Investigations
1st investigations to order
chest x-ray
Test
Patchy, nodular infiltrates; fibrosis in fibrotic HP.[38]
May be normal between episodes.
There are many other diseases with these chest x-ray findings, thus there is low specificity; the sensitivity is approximately 50%, but no methodical studies have been done.
Result
infiltrates, nodular or patchy; fibrosis
CT chest
Test
More accurate than chest x-ray.
CT findings may help to distinguish non-fibrotic HP from fibrotic HP, and HP from other interstitial lung diseases. May also provide prognostic information.[3]
Result
ground-glass shadowing/attenuation and poorly defined micronodules; features of fibrotic HP include mosaic attenuation, centrilobular nodules, and relative basal sparing
serum antigen-specific IgG or IgA
Test
The negative predictive value of a properly performed immunological test is excellent. However, the sensitivity and specificity for differentiating HP from other interstitial lung diseases is only 83% and 68%, respectively. Enzyme-linked immunosorbent assay methods may perform better than precipitin testing. It should be noted that a positive result does not automatically imply causation, and it is important to consider the result in relation to the history and radiological findings.[3][40]
Result
positive
pulmonary function test
Test
In acute symptomatic episodes of HP, the pulmonary function tests will show a restrictive pattern.
In fibrotic HP there may be a mixed picture with both obstructive and restrictive features.
Compared with patients with non-fibrotic HP, those with fibrotic HP usually have a lower vital capacity.[3]
Result
restrictive; mixed restrictive/obstructive
diffusing lung capacity of carbon monoxide
Test
Diffusing lung capacity of carbon monoxide is decreased in all forms of HP, but it may be normal between acute symptomatic episodes of HP.
Compared with patients with non-fibrotic HP, those with fibrotic HP usually have a lower diffusion capacity.[3]
Result
decreased
Investigations to consider
bronchoalveolar lavage
Test
May occasionally be necessary to confirm the diagnosis or exclude differentials.[3][33][39]
Fluid analysed for antibody to causative antigen and presence of other cells, which may help to exclude other causes of lung disease, in particular, sarcoidosis. In patients with suspected non-fibrotic HP, bronchoalveolar lavage may help to identify or exclude pulmonary infection.[3]
Typically, in patients with HP there is a lymphocytosis.[39]
Result
positive antibody + lymphocytosis (elevated CD8+ cells)
lung biopsy
Test
Lung biopsy may occasionally be performed when a multidisciplinary team cannot make a diagnosis based on clinical, laboratory, or imaging data (e.g., patients with atypical features or no history of exposure).[40] Surgical lung biopsy and transbronchial lung cryobiopsy can be considered.
Result
bronchocentric infiltrate consisting of lymphocytes, plasma cells, neutrophils, foamy macrophages, and non-caseating granulomas; in later stages interstitial fibrosis predominates
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