New Terminologies in Interstitial Lung Disease
The modern ILD classification has evolved to include several critical new descriptive terms that reflect disease behavior rather than just histologic patterns, most importantly: Progressive Pulmonary Fibrosis (PPF)/Progressive Fibrosing ILD (PF-ILD), Rapidly Progressive ILD (RP-ILD), Unclassifiable ILD, and Pleuroparenchymal Fibroelastosis (PPFE). These terms represent a paradigm shift toward phenotypic classification that directly impacts treatment decisions and prognosis 1.
Key New Terminology Categories
Progressive Pulmonary Fibrosis (PPF) / Progressive Fibrosing ILD (PF-ILD)
This represents the most clinically significant new terminology, defining a phenotype rather than a specific disease entity 1, 2.
Definition includes three components: symptom progression (worsening dyspnea or cough), physiologic decline (≥5% FVC decline over 12 months associated with ~2-fold mortality increase), and radiologic progression (extension of fibrosis on HRCT) 1, 2, 3.
Can occur across multiple ILD subtypes: fibrotic hypersensitivity pneumonitis, CTD-related ILD, fibrotic NSIP, fibrotic sarcoidosis, occupational ILD, and others—essentially any fibrotic ILD except IPF itself 1.
Critical therapeutic implication: Antifibrotic therapy (nintedanib or pirfenidone) slows annual FVC decline by approximately 44-57% in PPF regardless of underlying ILD subtype, representing a major treatment advance 1, 3.
Rapidly Progressive ILD (RP-ILD)
RP-ILD is a descriptive term of the modern era, initially coined to describe patients who progress from being well to respiratory failure (requiring high-flow oxygen or mechanical ventilation) within days to weeks 1.
Primary context: Used predominantly in MDA-5 positive inflammatory myopathy (MDA-5-IIM) 1.
Historical confusion: Previously termed "acute interstitial pneumonia," but overlapping terminology created diagnostic ambiguity 1.
Critical distinction: PPF/PF-ILD and RP-ILD are NOT interchangeable terms—they represent fundamentally different disease behaviors and timelines 1.
Treatment approach: Expert centers use tacrolimus and IVIG in critically ill RP-ILD patients, though published data remain limited 1.
Unclassifiable Interstitial Lung Disease
Unclassifiable ILD represents cases where multidisciplinary discussion cannot achieve diagnostic certainty >50%, accounting for approximately 5% of surgical lung biopsies 1.
Formal classification system: Four-tier diagnostic certainty framework established—confident diagnosis (>90% certainty), provisional high confidence (70-90%), provisional low confidence (50-70%), and unclassifiable (<50% certainty) 1.
Common causes of unclassifiability: Inadequate clinical/radiologic/pathologic data, major discordance between clinical-radiologic-pathologic findings, previous therapy substantially altering findings, or truly mixed/overlapping patterns 1.
Multidisciplinary discussion impact: Approximately 50% of pathologically unclassifiable biopsies can be reclassified after MDD incorporating clinical and radiologic data 1.
Biopsy technique consideration: Transbronchial cryobiopsy shows higher unclassifiable rates (17.2%) versus VATS biopsy (1.3%) due to limited tissue sampling 1.
Pleuroparenchymal Fibroelastosis (PPFE)
PPFE is now recognized as a specific rare entity, usually idiopathic, characterized by upper lobe predominant pleural and subpleural parenchymal fibrosis with elastosis 1.
Classification status: Elevated from unrecognized pattern to formal rare idiopathic interstitial pneumonia category in 2013 ATS/ERS update 1.
Can manifest PPF: Listed among fibrotic lung diseases capable of developing progressive pulmonary fibrosis phenotype 1.
Additional Evolving Concepts
Interstitial Lung Abnormalities (ILA)
ILA represents incidentally identified interstitial changes on CT (irregular vessels/airways, distorted lobular anatomy, ground-glass opacities) that are independent risk factors for mortality 1.
Progression rate: At least 40% of ILA cases show radiologic progression over 4-6 years of follow-up 1.
Clinical significance: Often associated with histologic fibrosis despite incidental discovery on imaging performed for other indications 1.
Etiology-Focused Nomenclature Movement
Recent expert opinion advocates shifting away from "idiopathic" terminology toward etiology-focused classification, as most IIPs have identifiable environmental, occupational, genetic, or systemic disease associations 4.
Rationale: The term "idiopathic" may promote diagnostic complacency rather than thorough etiologic investigation 4.
Modern reality: In the 21st century, disorders termed IIPs are rarely truly idiopathic but represent well-described patterns of lung injury with diverse identifiable etiologies 4.
Clinical Implications of New Terminology
Treatment Decision Framework
The PPF/PF-ILD phenotype now drives antifibrotic therapy decisions across multiple ILD subtypes, not just IPF 1, 3.
Specific indications: Nintedanib or pirfenidone recommended for progressive fibrotic hypersensitivity pneumonitis, CTD-related ILD, fibrotic NSIP, and other progressive fibrotic ILDs 1.
Monitoring requirements: Multi-dimensional assessment including symptom tracking, serial HRCT, pulmonary function testing, and 6-minute walk testing to identify progression 2.
Diagnostic Certainty Communication
The four-tier certainty framework standardizes communication between clinicians about diagnostic confidence, reducing heterogeneity in the "unclassifiable" category 1.
Goal: Decrease patients inappropriately categorized as unclassifiable without proper multidisciplinary evaluation 1.
Practical impact: Enables more precise prognostic discussions and treatment planning based on diagnostic certainty level 1.
Common Pitfalls to Avoid
Do not confuse RP-ILD with PPF/PF-ILD—these represent completely different disease trajectories (days-to-weeks versus months-to-years) requiring distinct management approaches 1.
Do not label cases "unclassifiable" without formal multidisciplinary discussion—approximately half can be classified when clinical and radiologic data are integrated 1.
Do not restrict antifibrotic therapy to IPF alone—the PPF phenotype justifies antifibrotic treatment across multiple ILD subtypes demonstrating progression 1, 3.
Do not assume "idiopathic" means investigation is complete—thorough evaluation for environmental, occupational, medication, genetic, and systemic disease causes is essential 4.