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Laboratory Diagnosis of Tuberculosis: Methods and Newer Technologies

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1. Specimen Collection

• Specimen 1: "On the spot" at health facility
• Specimen 2: Early morning sample (collected the next day)
• If the patient lives far, two spot specimens 1 hour apart are acceptable

• AFB smear microscopy (×3)
• Mycobacterial culture (×3)
• NAAT/PCR (×1, first specimen is sufficient per CDC/ATS/IDSA guidance)

• Murray & Nadel's Textbook of Respiratory Medicine, p. 1189
• Goldman-Cecil Medicine, p. 3256

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2. Smear Microscopy (AFB Staining)

2a. Ziehl-Neelsen (ZN) Stain (Carbol Fuchsin Method)

1. Fix smear by passing through flame 3×
2. Cover with carbol fuchsin, steam gently for 5 min (or 20 min in water bath)
3. Wash, then decolorize with 3% acid-alcohol until faint pink remains
4. Counter-stain with Loeffler's methylene blue for 1 min
5. Examine under 1000× oil immersion

• Smear is positive when ≥10,000 organisms/mL of sputum are present
• Sensitivity: ~20-80% of culture-positive patients; ~50% overall smear-negative
• Specificity: >95% when performed carefully
• One positive smear out of two is sufficient to declare smear-positive TB

2b. Kinyoun's Cold Method

2c. Fluorochrome (Auramine-Rhodamine / Truant Method)

• Stains with auramine O or auramine-rhodamine dyes
• Examined under fluorescence microscope - AFB appear bright yellow-orange on a dark background
• Advantage: Can scan at lower magnification → 5-10× larger field of view → 1-2 min per smear
• Sensitivity ~10% higher than conventional ZN
• Standard in clinical TB laboratories in high-income countries

2d. LED Fluorescence Microscopy

• Uses light-emitting diodes (LED) instead of mercury vapor lamps as light source
• LED lifespan: >50,000 hours vs. 200-300 hours for mercury vapor
• Less expensive, lower maintenance, no mercury toxicity
• WHO: diagnostic accuracy comparable to conventional fluorescence microscopy and superior to conventional ZN microscopy
• WHO recommends LED microscopy be phased in as an alternative to Z-N microscopy in both high- and low-volume labs

• Park's Textbook of Preventive & Social Medicine, p. 216
• Murray & Nadel's Textbook of Respiratory Medicine, p. 1190
• Medical Microbiology 9e, p. 264

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3. Culture Methods (Gold Standard)

3a. Solid Media

• Lowenstein-Jensen (LJ) medium: Most widely used; egg-based. M. tuberculosis produces rough, buff-colored, cauliflower-like colonies (eugonic growth) in 3-8 weeks
• Middlebrook 7H10 and 7H11 (agar-based)
• Advantages: less expensive, allows colony morphology observation; can differentiate M. tuberculosis from NTM by growth characteristics
• Growth time: typically 12-42 days

3b. Liquid Media (BACTEC/MGIT Systems)

• MGIT (Mycobacteria Growth Indicator Tube): Contains modified Middlebrook 7H9 broth with a fluorescence-based oxygen sensor at the bottom
• Liquid media detect M. tuberculosis within 10-14 days (vs. 3-8 weeks on solid media)
• Liquid media are ~10% more sensitive but have a higher contamination rate
• Growth in liquid media produces fluorimetric or colorimetric reactions detected by automated or manual systems
• BACTEC 460 TB (older radiometric system): used 14C-labeled substrate; M. tuberculosis metabolizes it and releases 14CO2, detected by instrument
• Modern BACTEC MGIT 960 is non-radiometric and fully automated; recommended for high-workload labs

• Murray & Nadel's Textbook of Respiratory Medicine, p. 1191
• Goldman-Cecil Medicine, p. 3256

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4. Identification of M. tuberculosis from Culture

4a. Phenotypic Methods

• Niacin production: M. tuberculosis produces niacin (unique among MTBC)
• Nitrate reduction: M. tuberculosis reduces nitrate to nitrite (positive)
• Catalase inactivation at 68°C: M. tuberculosis heat-labile catalase (negative at 68°C)
• Growth time: 12-42 days on solid media
• Colony morphology: rough, cauliflower-like, buff-colored, non-pigmented
• Cord factor (trehalose 6,6'-dimycolate): mycobacteria form "serpentine cords" on microscopy

4b. Molecular Identification Methods

• DNA probes (e.g., AccuProbe, Gen-Probe): Hybridize with rRNA of M. tuberculosis complex; highly specific
• Can be used directly on positive culture broth for rapid species identification

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5. Drug Susceptibility Testing (DST)

5a. Phenotypic (Culture-Based) DST

• Proportion method (on LJ or 7H10): Inoculate drug-containing and drug-free media; resistance defined as >1% growth on critical concentration drug medium
• BACTEC MGIT 960: Automated liquid culture DST; faster than solid media; recommended for first-line and second-line drugs including WHO Group A drugs (moxifloxacin, levofloxacin, bedaquiline, linezolid) and Group B drug clofazimine
• Microscopic Observation Drug Susceptibility (MODS): Uses a sealed 24-well plate with liquid media containing drugs; detects resistance via microscopic observation of cord formation; simultaneous TB identification and DST for rifampin and isoniazid
• Colorimetric redox assays: alamarBlue, resazurin, or tetrazolium added after antibiotic exposure; color change indicates viable mycobacteria = resistance. Sensitivity 98% (rifampin), 97% (isoniazid)
• Nitrate reductase assay (NRA): Detects nitrate-to-nitrite reduction; 97% sensitivity for rifampin resistance, 100% specificity

5b. Genotypic DST (Molecular Methods)

• rpoB gene (rifampin resistance-determining region): mutations in ~96% of rifampin-resistant strains
• katG and inhA genes: mutations in 65-75% of isoniazid-resistant strains
• gyrA/gyrB genes: mutations in 42-85% of quinolone-resistant strains
• rrs and eis genes: second-line injectable resistance

• Murray & Nadel's Textbook of Respiratory Medicine, p. 1193

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6. Nucleic Acid Amplification Tests (NAATs)

6a. Conventional PCR

• Amplifies M. tuberculosis-specific DNA sequences
• Sensitivity ~70% in smear-negative patients; >95% in smear-positive patients
• Distinguishes M. tuberculosis from NTM
• Can be performed on fixed tissue (reference labs) though with lower sensitivity

6b. CB-NAAT / Xpert MTB/RIF (GeneXpert) - WHO Priority Test

• Principle: Fully automated, self-contained, real-time PCR in a cartridge
• Concentrates MTB from sputum, isolates genomic DNA by sonication, amplifies by real-time PCR
• Detects M. tuberculosis complex AND rifampin resistance by identifying mutations in the rpoB gene using molecular beacon probes
• Results in 90 minutes from unprocessed sputum
• Sensitivity: >95% (smear-positive specimens); ~70% (smear-negative specimens); ~97% specificity
• Works on sputum, BAL, CSF, pleural fluid, tissue, stool (for pediatric TB)

• Preferred initial test for HIV-infected persons with suspected TB
• Preferred for MDR-TB risk patients
• Can replace two smear specimens for all suspected TB patients where feasible
• Patients diagnosed by Xpert should still submit sputum for culture and conventional DST

• AFB smear+ + NAAT+: Rapid confirmation of TB
• AFB smear+ + NAAT-: Suggests NTM (not M. tuberculosis)
• AFB smear- + intermediate/high clinical probability + NAAT+: Presumptive TB evidence
• AFB smear- + low probability + NAAT+: Interpret with caution (higher false-positive risk)
• NAAT does not replace culture (insufficient sensitivity for exclusion)

6c. Xpert MTB/RIF Ultra

• Second-generation cartridge with improved sensitivity, especially in smear-negative and extrapulmonary TB
• Lower limit of detection; better for paucibacillary disease

• Park's Textbook of Preventive & Social Medicine, p. 216-217
• Murray & Nadel's Textbook of Respiratory Medicine, p. 1189-1193
• Medical Microbiology 9e, p. 264

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7. Line Probe Assays (LPA)

1. Sputum decontaminated and concentrated
2. DNA extracted from smear-positive samples
3. PCR amplification of drug resistance-associated regions
4. PCR products reverse hybridized onto nitrocellulose strips containing specific probes
5. Color lines appear at probe positions indicating wild-type or mutant alleles

• Smear-negative samples are inoculated in liquid culture first, then LPA performed on the growth isolate
• Results in ~1-2 days (vs. weeks for phenotypic DST)
• WHO-endorsed for rapid MDR-TB and XDR-TB diagnosis

• Park's Textbook of Preventive & Social Medicine, p. 216-217

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8. Immunodiagnosis

8a. Tuberculin Skin Test (TST/Mantoux/PPD)

• Intradermal injection of 5 tuberculin units (TU) of PPD (purified protein derivative)
• Read at 48-72 hours by measuring the diameter of induration (not erythema)
• Skin test positivity develops 3-4 weeks after exposure to M. tuberculosis

• False positives: BCG vaccination, NTM infection
• False negatives: HIV/AIDS, severe TB itself, live-virus vaccination, corticosteroids, malnutrition
• Cannot differentiate active vs. latent TB
• Two-step testing recommended for healthcare workers (to detect "booster phenomenon")
• Remains the only field tool for measuring prevalence of TB infection in communities

8b. Interferon-Gamma Release Assays (IGRAs)

• ESAT-6 and CFP-10 are absent from BCG strains and most NTM → highly specific
• NOT affected by BCG vaccination status

• Single patient visit required
• Results within 24 hours
• Not affected by BCG vaccination
• More specific

• Blood sample must be processed quickly
• Requires laboratory infrastructure
• Only for latent TB (not active TB diagnosis)
• May be less accurate in HIV patients

• Medical Microbiology 9e, p. 264
• Park's Textbook of Preventive & Social Medicine, p. 218

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9. Non-Sputum Based / Point-of-Care Tests

9a. Urine Lipoarabinomannan (LAM) Assay

• LAM is a glycolipid of the MTB cell wall released from metabolically active or degenerating bacteria
• Detected in urine by lateral-flow immunochromatographic assay
• Determine TB LAM Ag (Alere): Only commercially available assay
  • Pooled sensitivity: 42% (HIV+); 62% in symptomatic HIV+ inpatients
  • Pooled specificity: 91%
  • Sensitivity increases as CD4 count decreases
• WHO recommendation: Initial test for all HIV-infected inpatients with TB symptoms (regardless of CD4 count), and HIV-infected outpatients with CD4 <100 cells/µL
• FujiLAM (FujiFilm): Next-generation assay using high-affinity monoclonal antibodies + silver-based amplification step; demonstrates improved sensitivity vs. Determine TB LAM

• Murray & Nadel's Textbook of Respiratory Medicine, p. 1193

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10. Newer and Emerging Technologies

10a. Next-Generation Sequencing (NGS) / Whole-Genome Sequencing (WGS)

• Provides comprehensive drug resistance profiles covering all anti-TB drug classes simultaneously
• Overcomes PCR-based limitations (which miss uncommon resistance mutations)
• Turnaround: ~48 hours from clinical specimen
• Deeplex Myc-TB (GenoScreen): Targeted NGS; identifies mutations in 18 drug resistance-associated gene targets covering 13 anti-TB drugs/drug classes (all first-line + most second-line: fluoroquinolones, injectables, bedaquiline, clofazimine, linezolid, ethionamide)
• DeepChek-TB (Advanced Biological Laboratories): Similar 13-drug coverage; automated raw data analysis
• Also enables molecular epidemiology and TB strain typing for outbreak investigation

10b. Targeted NGS / Amplicon Sequencing

• Sequences specific drug-resistance gene regions rather than whole genome
• Faster and cheaper than WGS while covering all relevant resistance loci

10c. Digital PCR

• Absolute quantification of M. tuberculosis DNA without reference standards
• Can detect very low bacterial loads (paucibacillary disease)
• Mentioned by Goldman-Cecil as a tool to decrease detection time

10d. CRISPR-Based Diagnostics (Emerging)

• SHERLOCK (Specific High-sensitivity Enzymatic Reporter UnLOCKing) and DETECTR platforms
• Combine isothermal amplification with CRISPR-Cas effectors for ultra-sensitive, rapid TB detection
• Currently in research/validation phase; not yet in routine clinical use

10e. Volatile Organic Compound (VOC) Analysis / "Electronic Nose"

• M. tuberculosis produces characteristic VOCs detectable in exhaled breath
• Allows non-invasive, non-sputum-based diagnosis
• Research stage

10f. Artificial Intelligence-Aided Chest X-Ray Analysis

• AI algorithms (e.g., CAD4TB) analyze CXR for TB-consistent patterns
• WHO-endorsed as a screening tool in high-burden settings
• Used to triage patients for further molecular testing

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11. Molecular Epidemiology / Strain Typing

• IS6110 RFLP: Classical method; uses restriction fragment length polymorphism of IS6110 insertion sequence (polymorphic in MTB)
• Spoligotyping: PCR-based; detects spacer sequences in the CRISPR-like direct repeat region of MTB; faster than RFLP; classifies into lineages (Beijing, Euro-American, etc.)
• MIRU-VNTR (Mycobacterial Interspersed Repetitive Unit - Variable Number of Tandem Repeats): PCR-based; 24-locus typing; discriminatory power comparable to IS6110 RFLP
• WGS: Highest resolution typing; increasingly replacing other methods in high-income countries

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Summary: Diagnostic Algorithm (NTEP/WHO)

Presumptive TB patient
        ↓
Sputum collection (2 specimens)
        ↓
AFB smear microscopy + CB-NAAT (Xpert MTB/RIF)
        ↓
Smear+ / NAAT+           Smear- / NAAT- but clinical suspicion
    ↓                              ↓
Start treatment         Culture (liquid + solid media)
DST (LPA / MODS / NGS)       ↓ (10-14 days)
                        Culture+ → Species ID + DST
                        Culture- → Consider extrapulmonary
                                   workup / empiric treatment
                                   in high-suspicion cases

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Quick Reference Table

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• Murray & Nadel's Textbook of Respiratory Medicine (Chapter 53, Laboratory Diagnostic Evaluation)
• Goldman-Cecil Medicine (Chapter 299, Active Tuberculosis)
• Park's Textbook of Preventive & Social Medicine (TB Diagnosis section, NTEP guidelines)
• Medical Microbiology 9e (Chapter 22, Immunodiagnosis and Microscopy)
• Recent PubMed reviews: Kontsevaya et al., Clin Microbiol Infect 2024 (PMID 37490968); Trajman et al., Lancet 2025 (PMID 40057344)