Overview
This lecture reviews the current, ongoing, and future approaches to tuberculosis (TB) diagnosis, emphasizing traditional, molecular, and immunological methods, with a focus on strengths, limitations, and innovations to improve detection and control.
TB Overview and Epidemiology
- TB is caused by Mycobacterium tuberculosis and related complex, mainly affecting the lungs (pulmonary TB, PTB).
- Extrapulmonary TB affects organs like pleura, lymph nodes, and is common in immunocompromised patients.
- Transmission is via inhaled aerosols; risk factors include HIV, diabetes, malnutrition, and incarceration.
- Latent TB infection (LTBI) shows no symptoms but can reactivate and transmit.
Traditional Diagnostic Methods
- Sputum smear microscopy (SSM): Detects acid-fast bacilli; fast and cheap but low sensitivity, cannot distinguish dead from live bacteria or mycobacterial species.
- Chest radiography (CXR): High sensitivity for PTB; cannot distinguish TB from non-tuberculous mycobacteria (NTM); AI improves screening accuracy.
Culture-Based Diagnosis
- Culture is the gold standard; requires BSL-3/4 lab and is time-consuming (up to 6 weeks).
- Selective media (Löwenstein–Jensen, Kudoh–Ogawa, Middlebrook, MGIT) enhance specificity and sensitivity.
- Cultures differentiate TB from NTM and are essential for drug susceptibility testing.
Molecular Diagnostic Techniques
- Nucleic acid amplification tests (NAATs): Rapid TB detection and, for some, drug resistance; examples include Xpert MTB/RIF, Truenat MTB, and cobas MTB.
- Line probe assays (LPAs): Identify TB and mutations for drug resistance within hours.
- Sequencing (WGS/NGS): Offers detailed resistance profiling, but is costly and requires infrastructure.
- MALDI-TOF MS & biosensors: Emerging rapid tools for species identification/drug resistance, though not yet routine.
Immunological Approaches
- Interferon-gamma release assays (IGRAs): Blood-based, unaffected by BCG vaccine, identify LTBI; more costly and require lab facilities.
- Tuberculin skin test (TST): Cheap, field-friendly, but affected by prior BCG vaccination.
- MTB antigen-based skin tests (TBST): Newer tests with improved specificity and sensitivity.
- Serological tests currently lack sufficient accuracy for TB diagnosis.
Ongoing and Future Diagnostic Research
- Innovations include: Fingerstick transcriptomic assays, LC-MS for TB/HIV peptides, lab-on-chip sample preparation, electronic nose for breath analysis.
- WHO is reviewing: New NAATs, culture-based susceptibility platforms, antigen-based rapid tests, and advanced IGRAs.
Key Terms & Definitions
- Pulmonary tuberculosis (PTB) — TB primarily affecting the lungs.
- Extrapulmonary TB — TB affecting organs other than lungs.
- Acid-fast bacilli (AFB) — Bacteria that retain certain stains after acid wash; characteristic of M. tuberculosis.
- NAATs — Tests amplifying bacterial DNA for rapid detection.
- Line probe assay (LPA) — DNA strip-based test to detect TB and drug resistance.
- Interferon-gamma release assays (IGRAs) — Tests measuring immune response to TB antigens.
- Latent TB infection (LTBI) — Non-active, non-symptomatic TB infection.
- MGIT — Mycobacteria Growth Indicator Tube for culture-based detection.
- Drug-resistant TB — TB strains resistant to one or more antibiotics.
Action Items / Next Steps
- Review Table 1 (WHO-recommended NAATs) and Table 2 (summary of diagnostic methods’ pros/cons) for exam preparation.
- Familiarize with both traditional and molecular diagnostic workflows and sample types.
- Follow updates on WHO reviews for new TB diagnostic tools.
- Read supplementary materials for deeper understanding of NTM diagnosis.