Discuss in short the role of a Chest X-ray in the diagnosis of pulmonary tuberculosis.

Principles of Chest Radiography

Chest X-rays utilize ionizing radiation to create images of the lungs, heart, and surrounding structures. Different tissues absorb radiation differently, resulting in varying shades of gray on the image. Bones appear white (radiopaque) due to high calcium content, while air-filled lungs appear dark (radiolucent). Pathological changes, such as inflammation or fluid accumulation, alter the density of tissues, leading to visible abnormalities.

Radiographic Findings in Pulmonary Tuberculosis

Primary Tuberculosis

Primary TB typically occurs in individuals with no prior exposure to *Mycobacterium tuberculosis*. Radiographic findings include:

• Ghon complex: A combination of a small, dense focus of infection in the lung periphery (Ghon focus) and enlarged hilar lymph nodes.
• Pleural effusion: Accumulation of fluid in the pleural space.
• Miliary TB: Characterized by numerous small, millet-seed-like nodules scattered throughout both lungs, representing widespread hematogenous dissemination of the bacilli.

Post-Primary (Reactivation) Tuberculosis

Post-primary TB arises from reactivation of latent TB infection, often in the apical and posterior segments of the upper lobes. Common radiographic features include:

• Cavitation: Formation of a hollow space within a lung lesion, a hallmark of post-primary TB.
• Infiltrates: Areas of increased density in the lung, representing inflammation and consolidation.
• Fibrosis: Scarring of lung tissue, resulting from chronic inflammation.
• Hilar lymphadenopathy: Enlargement of lymph nodes in the hilum (central region) of the lung.
• Pleural thickening: Thickening of the pleura, often due to prior inflammation.

Specific Patterns and Their Significance

Radiographic Pattern	Likely TB Stage/Type	Additional Notes
Ghon Complex	Primary TB	Often seen in children; may resolve without symptoms.
Apical Cavitation	Post-Primary TB	Highly suggestive of reactivation TB; often associated with cough and hemoptysis.
Miliary Pattern	Disseminated TB	Indicates widespread infection; often associated with fever and malaise.
Pleural Effusion	Primary or Post-Primary TB	Can be serous or exudative; requires further investigation to rule out other causes.

Limitations of Chest X-ray in TB Diagnosis

While valuable, chest X-rays have limitations:

• Low Sensitivity in Early/Mild Cases: Early TB or TB in immunocompromised individuals may not show obvious radiographic abnormalities.
• Non-Specific Findings: Many other conditions (pneumonia, lung cancer, fungal infections) can mimic TB on chest X-ray.
• Difficulty in Assessing Extent of Disease: Chest X-rays provide a two-dimensional view and may underestimate the extent of lung involvement.

Complementary Diagnostic Tests

Due to the limitations of chest X-ray, it is crucial to combine it with other diagnostic tests:

• Sputum Smear Microscopy: Detects acid-fast bacilli (AFB) in sputum.
• Sputum Culture: Confirms the presence of *Mycobacterium tuberculosis* and allows for drug susceptibility testing.
• Molecular Tests (e.g., GeneXpert MTB/RIF): Rapidly detects *Mycobacterium tuberculosis* and rifampicin resistance.
• Tuberculin Skin Test (TST) / Interferon-Gamma Release Assay (IGRA): Detects latent TB infection.
• High-Resolution Computed Tomography (HRCT): Provides more detailed images of the lungs, useful for evaluating subtle abnormalities.