Guideline Summary: Differentiating Inflammatory vs Infectious Causes of Bilateral Upper‑Lung Perihilar Opacities

1. Clinical Presentation & Temporal Patterns

Acute onset (< 2 weeks) of symptoms points toward bacterial infection or acute interstitial pneumonia in patients with new bilateral upper‑lung opacities【7】【8】.
Subacute presentation (weeks to 3 months) should raise suspicion for cryptogenic organizing pneumonia (COP), hypersensitivity pneumonitis (HP), or atypical infections【1】【2】【6】.
Chronic course (> 3 months) strongly suggests mycobacterial infection, chronic HP, or fibrotic inflammatory disease【7】【8】.

COP: Patchy consolidation in subpleural, peribronchial, or band‑like distribution that may migrate over time; HRCT pattern is highly suggestive of COP【1】【2】.
HP: Presence of centrilobular nodules with upper/middle‑lobe predominance, combined with the “three‑density sign” (mosaic attenuation, ground‑glass opacity, and air‑trapping) is highly specific for HP【5】【6】.
Early HP: Absence of honeycombing helps differentiate early HP from other fibrotic lung diseases【5】.
Sarcoidosis: Bilateral hilar lymphadenopathy and perilymphatic nodular distribution are characteristic imaging clues【7】.

No clinical improvement after 48–72 h of appropriate antibiotics strongly indicates a non‑infectious inflammatory process, prompting early consideration of corticosteroid therapy and tissue diagnosis【3】.

Bronchoscopy is recommended when there is lack of response to antibiotics within 48–72 h or when HRCT suggests an inflammatory pattern【3】【4】.
Transbronchial biopsy can confirm organizing pneumonia and exclude alternative diagnoses, with a favorable risk‑benefit profile【3】.

Corticosteroids are the cornerstone for inflammatory etiologies (COP and HP); oral prednisolone typically yields rapid clinical and radiological improvement within days to weeks【1】【2】【3】.
COP: > 80 % of patients achieve significant response to steroids; however, relapse is common and may require prolonged tapering【1】【2】.
HP: Antigen avoidance combined with corticosteroids leads to good outcomes【7】【8】.

COP: Excellent prognosis with > 80 % response rate to corticosteroids【3】.
HP: Generally favorable prognosis when the inciting antigen is removed and steroids are administered【7】【8】.
Acute interstitial pneumonia: Carries a high mortality risk (≥ 50 %) if present【1】【2】.

Do not initiate anti‑tuberculosis therapy based solely on radiographic opacities, as this can cause unnecessary drug toxicity and delay correct diagnosis【1】.
Avoid continuing empiric antibiotics beyond 48–72 h without clinical improvement, which postpones appropriate work‑up and corticosteroid initiation【3】.
Never overlook a detailed exposure history, which is essential for diagnosing HP and improving pre‑test probability【6】.