Tuberculosis Classification: Exam-Ready Framework
Primary Tuberculosis
Primary TB occurs in individuals without prior immunity, typically within the first 5 years after initial Mycobacterium tuberculosis exposure, most commonly affecting children and immunocompromised adults. 1
Key Clinical Features
- Develops in patients with no previous TB exposure or specific immunity 1
- Peak risk occurs during adolescence, though can happen at any age during childhood 1
- Positive tuberculin skin test (TST) or interferon-γ release assay (IGRA) develops as immunity forms 2, 1
- Symptoms may be minimal or absent initially 1
Radiographic Patterns
- Lymphadenopathy (hilar/mediastinal) is the hallmark finding 3
- Pulmonary consolidation (often middle or lower lobes) 3
- Pleural effusion 3
- Miliary pattern in immunocompromised patients with hematogenous dissemination 3
Clinical Significance
- High risk of immediate complications with significant morbidity and mortality if untreated 1
- Can progress to active disease or enter latent phase 1
- Early detection and treatment prevent both immediate complications and later reactivation 1
Latent Tuberculosis Infection (LTBI)
LTBI represents persistent immune response to M. tuberculosis antigens (positive TST/IGRA) without any clinical, microbiological, or radiological evidence of active disease. 2, 4
Defining Characteristics
- Completely asymptomatic—no cough, fever, night sweats, or weight loss 2, 4
- Positive immunologic test (TST ≥5mm, ≥10mm, or ≥15mm depending on risk factors; or positive IGRA) 5
- Normal chest radiograph or stable fibrotic changes only 3
- Non-contagious 6
- 5-10% lifetime risk of progression to active TB disease 7
Diagnostic Approach
- IGRA preferred over TST in individuals ≥5 years with BCG vaccination history or unlikely to return for TST reading 2
- TST interpretation cutoffs: ≥5mm (HIV+, recent contacts, immunosuppressed), ≥10mm (moderate risk), ≥15mm (low risk) 5
- Chest radiograph mandatory to exclude active disease before treatment initiation 8
Treatment Considerations
- Standard regimen: Isoniazid for 9 months 8
- Alternative: Rifampin for 4 months if isoniazid contraindicated 8
- Monitor for hepatotoxicity with liver function tests every 2-4 weeks during treatment 8
Post-Primary (Reactivation) Tuberculosis
Post-primary TB develops after a prolonged period of latent infection, typically occurring in the lung apices and posterior segments of upper lobes, representing reactivation rather than recent infection. 3, 9
Distinguishing Features
- Occurs in individuals with established immunity from prior infection 9
- Radiographic findings depend on host immunity level rather than time elapsed since infection 9
- Most common form in adults, particularly with immunosuppression, malnutrition, or aging 3
Classic Radiographic Patterns
- Cavitary lesions (hallmark finding)—presence affects treatment duration 3
- Apical and posterior upper lobe consolidations 3
- Centrilobular nodules in a "tree-in-bud" pattern 3
- Fibronodular opacities in upper lung zones 3
- Bronchogenic spread can occur with endobronchial disease 9
Clinical Presentation
- Constitutional symptoms: persistent cough >3 weeks, night sweats, weight loss, fever 5
- Highly contagious when cavitary disease present 3
- May present with atypical manifestations in immunocompromised patients 9
Diagnostic Confirmation
- Sputum analysis: smear, culture, and nucleic acid amplification testing 3
- CT useful when chest radiograph normal or inconclusive, and for assessing disease activity 9
- Stability of radiographic findings for 6 months distinguishes inactive from active disease 3
Critical Exam Pitfalls to Avoid
- Never dismiss positive TST in BCG-vaccinated patients—IGRA preferred in this population 2, 5
- Children <5 years with TB exposure require immediate evaluation and prophylaxis due to rapid progression risk 8
- Immunocompromised patients may present with atypical patterns (miliary, disseminated primary) rather than classic post-primary findings 3, 9
- Only measure induration (not erythema) when reading TST 5