Are ground glass opacities (GGOs) indicative of pneumonia?

Ground Glass Opacities and Pneumonia

Ground glass opacities (GGOs) are indicative of pneumonia but are nonspecific and can represent numerous other acute and chronic pulmonary processes, requiring clinical correlation and additional imaging features for accurate diagnosis.

GGOs in Infectious Pneumonia

Viral Pneumonia

• COVID-19 pneumonia characteristically presents with multiple patchy ground glass opacities in bilateral multiple lobules with peripheral distribution 1, 2
• The typical CT manifestation includes "paving stone-like" changes with fine-grid or small honeycomb-like thickening of interlobular septa 1
• Approximately 72% of COVID-19 patients demonstrate ground glass opacities on chest CT 2
• Other viral pneumonias including influenza and cytomegalovirus can also present with GGOs 3

Bacterial and Atypical Pneumonia

• Pneumocystis pneumonia (PcP) demonstrates diffuse bilateral perihilar infiltrates and patchy areas of ground-glass attenuation with peripheral sparing 1
• PcP may show cysts, septal thickening, consolidation, and centrilobular nodules 1
• In febrile neutropenic patients, GGOs can indicate various infectious etiologies requiring bronchoscopy and targeted antimicrobial therapy 1

Critical Differential Diagnoses Beyond Pneumonia

Organizing Pneumonia

• Cryptogenic organizing pneumonia (COP) characteristically demonstrates patchy consolidation commonly associated with ground-glass opacity 1
• Peripheral/peribronchovascular consolidation pattern should raise suspicion for organizing pneumonia 4
• Empiric corticosteroid trial should be considered when organizing pneumonia is suspected, with expected clinical improvement within 48-72 hours 4

Interstitial Lung Diseases

• Nonspecific interstitial pneumonia (NSIP) most commonly presents with bilateral ground-glass opacity, often with subpleural sparing 1, 5
• Extensive ground-glass opacification with subpleural sparing suggests fibrotic NSIP rather than usual interstitial pneumonia (UIP) 1
• Idiopathic pulmonary fibrosis may show mild GGOs, but extensive ground glass opacity (>30% of lung involvement) should prompt consideration of alternative diagnoses 5
• Acute exacerbation of IPF presents with bilateral ground-glass opacification with or without consolidation on a background of lung fibrosis 1

Other Non-Infectious Causes

• Pulmonary edema (both hydrostatic and permeability-related) presents with GGOs and requires cardiac function assessment 4, 5
• Drug-induced pneumonitis from antibiotics, molecular targeting agents, and immune checkpoint inhibitors can manifest as GGOs 4, 5
• Alveolar hemorrhage presents with bilateral patchy GGOs in middle and lower lung zones 5
• Hypersensitivity pneumonitis shows centrilobular nodules with ground glass opacity 4, 5

Diagnostic Approach for Persistent GGOs

Imaging Evaluation

• Obtain thin-section HRCT to characterize the GGO pattern and distribution, looking for crazy-paving pattern, peripheral and lower lung predominance, centrilobular nodules, and traction bronchiectasis 4
• Distribution pattern (diffuse, patchy, peripheral, basal) and associated findings (reticular opacities, honeycombing, traction bronchiectasis) are critical for narrowing differential diagnosis 5
• GGOs with reticular lines and traction bronchiectasis/bronchiolectasis indicate lung fibrosis 5

Timeline and Follow-up

• Expected resolution of post-pneumonic ground glass opacities occurs within 2-3 weeks after clinical symptom onset 4
• Serial HRCT at 4-6 week intervals should be performed when GGOs persist to document progression, stability, or resolution 4
• Follow-up thoracic CT scans should generally not be ordered less than 7 days after starting treatment, as pulmonary infiltrates may increase in volume during the first week despite effective therapy 1

Laboratory and Invasive Testing

• Repeat respiratory pathogen testing including atypical organisms and fungal antigens, and check inflammatory markers (CRP, ESR, procalcitonin) 4
• Lymphocyte count with absolute lymphocyte count <0.8 × 10⁹/L warrants particular attention 4
• Proceed to bronchoscopy with bronchoalveolar lavage when ground glass opacity persists beyond 4 weeks or clinical deterioration occurs 4
• Transbronchial biopsy may assist in ruling out lymphangitic spread, organizing pneumonia, or chronic drug-induced changes 4

Common Pitfalls to Avoid

• Do not delay bronchoscopy beyond 6-8 weeks in immunocompromised patients or those with progressive symptoms 4
• Do not overlook medication history, as drug-induced pneumonitis can occur with common antibiotics and requires drug discontinuation 4
• Do not assume all GGOs in the current era represent COVID-19; maintain a broad differential diagnosis including Pneumocystis pneumonia and other etiologies 6
• Approximately 23% of COVID-19 patients may have normal baseline chest CT, with GGOs appearing on follow-up imaging after 3-4 days 2