**3.4. Undifferentiated connective tissue disease (UCTD) and interstitial pneumonia with autoimmune features (IPAF)**

Besides classical, well-established CTDs, increasing attention has been paid to pulmonary involvement in undifferentiated CTD (UCTD) [15, 17, 28–30]. UCTD has been generally defined as a condition that presents with signs and symptoms suggestive of CTDs along with positive ANA but does not fulfill any rheumatology classification criteria for specific CTDs [30, 31]. Mosca et al. reported that 60% of patients with UCTD remain undifferentiated. When evolution to defined CTD occurs, it usually does within the first 5 years of disease. UCTD may develop into any of the CTDs, most often into SLE [29]. There are a large number of patients, in whom the IP appears to be the lone part for the clinically predominant manifestation of an occult CTD with subtle clinical features that suggest an autoimmune process but not meet established criteria for CTD, raising a controversy over the strategies for identification and classification of these patients. Well-organized prospective studies have been needed to better understand this entity of the lung disease and distinguish it from the ILD with well-defined CTD or IIP. Proposed terminology to classify such patients includes "undifferentiated CTD" [31], "lung-dominant CTD" [15], and "autoimmune-featured ILD" [32]. Recently, the "ATS/ ERS Task Force on Undifferentiated Forms of CTD-associated ILD" created consensus regarding the nomenclature and classification criteria for patients with IIP and features of autoimmunity and proposed the term "interstitial pneumonia with autoimmune features" (IPAF). The classification criteria require evidence of IP and are organized around three central domains: a clinical domain consisting of specific extra-thoracic features, a serologic domain consisting of specific autoantibodies, and a morphologic domain consisting of specific chest imaging, histopathologic, or pulmonary physiologic features [33]. Currently, it is not yet clear whether IPAF is a distinct phenotype of ILD or simply a part of IIP. Adopting IPAF classification may provide platform for the future study of a more uniform cohort, and prospective survey will be needed in determining efficacy of therapy and outcomes for the patients [34].
