This is a good way to help exclude other similar lung diseases like sarcoidosis, infection and Idiopathic pulmonary fibrosis. Lung biopsy
Lung biopsies can be diagnostic in cases of chronic hypersensitivity pneumonitis, or may help to suggest the diagnosis and trigger or intensify the search for an allergen. The main feature of chronic hypersensitivity pneumonitis on lung biopsies is expansion of the interstitium by lymphocytes accompanied by an occasional multinucleated giant cell or loose granuloma.When fibrosis develops in chronic hypersensitivity pneumonitis, the differential diagnosis in lung biopsies includes the idiopathic interstitial pneumonias. This group of diseases includes usual interstitial pneumonia, non-specific interstitial pneumonia and cryptogenic organizing pneumonia, among others.The prognosis of some idiopathic interstitial pneumonias, e.g. idiopathic usual interstitial pneumonia (i.e. idiopathic pulmonary fibrosis), are very poor and the treatments of little help. This contrasts the prognosis (and treatment) for hypersensitivity pneumonitis, which is generally fairly good if the allergen is identified and exposures to it significantly reduced or eliminated. Thus, a lung biopsy, in some cases, may make a decisive difference. Serum Precipitins
Assays for serum IgGs can aid in identifying possible antigenic exposures and are used as markers of exposure  However, there use is limited in making a definitive diagnosis because serum antibody levels are often elevated in those people who are exposed to an antigen but do not have the disease. Up to 90% of people exposed to the antigen have precipitins but only 50% of similarly exposed people who are asymptomatic have the same precipitins.
They have an insidious onset of cough, progressive dyspnea, fatigue, and weight loss. This is associated with partial to complete but gradual reversibility. Avoiding any further exposure is recommended. Clubbing is observed in 50% of patients. Tachypnea, respiratory distress, and inspiratory crackles over lower lung fields often are present. In fact, hypersensitivity pneumonitis can often resemble (IPF) in terms of pulmonary fibrosis and that many patients experience hypoxemia. Epidemiology
Although the prevalence of hypersensitivity pneumonitis is not established it is thought to be low. Data collection limitations are a result of difficulty in diagnosis, sub-clinical presentations that go undetected and variability in climate, region and proximity to local industries. The most common types are bird fanciers and farmers lung. Interestingly, cigarette smoking appears to be protective against the disease. Pathophysiology
Hypersensitivity pneumonitis is caused by an exaggerated immune response (hypersensitivity). Type III hypersensitivity and type IV hypersensitivity can both occur depending on the cause. In general, acute HP is suspected to be attributed to a type III hypersensitivity while the subacute and chronic types are suspected to be caused by T cell infiltration and granuloma formation. Because different people react variably to antigen exposure, the exact mechanism is unclear but genetic and host factors are likely at play. The two hit hypothesis is often toted in the literature to explain why some people have a normal reaction to an antigenic exposure without clinical findings while others experience an exaggerated immune response.