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Rapid Diagnostic Tests for Tuberculosis (TB)

Complete Guide for MD Examinations

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Overview

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Classification of TB Diagnostic Tests

TB Diagnostics
├── Conventional (Slow)
│   ├── Sputum Smear Microscopy (ZN stain / FM stain)
│   ├── Liquid Culture (MGIT) — Gold standard
│   └── Solid Culture (LJ medium)
└── Rapid Diagnostics
    ├── Molecular (NAAT-based)
    │   ├── Xpert MTB/RIF (GeneXpert)
    │   ├── Xpert MTB/RIF Ultra
    │   ├── Line Probe Assay (LPA) — FL-LPA & SL-LPA
    │   ├── TB-LAMP
    │   └── Truenat MTB
    └── Immunological (Indirect)
        ├── Tuberculin Skin Test (TST / Mantoux)
        └── Interferon Gamma Release Assay (IGRA)

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1. Xpert MTB/RIF (GeneXpert)

Principle

• Real-time PCR (hemi-nested) targeting the rpoB gene of Mycobacterium tuberculosis
• Simultaneously detects MTB AND rifampicin (RIF) resistance within 2 hours
• Fully automated, closed cartridge system — minimal biosafety requirement

WHO Endorsement

• First endorsed in 2010; WHO recommends as initial diagnostic test for all adults with pulmonary TB symptoms

Performance (Pulmonary TB vs. Culture)

Key Features

• Detects rifampicin resistance with ~94–97% sensitivity, ~98% specificity (proxy for MDR-TB)
• Results in ~2 hours (vs. weeks for culture)
• Applicable to: sputum, BAL, pleural fluid, CSF, lymph node aspirate, gastric lavage, urine, stool

Limitations

• Lower sensitivity in smear-negative and HIV-positive patients
• Cannot detect resistance to drugs other than RIF
• Cartridge cost limits use in resource-poor settings

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2. Xpert MTB/RIF Ultra

Key Improvements over Original Xpert

Clinical Importance

• Preferred for: HIV-positive patients, children, smear-negative TB, extrapulmonary TB
• WHO recommends Ultra over original Xpert as initial test (2021 guidelines)
• For TB meningitis (TBM): Sensitivity ~90% with Ultra on CSF

Special Use (per WHO 2023 TB/HIV Guidelines)

"In HIV-positive adults and children with signs and symptoms of disseminated TB, Xpert MTB/RIF may be used in blood as an initial diagnostic test for disseminated TB" (WHO 2023, TB/HIV Guidelines — conditional recommendation, very low certainty)

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3. Line Probe Assay (LPA)

Types

Principle

• Reverse hybridization on nitrocellulose strip
• PCR amplification → hybridization to probes on strip → color bands indicate mutations
• Manufacturers: Hain Lifescience (GenoType MTBDRplus/sl) and Nipro (Japan)

Indications

• FL-LPA: Smear-positive sputum or MTB cultures → detects MDR-TB
• SL-LPA: Confirmed MDR-TB isolates → detects pre-XDR and XDR-TB

Performance

Advantages

• Turnaround: 1–2 days
• Multiple resistance markers simultaneously
• Can be done directly on smear-positive sputum (no culture needed)

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4. TB-LAMP (Loop-Mediated Isothermal Amplification)

Principle

• Isothermal nucleic acid amplification (no thermocycler needed)
• Targets IS6110 and devR genes
• Visual readout via fluorescent dye under UV lamp
• Manufacturer: Eiken Chemical Co. (Japan)

Performance

Advantages

• No PCR machine required — field-deployable
• Results in 35–45 minutes
• Simpler infrastructure than Xpert

Limitations

• Does not detect drug resistance
• Manual steps increase contamination risk
• Less widely endorsed than Xpert/Ultra

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5. Truenat MTB / Truenat MTB Plus

Principle

• Chip-based, battery-operated real-time micro-PCR
• Detects MTB; Truenat MTB-RIF Dx reflex test detects RIF resistance
• Manufacturer: Molbio Diagnostics (India)

Key Points

• WHO-endorsed 2020 for pulmonary and extrapulmonary TB
• Portable — runs on solar power/battery → ideal for peripheral/primary care
• Turnaround: ~1 hour
• Performance comparable to Xpert

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6. Tuberculin Skin Test (TST / Mantoux Test)

Principle

• Intradermal injection of 5 TU PPD (purified protein derivative)
• Read at 48–72 hours → measure induration (not erythema)

Interpretation

Performance

• Sensitivity: ~64.7% (GITB context), overall ~70–80%
• Specificity: ~73.3% (GITB context), affected heavily by BCG vaccination

False Positives

• BCG vaccination (within past 10 years)
• Non-tuberculous mycobacterial (NTM) infections

False Negatives (Anergy)

• HIV/AIDS (CD4 <200)
• Severe malnutrition
• Corticosteroids / immunosuppressives
• Overwhelming TB (miliary, disseminated)
• Sarcoidosis
• Recent viral infections (measles, varicella)
• Very early infection (<8 weeks post-exposure)
• Elderly

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7. IGRA (Interferon-Gamma Release Assay)

Types

Principle

• Blood test measuring IFN-γ release from T-cells stimulated by TB-specific antigens (ESAT-6, CFP-10)
• These antigens are absent in BCG and most NTMs → higher specificity than TST

Performance

Advantages over TST

• Single visit (no 48–72 hr return)
• Not affected by BCG vaccination
• Better specificity in BCG-vaccinated populations

Limitations

• Cannot distinguish latent TB (LTBI) from active TB
• Expensive; requires lab infrastructure
• Indeterminate results in immunosuppressed patients
• Not reliable in children <5 years (limited data)

Key Point for Exams

Both TST and IGRA detect latent TB infection (LTBI) — they are NOT diagnostic for active TB. A positive result means infection, not disease.

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8. Comparison Table: All Rapid Tests at a Glance

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9. WHO Diagnostic Algorithm (High-Burden Settings)

Presumptive TB (cough >2 weeks + constitutional symptoms)
         ↓
Xpert MTB/RIF Ultra (preferred initial test)
    ├── MTB Detected + RIF Susceptible
    │        ↓
    │   Treat for Drug-Susceptible TB (DS-TB)
    │
    ├── MTB Detected + RIF Resistant
    │        ↓
    │   FL-LPA (confirm RIF + test for INH resistance)
    │        ↓
    │   SL-LPA (if MDR-TB confirmed → detect pre-XDR/XDR)
    │        ↓
    │   Culture + DST + Treat MDR/pre-XDR/XDR-TB
    │
    └── MTB Not Detected
             ↓
        Chest X-ray + Clinical assessment
             ↓
        If strong clinical suspicion → Culture + repeat Xpert

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10. Extrapulmonary TB (EPTB) — Xpert Use

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11. High-Yield Exam Points

1. Xpert MTB/RIF Ultra is now the WHO-preferred first-line test (over original Xpert) due to higher sensitivity in paucibacillary disease.
2. RIF resistance on Xpert = proxy for MDR-TB (RIF resistance almost always co-occurs with INH resistance in practice).
3. TST and IGRA diagnose LTBI, NOT active TB — this is the most commonly tested distinction.
4. False-negative TST occurs in miliary TB, HIV with low CD4, and recent infection (<8 weeks).
5. IGRA is preferred over TST in BCG-vaccinated individuals and healthcare workers.
6. LPA can be done directly on smear-positive sputum — does not require culture.
7. TB-LAMP is the only WHO-endorsed rapid test that does not require electricity or a PCR machine.
8. Truenat is notable as a portable, battery-operated Xpert alternative endorsed for primary care facilities.
9. ADA (adenosine deaminase) >40 U/L in pleural/pericardial/ascitic fluid is a rapid indirect marker for TB — not a molecular test but frequently tested.
10. Sensitivity hierarchy (smear-negative pulmonary TB): Ultra > Xpert > Smear microscopy.

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12. Drug Resistance Terminology

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