Do the CT findings of bilateral lower‑lobe mosaic attenuation, patchy ground‑glass opacities with interstitial septal and subpleural thickening, and pleural fibrosis represent an infectious process or are they more consistent with small‑airway disease such as hypersensitivity pneumonitis?

These CT findings are more consistent with small airway disease, particularly hypersensitivity pneumonitis, rather than a typical infectious process.

The combination of bilateral lower-lobe mosaic attenuation, ground-glass opacities with interstitial septal thickening, subpleural thickening, and pleural fibrosis strongly suggests hypersensitivity pneumonitis (HP) rather than infection. 1

Why This Represents Small Airway Disease (Likely HP)

The imaging pattern described contains multiple features that align with the 2020 ATS/JRS/ALAT diagnostic criteria for HP:

Key Diagnostic Features Present:

Mosaic attenuation - This represents heterogeneous lung attenuation from coexisting areas of ground-glass opacity adjacent to normal-appearing lung, which is a hallmark of small airway disease 1. In HP, this reflects lobules affected by pneumonitis alongside areas of air-trapping from bronchiolar obstruction 1.

Ground-glass opacities with interstitial septal thickening - This combination indicates parenchymal infiltration, which is one of the required HRCT abnormalities for diagnosing HP 1. The posterior basal distribution you describe is compatible with HP, which can show lower lobe predominance as a variant pattern 1.

Subpleural thickening and pleural fibrosis - These findings suggest chronicity and potential fibrotic changes, which would classify this as fibrotic HP rather than acute nonfibrotic HP 1, 2.

Why This is NOT Typical of Infection

Infections typically present with:

• Consolidation rather than mosaic attenuation
• Acute onset symptoms
• Absence of pleural fibrosis (unless chronic/organizing infection)
• Different distribution patterns

The presence of pleural fibrosis/thickening is particularly important - this indicates a chronic process rather than acute infection 1. While organizing infections can occasionally cause some of these findings, the constellation of mosaic attenuation, air-trapping features, and chronic fibrotic changes points away from infection.

Diagnostic Confidence Level

Based on the 2020 ATS/JRS/ALAT guidelines 1:

This pattern would be classified as "compatible with fibrotic HP" because:

• There is evidence of lung fibrosis (interstitial septal thickening, subpleural thickening, pleural fibrosis)
• Small airway disease indicators are present (mosaic attenuation, ground-glass opacities)
• The distribution shows lower lobe involvement, which is a recognized variant pattern 1

However, this is NOT classified as "typical HP" because typical fibrotic HP requires mid-lung zone predominance or random distribution with relative basal sparing 1. Your case shows lower lobe predominance.

Critical Next Steps for Definitive Diagnosis

To increase diagnostic confidence from "compatible" to "definite" HP:

1. Exposure history is essential - Systematically question about:

   • Bird exposure (pigeons, budgerigars, feather bedding)
   • Home environment (mold, water damage, humidifiers)
   • Occupational exposures (farming, painting, mushroom growing)
   • Seasonal pattern of symptoms 3

2. Expiratory CT images - If not already obtained, these are critical to demonstrate air-trapping, which would further support small airway disease 1. The guidelines specifically recommend both inspiratory and expiratory acquisitions 1.

3. Serum biomarkers:

   • Bird-specific IgG antibodies (if bird exposure suspected)
   • Trichosporon asahii antibody (for summer-type HP in endemic areas)
   • KL-6 levels (elevated in HP, may show seasonal variation) 3

4. Bronchoalveolar lavage (BAL) - Lymphocytosis >20% significantly increases diagnostic confidence for HP 1

Common Pitfall to Avoid

Do not dismiss HP simply because of lower lobe predominance. While classic fibrotic HP shows mid-lung or upper-lung predominance with basal sparing, lower lobe predominant disease is recognized as a compatible variant pattern in the guidelines 1. Studies show only 10% of fibrotic HP cases have upper lung predominance 1.

The "Three-Density Pattern"

If expiratory images demonstrate air-trapping creating three distinct densities (ground-glass, normal lung, and lucent areas with decreased vascularity), this would be highly specific for fibrotic HP and would upgrade the diagnosis to "typical fibrotic HP" 1, 4. This pattern has been shown to differentiate fibrotic HP from idiopathic pulmonary fibrosis 1.

The radiologist's interpretation correctly identifies small airway disease as the primary consideration, with HP as the leading differential diagnosis based on current diagnostic guidelines.