Pulmonary Function Test Reference Equations May Affect Classification of Restrictive Lung Disease Severity in Systemic Sclerosis.

Background: Interstitial lung disease (ILD) is a significant cause of morbidity and mortality in systemic sclerosis (SSc), particularly among Black patients. Pulmonary function tests (PFTs) are critical to screen for and monitor SSc-ILD. We examined whether race-specific and race-neutral PFT reference equations impact classification of restrictive lung disease (RLD) severity in Black and White patients with SSc.

Methods: Baseline percent predicted forced vital capacity (ppFVC) was calculated for self-identified Black (N=641) and White (N=2909) patients in the Johns Hopkins Scleroderma Center Research Registry using race-specific (Global Lung Initiative 2012 [GLI 2012], National Health and Nutrition Examination Survey III [NHANES]) and race-neutral (GLI Global) equations. The percentage of Black and White individuals who switched RLD severity categories (normal (ppFVC≥80%); mild (70≤ppFVC<80%), moderate (60%≤ppFVC<70%), severe (50%≤ppFVC<60%) or very severe (ppFVC<50%)) when using race-neutral versus race-specific equations was calculated. The percentage who would meet typical ppFVC thresholds for immunosuppression, clinical trial eligibility, and lung transplant referral was compared.

Results: Black individuals had lower absolute FVC values than White individuals. 47% (n=303) of Black individuals were reclassified as having more severe RLD and 17% (n=487) of White individuals were reclassified as having less severe RLD when using the GLI Global versus GLI 2012 equations. Statistically greater proportions of Black individuals met ppFVC thresholds for immunosuppression, clinical trial eligibility and lung transplant referral with race-neutral versus race-specific equations.

Conclusions: The use of race-specific PFT reference equations may result in systematic misclassification of ILD severity with potential impact on healthcare delivery and clinical trial eligibility.