The radiographic concept of "interstitial lung abnormalities" grew from the idea that, in the general population, many individuals probably have subtle lesions associated with early ILD that will progress over time. It was observed that peripheral and basilar fibrosis with honeycombing on computed tomography was suggestive of usual interstitial pneumonia and of idiopathic pulmonary fibrosis (IPF), whereas, patients with radiographic patterns not suggestive of IPF seem to have a less IPF-like course that is a bit more indolent. However, screening the entire population for ILD in this way is clearly not feasible. Thus, earlier detection of the underlying pathology of fibrosing lung disease in patients with ILD has been, and remains, somewhat of an unmet need. The concept of chronic fibrosing ILD emerged to consider whether patients with a progressive phenotype can be viewed as having IPF and treated as such. And, indeed, recent clinical trial data suggest that treating patients with chronic fibrotic disease like we treat IPF may have some benefit. 

A number of genes and biomarkers have been associated with chronic fibrosing ILDs, but we currently lack the ability to reliably predict who will develop severe, chronic fibrosing disease and who will not. Having the MUC5B promoter polymorphism increases the risk of having interstitial abnormalities and the risk of IPF. Although this association of MUC5B with IPF is strong and reproducible, MUC5B is a poor screening biomarker in that the polymorphism is present in at least 1 allele in wide swaths of the population, yet IPF does not occur all that frequently. KL-6 a mucin-like glycoprotein that may be overexpressed in a variety of ILDs. In addition to KL-6, a host of ILD biomarkers such as CA-125 and CA-19 and IPF biomarkers such as the matrix metalloproteinases, surfactant proteins, and immune dysregulation biomarkers (eg, CCL-18, CXL-13) have also been identified. However, research on identifying better predictors of short-term progression to identify patients before they progress continues.

Pulmonary hypertension (PH) has prognostic significance in ILD, and noninvasive modalities that might raise suspicion for the presence of PH in chronic lung disease include circulating biomarkers, pulmonary function testing, echocardiography, and imaging. Plasma levels of brain natriuretic peptide or N-terminal pro-brain natriuretic peptide are elevated in severe cases of PH but are less sensitive and specific for moderate PH and may be confounded by left heart abnormalities. But, in general, I think that brain natriuretic peptide is actually a fine biomarker for PH.

Some patients with IPF have been found to have very short telomeres, which is linked to poorer response to immunosuppressive therapy and poorer disease outcomes, so telomere length is another marker to watch. Proteomic profiling also holds promise for improved identification of additional proteins that could be relevant to IPF. A study from the American Journal of Respiratory and Critical Care Medicine has also found that several validated biomarkers in IPF are predictive of other fibrosing ILDs, underscoring the shared biology of progressing ILDs.