Do Non-Invasive Fibrosis Indices Reflect Interstitial Lung Disease and Pulmonary Function in Systemic Sclerosis?

Background/Objectives: Interstitial lung disease (ILD) is a major manifestation of systemic sclerosis (SSc). The Fibrosis-4 index (FIB-4) and aspartate aminotransferase-to-platelet ratio index (APRI) are indirect hepatic fibrosis indices of uncertain pulmonary relevance. We compared these indices by documented ILD status and explored associations with pulmonary function and historical forced vital capacity (FVC) changes. Methods: This retrospective observational study included 102 patients with SSc (61 with ILD, 41 without ILD). Indices were calculated from the last eligible laboratory assessment; ILD was classified using available high-resolution computed tomography records. Patients with relevant hepatic and laboratory confounders were excluded. Pulmonary associations were explored within SSc-ILD, including a sensitivity analysis restricted to ≤12 months between laboratory and follow-up pulmonary function testing. Results: Neither index differed significantly by ILD status (FIB-4, p = 0.91; APRI, p = 0.06). In logistic models adjusted for age and diffuse cutaneous phenotype, FIB-4 showed a borderline inverse association with documented ILD status only after adjustment for age and and diffuse cutaneous phenotype (adjusted odds ratio 0.247, 95% confidence interval 0.062–0.988; p = 0.048), whereas APRI did not (p = 0.064). Within SSc-ILD, neither index correlated significantly with FVC % predicted, diffusing capacity for carbon monoxide (DLCO) % predicted, forced expiratory volume in 1 s (FEV1) % predicted, or historical FVC changes. FIB-4 correlated inversely with absolute FVC (ρ = −0.53, p < 0.001), but this association was not significant after age and sex adjustment (p = 0.40). The absolute-FVC correlation persisted within the ≤12-month subgroup. Conclusions: Findings do not consistently support these indices as stand-alone indicators of SSc pulmonary involvement; prospective validation is needed.