Newer Methods in Diagnosis of Tuberculosis

Introduction

1. Improved microscopy techniques
2. Liquid culture systems (MGIT)
3. Nucleic Acid Amplification Tests (NAATs) — Xpert MTB/RIF & Ultra
4. Line Probe Assays (LPA)
5. Interferon-Gamma Release Assays (IGRAs)
6. New antigen-based skin tests (TBSTs)
7. Lateral Flow Lipoarabinomannan (LF-LAM) assay
8. Next-Generation Sequencing (NGS)
9. Whole genome sequencing / genotyping
10. Immunological & biomarker-based methods
11. Advanced imaging (PET-CT, MRI)

1. Improved Smear Microscopy

• Replaces traditional Ziehl–Neelsen staining as the WHO-recommended method
• Uses fluorescent auramine O dye; mycobacteria appear bright yellow-green on dark background
• Sensitivity ~10% higher than conventional ZN staining
• Light-emitting diode (LED) fluorescence microscopes (life span >50,000 hours, lower cost) have replaced expensive mercury vapor lamps
• Slides can be scanned at lower magnification, making reading faster
• Sensitivity still requires ~10,000 bacilli/mL; technique improves throughput but not fundamental detection limit

2. Liquid Culture — MGIT (Mycobacteria Growth Indicator Tube)

• Liquid media (Middlebrook 7H9 broth) with fluorescent oxygen-quenching indicator
• Detects as few as 10–1,000 viable bacilli/mL (vs. 10,000 for smear)
• Results in 1–3 weeks vs. 2–8 weeks on solid media (Löwenstein–Jensen, Middlebrook 7H10/7H11)
• The BACTEC MGIT 960 system is the most widely used automated liquid culture platform — considered the current gold standard for culture diagnosis
• Automated monitoring eliminates subjective reading
• Suitable for primary isolation and drug susceptibility testing (DST)
• Limitation: higher contamination risk than solid media; expensive

3. Nucleic Acid Amplification Tests (NAATs)

a) Xpert MTB/RIF (Cepheid)

• Automated, cartridge-based, real-time PCR (hemi-nested PCR targeting rpoB gene)
• Detects M. tuberculosis complex AND rifampicin resistance simultaneously
• Results within ~90 minutes ("near-patient" point-of-care test)
• WHO-recommended as the initial diagnostic test where available
• Sensitivity: ~88% overall; higher in smear-positive (98%), lower in smear-negative (~68%); ~30% sensitivity for pleural fluid but higher on pleural biopsy
• Specificity: >99%
• Key advantages: closed system (no cross-contamination), minimal biosafety requirement, usable at peripheral/district level
• Limitation: cannot replace culture; negative result does not exclude TB (insufficient sensitivity); expensive consumables

b) Xpert MTB/RIF Ultra (Cepheid)

• Next-generation assay with improved sensitivity (especially smear-negative and paucibacillary disease)
• Two amplification targets (IS6110 and IS1081) in addition to rpoB
• Sensitivity increased by ~5% over original Xpert in smear-negative cases
• A 2025 Cochrane systematic review (PMID 41128098) confirms higher sensitivity in children vs. standard Xpert
• Slight reduction in specificity vs. original (more "trace" positive calls in treated patients)
• WHO-endorsed to replace original Xpert MTB/RIF

c) Other NAATs

• Truenat MTB / MTB Plus (Molbio Diagnostics): chip-based PCR platform, point-of-care, battery-operated — valuable in resource-limited/remote settings
• Abbott RealTime MTB: laboratory-based NAAT on the m2000 platform with higher throughput
• All NAATs can distinguish M. tuberculosis from non-tuberculous mycobacteria (NTM)

4. Line Probe Assays (LPA)

• DNA strip-based hybridization (reverse hybridization) after PCR amplification
• GenoType MTBDRplus (Hain Lifescience): detects resistance to isoniazid (katG, inhA mutations) and rifampicin (rpoB mutations) — MDR-TB detection
• GenoType MTBDRsl: detects resistance to fluoroquinolones (gyrA/gyrB) and second-line injectable agents — XDR-TB detection
• Can be performed directly on smear-positive sputum specimens — results in 24–48 hours
• WHO-endorsed for rapid MDR and XDR-TB detection
• Limitation: detects only known resistance mutations (~96% of rifampicin resistance from rpoB, 65–75% of isoniazid resistance); misses novel mutations

5. Interferon-Gamma Release Assays (IGRAs)

Available Platforms:

Key Features:

• Greater specificity than TST (not confounded by BCG or most NTM)
• Single patient visit (no 48–72 hour return)
• Measures recirculating memory CD4+ T cells (and CD8+ in QFT-Plus)
• Not affected by BCG vaccination
• Indications: Preferred over TST for persons >5 years in the USA; TST preferred in <5 years (limited IGRA data in children)
• Limitation: Cannot differentiate latent TB infection (LTBI) from active TB; reduced sensitivity in immunosuppressed (HIV, high-dose steroids); indeterminate results in patients with anti-IFN-γ autoantibodies; requires lab infrastructure and cold chain

6. New Antigen-Based Skin Tests (TBSTs)

• Combine the simplicity of TST with the specificity of IGRAs
• Use ESAT-6 and CFP-10 antigens (like IGRAs) instead of non-specific PPD
• Three commercial TBSTs evaluated by WHO: Cy-Tb (Serum Institute), ESAT-6/CFP-10 skin test (C-Tb by Statens Serum Institut), Diaskintest (Russia)
• WHO assessment: accuracy similar to IGRAs and superior to TST
• Useful in PLWH, BCG-vaccinated persons, and children
• Advantage: field-deployable, no blood draw needed; single dose intradermally
• Read at 48–72 hours like TST, but positive in LTBI even in BCG-vaccinated

7. Lateral Flow Lipoarabinomannan (LF-LAM) Assay

• Urine antigen detection test — detects lipoarabinomannan (LAM), a lipopolysaccharide shed from mycobacterial cell walls of metabolically active or damaged bacilli
• Point-of-care, rapid (25 minutes), no laboratory equipment needed
• Commercial test: Alere Determine TB LAM Ag; newer: AlereLAM Ultra (improved sensitivity)
• WHO endorsement (2015, updated 2019): Recommended specifically for:
  • HIV-positive patients with suspected TB AND CD4 ≤ 100 cells/µL, OR
  • Seriously ill HIV patients (regardless of CD4 count)
• Sensitivity: 40–56% in HIV+ patients with low CD4; very low sensitivity in HIV-negative patients
• Specificity: ~95%
• Advantage: non-sputum test, avoids aerosol transmission risk, easy to collect urine
• Limitation: poor sensitivity in HIV-negative and immunocompetent patients; not recommended as standalone test

8. Genotypic Drug Susceptibility Testing (DST) — Molecular Methods

PCR-Based (Targeted):

• Direct detection of resistance-associated mutations in clinical specimens
• rpoB (rifampicin resistance — present in 96% of resistant strains)
• katG / inhA (isoniazid resistance — 65–75% of resistant strains)
• gyrA / gyrB (fluoroquinolone resistance — 42–85% of resistant strains)
• Faster than phenotypic culture-based DST

Next-Generation Sequencing (NGS):

• Targeted NGS assays now commercially available:
  • Deeplex Myc-TB (GenoScreen): targets 18 drug resistance gene loci; detects resistance to 13 anti-TB drugs (all first-line + most second-line: injectables, fluoroquinolones, bedaquiline, clofazimine, linezolid, ethionamide)
  • DeepChek-TB (Advanced Biological Laboratories): same 13 drug classes
• Performed directly on clinical specimens; results within 48 hours
• Whole Genome Sequencing (WGS): Comprehensive profiling of entire MTB genome; detects all known and novel mutations; used in reference laboratories for outbreak investigation, strain typing, and complex resistance profiling
• Restriction Fragment Length Polymorphism (RFLP): Epidemiological typing — compares strains in outbreak settings (older method largely replaced by WGS)
• Limitation of all genotypic methods: Cannot detect drug-resistant strains harboring novel/unknown mutations → phenotypic DST remains essential to exclude drug resistance

9. Automated Liquid Culture DST Systems

• BACTEC MGIT 960 and BacT/ALERT 3D: automated liquid culture-based susceptibility testing
• Colorimetric methods (alamarBlue/resazurin/tetrazolium): reduction of redox indicator by viable mycobacteria → color change = resistance
  • Sensitivity/specificity: 98%/99% for rifampicin; 97%/98% for isoniazid
• Nitrate Reductase Assay (NRA): M. tuberculosis reduces nitrate → nitrite (Griess reaction) — highly sensitive/specific, low-cost, applicable in resource-limited settings
  • Sensitivity 97%, specificity 100% for rifampicin resistance

10. Advanced Imaging

FDG-PET/CT:

• Fluorodeoxyglucose-PET/CT used to differentiate active from inactive/old TB and monitor treatment response
• Active TB lesions show high FDG uptake; sensitivity and specificity reported at 100% using maximal SUV threshold ≥1.05
• Useful in smear/culture-negative cases, extrapulmonary TB assessment, and monitoring therapeutic response
• Limitations: expensive, radiation, limited availability

MRI:

• Superior to CT for pleural involvement, pleural effusion characterization, and caseous necrosis/liquefaction
• Comparable to CT for lymphadenopathy and consolidation; lower for pulmonary nodules
• Preferred in vulnerable populations (pregnant women, children) due to absence of ionizing radiation

11. Whole-Blood Multiplex Transcriptomic Signatures (Emerging)

• Host blood gene expression signatures (transcriptomic biomarkers) under investigation as TB screening tools
• Aim to detect active TB from LTBI and healthy contacts before symptoms
• Not yet in routine clinical use — area of active research

Summary Table — Newer TB Diagnostics

Conclusion

• Harrison's Principles of Internal Medicine 22E (2025), Ch. 183, pp. 1436–1438
• Murray & Nadel's Textbook of Respiratory Medicine, Ch. 53, pp. 1187–1196
• Tietz Textbook of Laboratory Medicine, 7th Ed., pp. 1313–1316
• Recent evidence: Cochrane SR on Xpert Ultra in children — PMID 41128098 (2025); Meta-analysis of Xpert for intestinal TB — PMID 37531292 (2024)