Background & Nuances

♫ Not atelectasis, not yet an ILD ♫

Interstitial lung abnormality has been defined by a 2025 ATS Clinical Statement, which built on a prior Fleischner Society Position Paper. In addition to providing a rich definition for ILA, the Clinical Statement made several recommendations. Key points below.

Specifically, the committee defined an ILA as nondependent, bilateral, parenchymal abnormalities detected on CT, including ground-glass or reticular abnormalities, lung distortion, traction bronchiectasis, and/or honeycombing, involving >5% of a lung zone by visual estimate but without meeting the criteria for ILD.

• Nondependent refers to parts of the lung that are less influenced by gravity during scan acquisition; this may include abnormalities that are present in dependent locations on supine imaging but persist on prone imaging; this helps exclude atelectasis, which occurs in dependent regions when supine and can mimic reticulation (but should disappear when a patient is proned)

• Lung zone refers to the upper, middle, and lower lung zones, which are demarcated by the levels of the inferior aortic arch and right inferior pulmonary vein.

Why bilateral?

The rationale for requiring bilateral findings is that for patients with low-risk for ILD, there are causes of pulmonary fibrosis which are generally non-progressive and unilateral, such as post-infectious or aspiration-related abnormalities.

So… when does the ILA become an ILD?

Meeting any of these criteria transitions the finding from an ILA to ILD.

• Symptoms: If a qualified clinician attributes any symptoms to the abnormal finding, it is no longer an ILA (i.e., ILA by definition is asymptomatic)

• Physiology:

  • FVC, TLC, or DLCO below LLN, which is thought to be due to the imaging finding (i.e., not due to co-morbid disease, high BMI, etc.)

  • Meets criteria for PPF, and meeting criteria is felt to be due to the imaging finding

• Imaging:

  • Fibrotic abnormalities (reticulation with traction bronchiectasis and/or honeycombing) in >5% of the total lung volume by visual estimate

  • Progressive fibrotic abnormality

  • Presence of major fibrotic imaging pattern (UIP/probable UIP, fibrotic HP, fibrotic NSIP)

• Pathology:

  • Presence of a major fibrotic ILD histologic pattern (i.e., UIP/probable UIP, fibrotic HP, or fibrotic NSIP)

Should anyone be screened for ILAs?

Per the 2025 ATS Clinical Statement, yes! The committee suggested screening with CT Chest for the following groups:

• Adults with SARDs known to be associated with an increased risk of ILD (RA, SSc, IIM, MCTD, SjD)

• Adults >50 years old, with at least two genetically related first- or second-degree relatives with fibrotic disease (i.e., who meet criteria for familial pulmonary fibrosis)

What should be done with ILA?

Once an CT Chest demonstrates ILA, the committee recommended some next steps (if not already performed):

• Assess explicitly for cough and dyspnea on exertion (i.e., which, if attributable to the finding, would make it not an ILA):

• Obtain a full PFT, including evaluation of FVC, TLC, and DLCO

• Perform a follow up chest CT 2-3 years after the baseline scan; a shorter interval (e.g., 12 months) may be appropriate in some clinical contexts