16th January 2024

Diagnosis

There is no single gold standard diagnostic test for HP. Diagnosis is based on detailed exposure history, clinical presentation, and a combination of investigations. 

Laboratory tests

• Serum IgG (precipitins) against causative antigens
  • Only indicates exposure to specific antigens rather than diagnosis of HP
  • High false negative rate
  • The specific causative antigen may not be represented in the testing panel due to the vast range of causative agents.

Pulmonary function tests

• Non-fibrotic HP demonstrates a restrictive pattern
• Fibrotic HP- demonstrates both an obstructive and restrictive features

Imaging 

Chest radiography

• Chest x-rays may be normal 
• Where changes are present (more common in fibrotic HP), there are predominantly upper zone reticular interstitial changes with lung volume loss

High-resolution CT scan (HRCT)

• Typical findings in HP include:
  • Centrilobular ground-glass nodules due to bronchiolar inflammation
  • Air trapping occurs following inflammation of small airways, leading to retained air
  • Ground glass changes– interstitial inflammation leads to increased lung density

In HP, HRCT typically demonstrates a “three density pattern” which describes varying areas of attenuation- ground glass changes, decreased lung density (due to air trapping) and also areas of normal lung parenchyma. This is highly specific for HP. Such changes overlap with other forms of ILD. Sparing of bases and air trapping favours a diagnosis of HP.

Bronchoalveolar lavage (BAL)

• Flexible bronchoscopy with BAL typically displays lymphocytosis >30% in HP
• This helps to exclude other lung diseases which may present similarly to HP
• A low CD4+/CD8 + lymphocytosis is suggestive of HP
• Sarcoidosis presents with a high CD4+/CD8+ ratio of >3.5

Lung biopsy

• This should only be considered if a definitive diagnosis cannot be reached despite the above investigations.
• Lung biopsy histology shows a mixture of lung fibrosis, alveolitis and non-caseating granuloma formation.