Tuberculosis (TB) Diagnosis

Overview

1. Clinical Suspicion - When to Investigate

• Persistent cough (productive, sometimes bloody) - the most common symptom
• Systemic symptoms: fever, night sweats, weight loss
• Hemoptysis (indicates extensive disease)
• Note: 10-25% of bacteriologically confirmed TB patients do NOT report cough, so a broader symptom screen (any cough + fever + sweats + weight loss) is recommended in high-risk groups

• HIV infection / immunosuppression
• Close contact with confirmed TB case
• Origin from or travel to high-incidence country
• Homelessness, incarceration, healthcare worker exposure
• Diabetes, malnutrition, substance use

• Murray & Nadel's Textbook of Respiratory Medicine, p. 1187-1188

2. Chest Imaging

• Primary TB: parenchymal infiltrates, ipsilateral hilar/mediastinal lymphadenopathy, pleural effusion, miliary pattern
• Post-primary (reactivation) TB: upper lobe infiltrates (apical/posterior segments), cavitation, fibrosis, bronchiectasis
• Active vs. inactive: cavitation and air-space disease suggest active disease; calcified nodules (Ghon focus), fibrosis suggest healed disease

• Superior to CXR for visualizing the entire pleural space and parenchyma
• Best for distinguishing parenchymal from pleural disease
• Can show tree-in-bud pattern, centrilobular nodules - classic for endobronchial spread

• Murray & Nadel's Textbook of Respiratory Medicine, p. 1187-1190

3. Microbiologic Diagnosis

Specimen Collection

• At least 2-3 sputum specimens should be collected for AFB smear microscopy and mycobacterial culture
• Specimens should ideally be collected on separate days (morning sputum has highest yield)
• For patients unable to produce sputum: sputum induction (3-5% hypertonic saline via ultrasonic nebulizer) is the first choice - well tolerated, good yield
• If sputum induction fails: fiberoptic bronchoscopy with BAL ± transbronchial biopsy (especially useful in HIV patients with negative smears, and in miliary TB)
• Gastric lavage (early morning, before eating) - lower yield, used mainly in children

AFB Smear Microscopy

• Historically the first step; now performed alongside NAAT on the initial specimen
• Stains: Ziehl-Neelsen (carbol fuchsin, light microscopy) or auramine-rhodamine (fluorescence microscopy - faster, higher sensitivity)
• Sensitivity: ~45-80% in pulmonary TB (depends on bacillary load); much lower in paucibacillary disease (HIV, extrapulmonary TB)
• Specificity is limited - cannot distinguish M. tuberculosis from NTM (nontuberculous mycobacteria)
• Positive smear = presumptive TB; requires confirmation by culture or NAAT
• A positive smear in a TB-endemic setting from a symptomatic patient = strong presumptive evidence
• Murray & Nadel's Textbook of Respiratory Medicine, p. 1190

Mycobacterial Culture

• Gold standard for definitive TB diagnosis and drug susceptibility testing (DST)
• Liquid media (MGIT, BACTEC): results in 10-14 days; more sensitive than solid media
• Solid media (Lowenstein-Jensen): 3-8 weeks; less common now as primary method
• Speciation by probes or MALDI-TOF distinguishes M. tuberculosis complex from NTM
• Culture is essential for DST even when NAAT confirms TB
• In high-incidence, low-resource settings, culture is often not routinely available

Nucleic Acid Amplification Testing (NAAT)

• Automated real-time PCR; detects M. tuberculosis complex AND rifampin resistance in ~90 minutes
• WHO recommendation: Preferred initial test for suspected pulmonary TB in patients with HIV, risk factors for MDR-TB, or serious illness; also recommended as the initial test in place of smear microscopy wherever feasible
• U.S. guidelines (CDC/ATS/IDSA): NAAT recommended on at least the first sputum specimen; interpret alongside smear results
• Smear-positive + NAAT-positive: Rapid confirmation of TB diagnosis
• Smear-negative + NAAT-positive (intermediate/high pretest probability): Presumptive evidence of TB
• Smear-negative + NAAT-negative: Does not exclude TB; sensitivity of current NAATs is insufficient to replace culture
• For pleural TB: Xpert has higher sensitivity on pleural biopsy than on pleural fluid
• Patients confirmed by Xpert MTB/RIF should still submit sputum for smear and culture (especially if rifampin resistance detected) to enable treatment monitoring and full DST

• Nucleic acid probes: Used to identify organism in culture
• RFLP / whole genome sequencing (WGS): Strain comparison in epidemiologic studies
• Gene-based susceptibility testing: Rapid detection of resistance mutations (rpoB for rifampin, katG/inhA for isoniazid, etc.)
• Murray & Nadel's Textbook of Respiratory Medicine, p. 2491-2492; Harrison's Principles of Internal Medicine 22E (2025), p. 1437

4. Diagnosis of TB Infection (Latent TB / LTBI)

Tuberculin Skin Test (TST / Mantoux)

• PPD (purified protein derivative): 5 TU injected intradermally; read at 48-72 hours
• Measures T cell response to mycobacterial antigens
• Interpretation (induration size):
  • ≥5 mm: HIV+, close contacts, immunosuppressed, fibrotic changes on CXR
  • ≥10 mm: Recent immigrants, residents/staff of high-risk settings, children <4 years, injection drug users
  • ≥15 mm: No risk factors (general population)
• Limitations:
  • False positives: BCG vaccination, NTM infection, boosting phenomenon
  • False negatives: Immunosuppression, HIV, overwhelming TB, anergy
  • Cannot distinguish active TB from LTBI
  • Operator-dependent; batch-to-batch PPD variation

Interferon-Gamma Release Assays (IGRAs)

• QuantiFERON-TB Gold Plus (QFT-Plus): Whole-blood ELISA measuring IFN-γ; includes TB7.7 antigen; also measures CD8+ T cell response (useful in early infection and reactivation)
• T-SPOT.TB (Oxford Immunotec): ELISPOT assay

• Not affected by prior BCG vaccination
• Requires single patient visit
• Less subjective reading
• Useful in HIV-positive patients, children, and BCG-vaccinated persons

• Use ESAT-6 and CFP-10 (like IGRAs) but delivered as skin tests
• WHO assessment: Accuracy similar to IGRAs and greater than TST
• Combine TST simplicity with IGRA specificity

• Cannot distinguish LTBI from active TB
• Cannot identify individuals at highest risk of progression
• Cannot distinguish new from old infection
• Reduced sensitivity in immunocompromised patients
• Harrison's Principles of Internal Medicine 22E (2025), p. 1437-1438

5. Extrapulmonary TB Diagnosis

6. Drug Susceptibility Testing (DST)

• Molecular rapid DST at diagnosis: at minimum rifampin resistance (rpoB mutations); isoniazid resistance (katG, inhA)
• If rifampin resistance detected: full second-line DST
• Phenotypic DST (liquid culture): full susceptibility profile
• Resistance mutations (Harrison's 2025):
  • Rifampin: rpoB (95%)
  • Isoniazid: katG (50-95%), inhA promoter (up to 45%)
  • Pyrazinamide: pncA (up to 98%)
  • Ethambutol: embB (50-65%)
  • Fluoroquinolones: gyrA-gyrB (75-95%)
  • Aminoglycosides: rrs gene (up to 80%)

7. Diagnostic Algorithm Summary

Clinical suspicion (symptoms + risk factors)
        ↓
Chest X-ray / CT
        ↓
Collect ≥2-3 sputum specimens
        ↓
AFB smear + NAAT (Xpert MTB/RIF) simultaneously on first specimen
Culture on all specimens (liquid + solid media)
        ↓
Smear(+)/NAAT(+) → Presumptive TB → Start treatment + await culture/DST
Smear(-)/NAAT(+) → Presumptive TB (if intermediate/high risk)
Smear(-)/NAAT(-) → Culture still needed; if sputum unavailable → bronchoscopy/induced sputum
        ↓
Culture confirms species + DST guides regimen

TST or IGRA (preferred in BCG-vaccinated or immunocompromised)
Positive → Rule out active TB → Treat LTBI if indicated

Key Points

• Xpert MTB/RIF is the WHO-recommended first-line test where available; detects TB and rifampin resistance in 90 minutes
• Culture remains the gold standard - never omit it even if NAAT is positive, because DST requires it
• A negative NAAT does not exclude TB - current NAATs lack sufficient sensitivity to replace culture
• IGRAs are preferred over TST in BCG-vaccinated individuals and immunocompromised patients; new TBSTs (ESAT-6/CFP-10 skin tests) offer comparable accuracy with simpler delivery
• 10-25% of confirmed TB cases lack cough - use a broader symptom screen in high-risk populations
• Murray & Nadel's Textbook of Respiratory Medicine, pp. 1187-1192
• Harrison's Principles of Internal Medicine 22E (2025), pp. 1435-1441