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Non-Tuberculous Mycobacteria (NTM) — Diagnosis: Full MD Exam Guide

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1. Introduction & Classification

• Atypical mycobacteria
• Environmental mycobacteria
• MOTT (Mycobacteria Other Than Tuberculosis)

Runyon Classification (Classic Exam Table)

Exam tip: MAC (Group III) is the most common NTM causing pulmonary disease in immunocompetent adults.

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2. Epidemiology & Risk Factors

Host Risk Factors

• Structural lung disease: bronchiectasis, COPD, prior TB, pneumoconiosis, CF
• Lady Windermere syndrome: slender, elderly women with scoliosis/pectus excavatum
• Immunosuppression (HIV/AIDS — CD4 <50 for disseminated MAC)
• Use of inhaled corticosteroids

• HIV with CD4 <50 cells/µL (MAC most common)
• Primary immunodeficiencies (IFN-γ/IL-12 pathway defects)
• Anti-IFN-γ autoantibodies (Southeast Asian patients)

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3. Clinical Presentations

A. Pulmonary Disease (Most Common Form)

B. Lymphadenitis

• Most common NTM disease in children
• Cervical > submandibular nodes (unilateral, non-tender)
• Species: MAC > M. scrofulaceum > M. kansasii
• Violaceous skin, may spontaneously drain (scrofula-like, but NTM in immunocompetent child)
• Exam key: NTM lymphadenitis in a child — not MTB; treat with surgical excision

C. Skin and Soft Tissue / Bone

• Buruli ulcer: M. ulcerans — painless necrotic ulcer, Africa; toxin mycolactone
• Fish tank granuloma: M. marinum — nodular lesion on hand/elbow after aquarium exposure
• Rapidly growing mycobacteria (RGM): M. abscessus, M. fortuitum — post-surgical/injection abscesses

D. Disseminated MAC (HIV)

• CD4 <50 cells/µL
• Fever, night sweats, weight loss, hepatosplenomegaly
• Very high LDH and alkaline phosphatase
• Pancytopenia (bone marrow infiltration)
• Diagnosis: blood culture (lysis-centrifugation) or bone marrow biopsy

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4. Diagnosis — The Core Exam Topic

ATS/IDSA Diagnostic Criteria for NTM Pulmonary Disease (2020 Guidelines)

Treatment of Nontuberculous Mycobacterial Pulmonary Disease, ATS/ERS/ESCMID/IDSA, p. 2

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✅ CLINICAL Criteria (BOTH required):

1. Pulmonary symptoms (cough, hemoptysis, dyspnea) AND
2. Radiographic findings: nodular or cavitary opacities on CXR, OR HRCT showing multifocal bronchiectasis with multiple small nodules
3. Appropriate exclusion of other diagnoses (TB, fungi, malignancy)

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✅ MICROBIOLOGIC Criteria (ONE of the following):

Critical exam point (ATS 2020, p. 2): A single positive sputum culture is NOT diagnostic — clinically significant MAC pulmonary disease is unlikely with 1 positive culture but up to 98% likely with ≥2 positive cultures of the same species.

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Diagnostic Workup — Step by Step

Step 1: Clinical suspicion
        ↓
Step 2: Chest X-ray + HRCT chest
        ↓
Step 3: Sputum × 3 (AFB smear + Mycobacterial culture)
        ↓ (if sputum non-diagnostic)
Step 4: Bronchoscopy → BAL / bronchial wash for AFB culture
        ↓ (if still needed)
Step 5: CT-guided lung biopsy / transbronchial biopsy
        ↓
Step 6: Species identification (nucleic acid probes, HPLC, 16S rRNA gene sequencing)
        ↓
Step 7: Drug susceptibility testing (DST)

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Key Laboratory Investigations

Exam trap: IGRA (QuantiFERON-TB Gold) is negative in NTM — this helps differentiate NTM from MTB. TST can show weak positivity with some NTM species.

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HRCT Findings — High-Yield

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5. Specific NTM Species — Diagnosis Pearls

Exam tip for M. marinum: Culture must be done at 30°C, not 37°C — it will not grow at body temperature. Exam tip for M. haemophilum: Requires hemin-supplemented media — lab must be alerted.

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6. Differentiating NTM from MTB — Exam Table

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7. Special Scenarios

Hot Tub Lung (M. avium)

• Hypersensitivity pneumonitis from MAC aerosolized in hot tubs
• Dyspnea, cough, fever after hot tub exposure
• HRCT: diffuse ground glass + nodules
• Diagnosis: BAL + culture + clinical history
• Treatment: Avoidance (± steroids) — antituberculars not always needed

Lady Windermere Syndrome

• Elderly, slender women; right middle lobe/lingula bronchiectasis
• MAC most common
• Chronic productive cough; no underlying lung disease
• Caused by deliberate cough suppression → impaired mucociliary clearance

NTM in Cystic Fibrosis (ATS/IDSA Guidelines, p. 8)

• M. abscessus most important — can prevent lung transplantation
• Screening with annual sputum AFB cultures recommended
• Diagnosis: same ATS/IDSA criteria apply
• Treatment challenging — multidrug regimens, surgical debridement

Disseminated MAC in HIV

• CD4 <50 → start MAC prophylaxis (azithromycin weekly)
• Diagnosis: blood cultures (lysis-centrifugation) — most sensitive
• Features: fever, wasting, hepatosplenomegaly, markedly elevated ALP

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8. Key Exam Points — Quick Recall

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9. Summary Diagnostic Algorithm

Suspected NTM disease
         │
    ┌────▼────┐
    │ Clinical │ Symptoms + CXR/HRCT (nodules, bronchiectasis, cavities)
    └────┬────┘
         │
    ┌────▼────────────┐
    │ Sputum AFB ×3   │ Culture on Lowenstein-Jensen / MGIT
    └────┬────────────┘
         │
   ┌─────▼──────┐
   │ ≥2 positive │ → Identify species (AccuProbe / 16S rRNA)
   │ same species│ → DST → Diagnose NTM-PD ✓
   └─────┬───────┘
         │ If non-diagnostic
    ┌────▼──────────────┐
    │ Bronchoscopy BAL  │ ≥1 positive culture → Diagnose NTM-PD ✓
    └────┬──────────────┘
         │ Still inconclusive
    ┌────▼──────────────┐
    │ Lung biopsy       │ Granulomas/AFB + ≥1 culture → Diagnose ✓
    └───────────────────┘

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