFERON-TB Gold and T-SPOT.TB, sputum smear microscopy, culture tests, and nucleic acid amplification tests (NAATs) such as Xpert MTB/RIF, as recommended by the official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention clinical practice guidelines 1.
These markers work by either detecting the immune response to TB infection or directly identifying the presence of Mycobacterium tuberculosis in clinical specimens, providing clinicians with complementary tools for accurate diagnosis. The TST measures immune response to TB proteins through skin reaction, while IGRAs detect T-cell response to TB antigens in blood samples. Sputum microscopy directly identifies acid-fast bacilli under microscope examination, and culture tests remain the gold standard by growing TB bacteria from patient samples, though results take 2-8 weeks. Molecular tests like Xpert MTB/RIF can detect TB DNA and rifampin resistance within hours.
Some key points to consider when using these markers include:
- The sensitivity and specificity of the TST, as well as its interpretation, can be affected by various factors such as prior BCG vaccination and immunosuppression 1.
- IGRAs, such as QuantiFERON-TB Gold and T-SPOT.TB, can be used as an alternative to the TST, especially in individuals with a history of BCG vaccination or in those who are immunocompromised 1.
- NAATs, such as Xpert MTB/RIF, can provide rapid results and are useful for detecting TB DNA and rifampin resistance, but may not detect paucibacillary pulmonary TB 1.
- Additional markers, such as chest X-rays and lipoarabinomannan (LAM) urine tests, can be used to identify lung abnormalities and detect TB in HIV patients, respectively.
Overall, the choice of diagnostic marker(s) will depend on the individual patient's clinical presentation, medical history, and risk factors, as well as the availability of resources and expertise. It is essential to use the most recent and highest-quality evidence, such as the 2017 official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention clinical practice guidelines 1, to guide the diagnosis and management of TB.
From the Research
TB Markers
The common tuberculosis (TB) markers used for diagnosis include:
- Tuberculin skin test (TST) 2, 3, 4, 5, 6
- Interferon-gamma release assays (IGRAs) 2, 3, 4, 5, 6
- Enzyme-linked immunospot (ELISpot) 2
- Enzyme-linked immunosorbent assay (ELISA) 2
- QuantiFERON-TB Gold 2, 4, 5, 6
- T-SPOT.TB 2, 5, 6
Characteristics of TB Markers
The characteristics of these markers are:
- TST: low specificity due to cross-reactivity with BCG vaccine and non-tuberculous mycobacteria 3, 4
- IGRAs: higher specificity than TST, especially in BCG-vaccinated individuals 3, 4, 5, 6
- ELISpot and ELISA: measure interferon-gamma release in response to TB-specific antigens 2
- QuantiFERON-TB Gold and T-SPOT.TB: commercial IGRAs with high sensitivity and specificity 5, 6
Advantages and Disadvantages of TB Markers
The advantages and disadvantages of these markers are:
- TST: widely available, but low specificity and sensitivity 2, 3, 4, 5, 6
- IGRAs: higher specificity and sensitivity than TST, but more expensive and require specialized equipment 3, 4, 5, 6
- ELISpot and ELISA: require specialized equipment and trained personnel 2
- QuantiFERON-TB Gold and T-SPOT.TB: easy to use and interpret, but more expensive than TST 5, 6