Table 1– Main reasons for interstitial lung disease being unclassified in the series of Ryerson et al. [11]
No biopsy performed or biopsy noncontributory (unclassified or unclassifiable clinical/radiological condition)Biopsy not proposed by physician (stable or mild disease with biopsy outweighing the anticipated benefit; other reasons)
Contraindication or too old to biopsy
Biopsy denied by the patient
Sampling not contributory (insufficient tissue, inadequate site of biopsy, endstage lung)
Overlapping histological features (unclassifiable histology)Nonspecific interstitial pneumonia – usual interstitial pneumonia
Hypersensitivity pneumonitis – usual interstitial pneumonia
Major discrepancy between clinical, imaging and histological features (unclassifiable clinical/radiological/pathological condition)Stable disease – usual interstitial pneumonia histological pattern
Other situations
Uncertain aetiology (unclassifiable clinical condition)Unclear diagnostic boundary with connective tissue disease – interstitial lung disease
Unclear diagnostic boundary with hypersensitivity pneumonitis