17th January 2024

Differential diagnoses

HP should be considered as part of the initial work up for any patient presenting with confirmed exposure to one of the identified causative allergens, alongside clinical findings. Other conditions to consider include:

1. Sarcoidosis – this condition usually demonstrates symptoms and signs not limited to the respiratory system, e.g., erythema nodosum. A transbronchial biopsy may be needed in some cases to accurately diagnose HP.
2. Idiopathic pulmonary fibrosis (IPF) – in IPF, HRCT chest would show more evidence of abnormalities in the lower zones of the chest rather than the upper zones, and centrilobular nodules would likely not be seen. However, the imaging can look very similar and respiratory opinion may be needed. Additionally, BALS in IPF would not show lymphocytosis.
3. Pneumoconiosis – this usually only occurs in patients with an obvious exposure history for this condition, e.g., miners who worked with mineral dust. Chest imaging tends to show signs of asbestosis and silicosis, e.g., calcified hilar lymph nodes.
4. Viral pneumonia – the symptoms of viral pneumonia tend to last for some time (at least a week) and would not repeatedly occur once resolved. In HP, symptoms may last only a couple of days but occur repeatedly in an episodic fashion every time the patient is exposed to the allergen. A respiratory virus panel can be helpful to deduce if any viral infection is present.
5. Other more common respiratory diseases, e.g., asthma, COPD – these should also be considered if the history does not contain any clear evidence of an allergen and other suspicious factors are present, e.g., history of atopy for asthma, smoking history in COPD, etc., alongside imaging and other initial investigations.