The Challenge of Incidentally Detected Interstitial Lung Abnormalities

Aaron B. Holley, MD


January 27, 2023

Clinicians working within the US healthcare system order CTs; it's just what we do, and we do it a lot. This isn't necessarily bad, but an inevitable by-product is the pandemic of incidental findings. One underrecognized but frequent "incidentaloma" on CT is an interstitial lung abnormality (ILA). The Fleischner Society defines an ILA as honeycombing, traction bronchiectasis, parenchymal distortions, and reticular abnormalities that take up more than 5% of a particular lung zone in a patient without a clinical diagnosis of interstitial lung disease (ILD). In essence, ILAs are both a radiographic and a clinical diagnosis.

ILAs are common. Depending on population characteristics, ILAs occur at a prevalence of up to 10%. With the advent of lung cancer screening and advances in CT technology, we're now inundated with detailed images of lung parenchyma in older smokers who are at high risk for respiratory disease. The resulting opportunity for early identification of disease is as exciting as the risk for overdiagnosis, excessive testing, and unnecessary treatment is frightening. Early diagnosis remains critical for preventing irreversible respiratory disease. But as with any disease process, when we attempt to detect pathology before it has become apparent, the line between benign change and true abnormality is blurred.

Such is the challenge with ILAs. Past studies have shown an association between ILAs and morbidity and mortality, but considerable uncertainty persists over what the ILAs represent and how they should be managed. A recent study published in the American Journal of Respiratory and Critical Care Medicine (AJRCCM) helps provide some clarity. The authors used data from the COPDGene cohort to correlate ILAs with lung testing, and functional and respiratory outcomes. Consistent with other studies, they found that approximately 10% of the COPDGene patients that they examined had ILAs on CT and half of those met their criteria for "suspected ILD." Suspected ILD was defined radiographically (definite fibrosis) and on lung function testing (abnormal forced vital capacity [FVC] or diffusing capacity of the lungs for carbon monoxide [DLCO]). The patients with suspected ILD had worse clinical outcomes; being a Black individual, pack-years of smoking, and GOLD stage on spirometry were independently associated with suspected ILD.

This type of study is urgently needed. Given their high prevalence, we're in dire need of a valid model for risk stratifying ILAs. The authors of this study have moved us closer, but we've still got a long way to go. The AJRCCM study has significant limitations. First, although patients with previous documentation of ILD were excluded from COPDGene, no formal, multidisciplinary assessment was performed; therefore, some of the patients labeled as having ILA probably had diagnosable ILD. Their possible inclusion would falsely increase the prevalence of clinically important ILAs and exaggerate the relationship between ILAs and clinical outcomes.

The rhetorical gymnastics performed throughout the paper are necessary yet problematic. "Suspected ILD" is not a recognized diagnosis and the definition is therefore arbitrary. To the extent that "suspected ILD" requires an abnormality on spirometry or DLCO, one could argue it's the lung function changes and not the radiographic findings that are driving the differences. In fact, "suspected ILD" was defined by lung function more often than radiographic criteria (16% had definite fibrosis on CT, 57% had an abnormal FVC, and 67% had an abnormal DLCO). Patients with ILAs without suspected ILD had outcomes that weren't statistically different from those with no ILAs at all, implying that the lung testing and not the ILA is the better discriminator. Regardless, this leads us back to where we started before this paper was published: ILAs require lung function testing and referral to a pulmonologist for proper risk stratification. An accompanying editorial highlights these and other limitations.

One particular problem that isn't addressed by the authors or the editorial is their findings on race. The authors concluded that Black persons with ILAs are more likely to have "suspected ILD." However, their definition suffers from an insidious form of incorporation bias generated by the way they handled their DLCO reference values. The Global Lung Function Initiative equations they used were derived exclusively from White persons. In accordance with the recent American Thoracic Society/European Respiratory Society (ATS/ERS) statement on lung testing, the authors did not apply a fixed correction factor to adjust for race. Without such an adjustment, Black persons would be biased toward having lower percent predicted values for DLCO. In short, self-identified Black individuals would be more likely to have a predicted DLCO of less than 70% and to therefore meet criteria for "suspected ILD." The resulting effects on biologic plausibility, causal inference, and the strength of the relationship between "suspected ILD" and clinical outcomes will vary by whether the association between race and lung function is considered a product of inherent biologic variability or a result of external (socioeconomic and environmental) effects.

In summary, ILAs remain a challenge for radiologists, primary care providers, pulmonologists, and anyone else who orders a CT of the lungs. Despite its limitations, I believe the recently published AJRCCM paper pushes us forward conceptually. Perhaps its most important contribution is showing that 50% of ILAs are clinically insignificant by definition. This offers further reassurance that a subset of ILAs can be dismissed. Now, all we need is an easy, cost-effective, and efficient way to identify this subset.

Aaron B. Holley, MD, is a professor of medicine at Uniformed Services University in Bethesda, Maryland, and a pulmonary/sleep and critical care medicine physician at MedStar Washington Hospital Center in Washington, DC. He covers a wide range of topics in pulmonary, critical care, and sleep medicine.

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