Differentials
Viral pneumonia
SIGNS / SYMPTOMS
Viral pneumonia typically lasts a week or so. Acute symptoms of HP last only a few days and recur with each additional exposure. Ordinarily, a person should not have repeated episodes of viral pneumonia.
INVESTIGATIONS
Bronchoalveolar lavage of both HP and viral pneumonia will demonstrate increased CD8+ cells; molecular diagnosis can identify common viral causes of pneumonia such as influenza A and B; respiratory syncytial virus; parainfluenza 1, 2, and 3; picornavirus; coronavirus; and adenovirus.
Sarcoidosis
SIGNS / SYMPTOMS
HP does not have extrapulmonary manifestations.
INVESTIGATIONS
Sarcoidosis tends to have an increased CD4/CD8 ratio following bronchoalveolar lavage.
Biopsy may be necessary to differentiate.
Bronchiolitis obliterans
SIGNS / SYMPTOMS
Can resemble fibrotic HP.
INVESTIGATIONS
The diffusing lung capacity of carbon monoxide is normal and pulmonary function tests show an obstructive pattern.
Idiopathic pulmonary fibrosis
SIGNS / SYMPTOMS
There are no differentiating signs or symptoms.
Desquamative interstitial pneumonia
Pneumoconioses
SIGNS / SYMPTOMS
Patients should have appropriate exposure history (mineral dust or a metal).
INVESTIGATIONS
Chest x-ray not likely to demonstrate only fibrosis; there may be pleural calcification (asbestosis) or eggshell calcification of hilar lymph nodes (silicosis).
Neutrophils and causative dust particles may be found in bronchoalveolar lavage.
Silo filler's lung
SIGNS / SYMPTOMS
May appear similar to acute symptomatic episodes of HP; generally symptoms last longer as they are caused by chemical inhalation toxicity; occurs on first exposure as sensitisation is not required.
INVESTIGATIONS
Bronchiolar lavage tends to demonstrate increased neutrophils, as opposed to lymphocytosis.
Organic toxic dust syndrome
SIGNS / SYMPTOMS
May appear similar to acute symptomatic episodes of HP; generally symptoms last longer as they are caused by chemical inhalation toxicity; occurs on first exposure as sensitisation is not required.
INVESTIGATIONS
Bronchoalveolar lavage tends to demonstrate increased neutrophils, as opposed to lymphocytosis.
Metal fume fever
SIGNS / SYMPTOMS
May appear similar to acute symptomatic episodes of HP; generally symptoms last longer as they are caused by zinc oxide or magnesium oxide; occurs on first exposure as sensitisation is not required.
INVESTIGATIONS
Bronchoalveolar lavage tends to demonstrate increased neutrophils, as opposed to lymphocytosis.
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