Is a patient with a history of exposure to Tuberculosis (TB) and presenting with ground glass opacities with a mosaic pattern on chest Computed Tomography (CT) scan likely to have active TB?

Ground Glass Opacities with Mosaic Pattern: Active TB Assessment

Ground glass opacities with a mosaic pattern on chest CT are NOT typical for active tuberculosis and should prompt consideration of alternative diagnoses, particularly MAC hypersensitivity-like lung disease (hot tub lung), small airways disease, or vascular pathology. 1

Why This Pattern is Atypical for Active TB

The classic CT findings of active postprimary pulmonary tuberculosis are centrilobular nodules with tree-in-bud appearance, lobular consolidation, cavitation, and bronchial wall thickening—not a mosaic pattern. 2 While ground glass opacities can occur in severe disseminated TB (miliary or bronchogenic spread), they typically present as diffuse bilateral areas mixed with small nodular lesions, not as a mosaic pattern. 3

Understanding the Mosaic Pattern

A mosaic pattern reflects heterogeneous lung attenuation and has three primary etiologies:

• Small airways disease (most common): The pulmonary vessels within lucent regions are smaller than vessels in opaque areas, with air trapping on expiratory CT scans 4
• Pulmonary vascular disease: Similar vessel size discrepancy but without air trapping 4
• Infiltrative ground-glass disease: Vessels remain uniform in size throughout different attenuation regions 4

MAC Hypersensitivity-Like Disease: The Key Differential

When ground glass opacities occur with a mosaic pattern, MAC hypersensitivity-like lung disease is a critical consideration, particularly with hot tub exposure history. 1 This presentation includes:

• Subacute onset of dyspnea, cough, and fever 1
• Diffuse infiltrates with prominent nodularity and ground glass opacities with mosaic pattern on HRCT 1
• Nonnecrotizing granulomas that are centrilobular and bronchiocentric (distinguishing it from sarcoidosis) 1
• MAC isolates from sputum, bronchoalveolar lavage, or hot tub water 1

Diagnostic Algorithm for TB Exposure with This CT Pattern

Step 1: Assess clinical presentation

• Active TB typically presents with chronic cough, hemoptysis, night sweats, weight loss, and upper lobe predominant disease 5
• Subacute dyspnea with hot tub exposure suggests MAC hypersensitivity 1

Step 2: Obtain microbiological confirmation

• Collect three sputum specimens for AFB smear and culture (essential regardless of imaging pattern) 5
• Consider bronchoscopy with BAL if sputum non-diagnostic 5
• Request mycobacterial culture to distinguish MTB from MAC 1

Step 3: Perform expiratory CT scans

• Paired inspiratory/expiratory CT distinguishes small airways disease (air trapping present) from vascular disease (no air trapping) 4
• This helps differentiate the mosaic pattern etiology 4

Step 4: Investigate alternative exposures

• Hot tub use (MAC hypersensitivity) 1
• Immunosuppressive medications, particularly anti-TNF agents or checkpoint inhibitors 1
• Occupational or environmental exposures 6

Critical Pitfalls to Avoid

Do not assume active TB based solely on exposure history when imaging is atypical. Ground glass with mosaic pattern is a nonspecific finding with a broad differential diagnosis including viral pneumonias, vaping injury, pulmonary hemorrhage, and pulmonary edema. 7

Do not delay microbiological workup. Even with atypical imaging, bacteriological confirmation remains essential—37% of culture-positive TB cases are smear-negative. 5

In immunocompromised patients (particularly those on checkpoint inhibitors or anti-TNF therapy), maintain heightened suspicion for both TB reactivation and opportunistic infections like MAC. 1 These patients may present with atypical radiographic patterns and require aggressive diagnostic evaluation including bronchoscopy. 1

When Active TB Remains Possible Despite Atypical Pattern

Active TB can occasionally present with ground glass opacities in severe disseminated disease (1.7% of active TB cases present with acute respiratory failure). 3 However, these cases typically show:

• Bilateral small nodular lesions (94% of cases) 3
• Consolidation (76% of cases) 3
• Miliary micronodules or tree-in-bud pattern on HRCT (not isolated mosaic pattern) 3

If TB exposure is documented and clinical suspicion remains high despite atypical imaging, initiate respiratory isolation and proceed with full microbiological workup including bronchoscopy. 5 Do not exclude TB based on imaging alone when epidemiologic and clinical factors suggest active disease. 5