Background: Multidisciplinary discussion (MDD) is the gold standard for diagnosis of interstitial lung disease (ILD). However, in cases with 50% or less diagnostic confidence or possible new entities (smoking-related ILD; SR-ILD and NSIP with OP), a diagnosis of unclassifiable ILD (UC-ILD) is made. Moreover, the impact of the ATS/JRS/ALAT guideline for hypersensitivity pneumonitis (HP) on the overall ILD diagnosis is unclear.

Objective: To investigate disease frequencies of ILD including new entities in an interim analysis result.

Method: This prospective nation-wide registry (PROMISE study) enrolled cases from 133 referral respiratory hospitals in Japan from September 2020 to October 2022, for which central MDDs were conducted through an online MDD platform. This study provides 3-year follow-up.

Results: This study included 2,741 patients with a mean age of 69.1 years, of whom 63.7% were male, 9.2% surgical lung biopsy, and 24.0% cryobiopsy. Based on MDD made within a mean of 8 days, frequencies of idiopathic pulmonary fibrosis, UC-ILD, HP, and connective tissue disease related ILD were 23.8%, 24.9%, 22.2%, and 13.8%, respectively. Non-IPF fulfilled progressive fibrosing ILD was seen in 13.7% of cases by enrollment. The reason for diagnosis of UC-ILD was diagnostic confidence of 30 to 50% in 29%, NSIP with OP in 18 %, and SR-ILD in 17%. MDD resulted in a change in 47% of institutional diagnoses, and in 83% of institutional UC-ILD. 33% of MDD-IPF and 57% of MDD-HP showed discordant institutional diagnosis.

Conclusion: This registry revealed the relative distribution of specific ILD, the details of UC-ILD, and the differences between central MDD and institutional diagnosis.