Systematic Literature Review
A flow diagram of the publications identified for inclusion is presented in Fig. S1 in the electronic supplementary material. In total, 1633 publications were identified through literature searches conducted in MEDLINE and Embase. After the removal of 340 duplicates, 1251 publications were excluded on the basis of their abstracts, as they either did not report on the outcomes of interest or were not conducted in North America or Europe. Of the remaining 42 publications, 33 did not meet the eligibility criteria. Web-based searches and manual review of reference lists yielded an additional seven references, providing a total of 16 publications from which data were extracted [1,2,3, 18,19,20,21,22,23,24,25,26,27,28,29,30]. Six of the studies were conducted in the USA and 10 were conducted in Europe; 10 were cohort studies and six had a cross-sectional design; studies were conducted between 1988 and 2013 and published between 1990 and 2013. Of the 16 identified publications, five reported data on overall ILD prevalence and 11 reported data on the prevalence of individual ILDs or the proportion of individual ILDs in the prevalent ILD population (Table S3).
Overall ILD prevalence, as reported in five identified publications from the literature review, ranged from 6.3 to 76.0 per 100,000 people in four studies in Europe [1, 24, 26, 30] and was 74.3 per 100,000 people in one study in the USA  (Fig. 2 [1, 21, 24, 26, 30]). Two of these studies also reported the prevalence of progressive fibrosing ILD specifically [21, 24]. The first study, conducted in France, estimated fibrotic idiopathic interstitial pneumonia (including IPF, iNSIP and cases with the International Classification of Diseases [ICD] 10th Revision code for pulmonary fibrosis) prevalence at 12.6 per 100,000 people . The second study, conducted in New Mexico, USA, estimated the prevalence of ILD with pulmonary fibrosis (based on the presence of ICD code 515) as 29.0 per 100,000 for men and 27.0 per 100,000 for women .
The prevalence of other individual ILDs was reported (or calculated on the basis of the proportion of patients with individual ILDs within the ILD population) in 11 identified publications from the literature review and varied up to 21.5 per 100,000 people. Some studies reported zero prevalence for particular ILDs. The estimated prevalence or prevalence range for each ILD, including data from studies that reported prevalence estimates for individual ILDs, is presented in Fig. 3 [2, 16, 19,20,21,22,23,24,25,26, 30].
Five additional publications were identified that did not report prevalence data, but did assess the proportion of patients diagnosed with individual ILDs within the prevalent ILD population [3, 18, 27,28,29], with IPF (27.0–32.5%) and sarcoidosis (33.7–44.7%) being the most commonly reported (Table S1).
Estimates of Progression
Excluding IPF (which is by definition progressive), 13–40% of patients with ILD were estimated to develop a progressive fibrosing phenotype, depending on the underlying disease (Table 1) [17, 31,32,33,34]. Four studies describing the proportion of patients with RA-ILD , SSc-ILD , polymyositis/dermatomyositis-associated ILD  and sarcoidosis  who had a progressive fibrosing phenotype were identified in the literature (Table 1). Two of these studies used pre-specified criteria (diffusing capacity of the lung for carbon monoxide [DLCO] falling below 40% predicted ; relative decline in FVC of at least 10% or in DLCO of at least 15% ) to define a progressive fibrosing phenotype. While progression was not defined in the other two studies, patients whose disease progressed were identified retrospectively on the basis of worsening lung function  or worsening respiratory symptoms .
Prevalence of Progressive Fibrosing ILDs
Of the five studies that reported overall ILD prevalence, two also reported the prevalence of various individual ILDs, such as RA-ILD [26, 30], and two reported the proportion of patients with individual ILDs within the ILD population [21, 24], thus enabling prevalence estimates to be calculated. For these four studies, for which the prevalence of individual ILDs could be determined [21, 24, 26, 30], calculated prevalence estimates for individual ILDs with a progressive fibrosing phenotype varied up to 16.7 per 100,000 people (Table 2) [21, 24, 26, 30]. The collective estimated prevalence of progressive fibrosing ILDs ranged from 2.2 to 20 per 100,000 people in Europe [24, 26, 30] and was 28 per 100,000 people in the USA .