Can CT distinguish hypersensitivity pneumonitis from idiopathic pulmonary fibrosis?

The clinical management of patients with idiopathic pulmonary fibrosis differs markedly from that of patients with hypersensitivity pneumonitis. However, the two diseases often cannot be differentiated on clinical grounds. The purpose of this study was to establish whether CT can be used to make the distinction.

Thirty-six patients with idiopathic pulmonary fibrosis and 27 patients with hypersensitivity pneumonitis were studied. All diagnoses were confirmed or supported by open lung biopsy. Three of the patients with idiopathic pulmonary fibrosis had desquamative interstitial pneumonia, and the remainder had usual interstitial pneumonia. In 19 of the 27 patients with hypersensitivity pneumonitis, the disease was chronic (symptoms lasting more than 1 year), while eight had acute or subacute symptoms. Two radiologists, who had not previously seen any of the cases and were blinded to the diagnosis, reviewed the CT images by consensus. The extent and distribution of CT features (including ground-glass attenuation, honeycombing, and micronodules) were recorded. In each case, a CT diagnosis was made, and the level of diagnostic confidence was recorded.

A CT diagnosis was made with a high level of confidence in 39 (62%) of 63 patients. In these patients, the CT diagnosis was correct in 35 cases (90%): 23 of 26 patients with a CT diagnosis of idiopathic pulmonary fibrosis and 12 of 13 patients with a CT diagnosis of hypersensitivity pneumonitis. In the three patients with desquamative interstitial pneumonia, the CT diagnoses were probable hypersensitivity pneumonitis in two cases and definite hypersensitivity pneumonitis in one case. Of the 19 patients with chronic hypersensitivity pneumonitis, only seven had a definite diagnosis of hypersensitivity pneumonitis based on CT findings; three had a definite diagnosis of idiopathic pulmonary fibrosis. The patients with idiopathic pulmonary fibrosis and usual interstitial pneumonia were more likely to have honeycombing and peripheral or lower lung zone predominance of disease, and less likely to have micronodules, than were patients with chronic hypersensitivity pneumonitis. The patients with idiopathic pulmonary fibrosis and desquamative interstitial pneumonia had widespread ground-glass opacity indistinguishable from some cases of acute or subacute hypersensitivity pneumonitis.

Our results show that CT can be used to distinguish idiopathic pulmonary fibrosis from hypersensitivity pneumonitis in most but not all cases. Desquamative interstitial pneumonia cannot reliably be distinguished from acute or subacute hypersensitivity pneumonitis. Chronic hypersensitivity pneumonitis may have findings identical to those of usual interstitial pneumonia. Lung biopsy should still be considered the gold standard for diagnosis of interstitial lung disease.