SSc-ILD Severity and Progression Still Lack a Shared Clinical Standard
Systemic sclerosis-associated interstitial lung disease (SSc-ILD) remains difficult to classify because its trajectory rarely follows a single, predictable path. Some patients decline quickly, others remain stable for long periods, and many fall somewhere in between, which makes borrowed definitions from idiopathic pulmonary fibrosis a poor fit for everyday rheumatology and pulmonary care.
A systematic literature review published in RMD Open catalogued how investigators have defined severity, progression, and outcome in SSc-ILD across the published literature. The central finding was blunt: the field is still working with a patchwork of criteria rather than a consensus framework that can reliably separate severe disease from milder forms or identify true progression in a reproducible way.
What the Review Examined Across More Than 9,000 Records
The review followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidance and searched Medline, Embase, Web of Science, and the Cochrane Library through 1 August 2023. Investigators screened 9,054 papers and ultimately included 342 studies. That volume alone reflects how active the field has become, but it also reveals a deeper problem: large numbers of studies have not converged on common language or common thresholds.
Across the included literature, the most frequently used tools for defining SSc-ILD progression and severity were combined changes in carbon monoxide diffusing capacity (DLCO) and forced vital capacity (FVC), isolated changes in FVC or DLCO, extension on high-resolution CT (HRCT), and composite algorithms that mixed pulmonary function tests, clinical signs, and HRCT findings. Mortality was the most commonly reported long-term endpoint, whether all-cause or ILD-related.
A single physiology-driven metric has not been enough.
Why FVC, DLCO, and HRCT Get Used So Often
The repeated reliance on FVC and DLCO is understandable. FVC captures restrictive ventilatory loss, while DLCO reflects gas-transfer impairment that may worsen as the interstitium thickens and pulmonary vascular involvement emerges. HRCT adds an anatomic layer, showing the extent and pattern of fibrotic change directly rather than inferring injury through spirometry alone. In a disease like SSc-ILD, where fibrosis, vasculopathy, and inflammatory injury can evolve at different speeds, clinicians naturally look to a combined physiologic and imaging approach.
Still, the review makes clear that these tools are being assembled in many different ways. Some studies emphasized absolute changes, others relative changes, and others mixed pulmonary function trends with radiographic extent or clinical judgment. That variety creates a serious problem for clinical trials and routine practice: two studies can both claim to evaluate progression while using definitions that are not interchangeable.
Heterogeneity Has Direct Consequences for Trials and Bedside Decisions
The absence of a standard definition affects more than academic comparison. It changes who gets labeled severe, who qualifies for treatment escalation, and how quickly a patient is considered to have failed therapy. In a disorder with variable tempo, a threshold that is too permissive risks overcalling progression, while one that is too strict may miss patients who are quietly worsening until reserve is already lost.
This matters especially in SSc-ILD because the disease is not continuously progressive in the same way as idiopathic pulmonary fibrosis. Some patients experience early decline followed by stabilization, and others may show more gradual deterioration. A definition built around a fixed slope of decline can misread that biology. Composite algorithms may be more forgiving of the disease’s complexity, but they are only useful if the field agrees on how to build them and what degree of change should trigger action.
Mortality Remains the Most Common Long-Term Endpoint
The review found that mortality was the most frequently reported long-term outcome, either as all-cause mortality or ILD-related mortality. That is not surprising. When the lung disease advances far enough, the endpoint is no longer only about spirometric drift or CT extension; it becomes about survival, respiratory failure, and the broader burden of systemic sclerosis.
Yet mortality alone does not solve the classification problem. It is too late an event for many clinical decisions and too blunt an outcome for measuring whether a therapy slows the disease at a stage where intervention still matters. A patient can worsen substantially long before mortality becomes visible in the data. For clinical trials, that means progression definitions must function as surrogate outcomes that are sensitive enough to detect meaningful change without being so noisy that they blur true treatment effects.
The Need for a Consensus Definition of Severe SSc-ILD
The most important conclusion from this review is not simply that definitions vary. It is that the field lacks a standardized, disease-specific framework for severe SSc-ILD and for progression that can be used consistently in both trials and clinical care. The authors argue for a consensus definition that aligns severity with a disease-specific progression construct, creating a surrogate outcome that can support trial design and bedside risk stratification.
That call is clinically sensible. SSc-ILD is not just another fibrotic lung disease with an autoimmune label attached to it. The lung involvement sits inside a broader systemic disorder in which skin thickening, vascular disease, and internal organ complications can all influence outcomes. A severity definition that ignores that context will continue to produce inconsistent enrollment criteria, inconsistent endpoints, and inconsistent treatment thresholds.
The review also exposes a practical research burden: future studies need not only better biomarkers and cleaner imaging metrics, but also agreement on how much change matters. Without that, one trial’s progression may be another trial’s noise.
What This Means for Rheumatology and Pulmonology Practice
For clinicians managing SSc-ILD, the findings support a cautious approach when interpreting serial FVC, DLCO, and HRCT changes in isolation. A small decline in one metric may be important, but its meaning depends on baseline severity, the pattern of change over time, and the broader clinical picture. Shortness of breath, oxygen need, exercise tolerance, radiographic progression, and pulmonary function trends all belong in the same conversation.
The review does not provide a new diagnostic threshold or a ready-made severity score. It does something more uncomfortable and more useful: it shows that the field is still speaking several different measurement languages at once. Until those languages are unified, comparisons across studies will remain difficult, and clinical trial endpoints will continue to vary in ways that limit interpretation.
Any future consensus will need to be disease-specific, reproducible, and sensitive to the non-linear course of SSc-ILD. Without that, progression will remain a moving target rather than a dependable clinical endpoint.