Navigating the Biomarker Landscape in Still's Disease and Macrophage Activation Syndrome: Toward Precision Diagnosis and Management

Still's disease occupies a distinctive nexus between autoinflammatory and autoimmune biology, presenting with systemic inflammation, quotidian fever, evanescent rash, and arthritis. Macrophage activation syndrome (MAS), a fulminant cytokine storm phenotype, remains its most feared complication 1. Although the immunopathologic complexity of Still's disease is increasingly recognized, the translation of biomarker research into diagnostic precision, phenotypic stratification, and therapeutic decision-making continues to fall short of clinical needs. This gap has become more evident as accumulating data demonstrate the discriminative and prognostic utility of interleukin-18 (IL-18), CXCL9, S100 family proteins, and glycosylated ferritin in defining disease activity and anticipating the risk of MAS 2-5. The purpose of this editorial is to conceptualize emerging biomarkers as dynamic biological readouts and risk stratification tools, and to identify the key methodological and translational challenges that must be addressed before these biomarkers can be integrated into routine practice. IL-18 has emerged as a cornerstone biomarker in Still's disease 6, 7. Unlike IL-1β, whose serum detection is often unreliable, and IL-6, which frequently correlates with C-reactive protein (CRP) and may offer limited incremental value in routine clinical settings, IL-18 exhibits distinct diagnostic specificity and prognostic utility 8. Serum IL-18 concentrations are significantly higher in patients with active systemic juvenile idiopathic arthritis (sJIA) or adult-onset Still's disease (AOSD), often exceeding 5000 pg/mL, and may predict MAS development and resistance to therapy 6, 9. Incorporating elevation of IL-18 into International League of Associations for Rheumatology (ILAR) and Pediatric Rheumatology International Trials Organization (PRINTO) criteria could significantly enhance diagnostic accuracy 4. However, it is critical to emphasize that these specific thresholds are highly dependent on the assay platform, calibration standards, and pre-analytical conditions. Therefore, inter-laboratory harmonization and rigorous local validation are required before these numerical cut-offs can be applied broadly in clinical practice. In addition, assays typically quantify total IL-18, which encompasses both free IL-18 and the fraction bound to IL-18 binding protein (IL-18BP). Emerging evidence suggests that free IL-18 may better reflect biologically active inflammation and correlate more closely with disease activity 6. In clinical settings where free IL-18 measurement is unavailable, clinicians should interpret total IL-18 levels in conjunction with downstream pathway markers. Specifically, the simultaneous elevation of total IL-18 and CXCL9 serves as a pragmatic proxy for bioavailable IL-18 activity and active systemic inflammation. Conversely, isolated total IL-18 elevation without a corresponding CXCL9 response may suggest the buffering effect of IL-18BP, necessitating a more nuanced assessment of clinical disease activity. Overall, the role of IL-18 as a bridge between innate immunity and interferon-γ (IFN-γ) production positions it as a critical biomarker for both Still's disease and MAS 3, 6. Nevertheless, IL-18-driven inflammation represents only one dominant axis within a broader cytokine network, and its interpretation must be integrated with parallel inflammatory pathways, treatment context, and clinical evolution. Other biomarkers enhance diagnostic certainty and disease monitoring. Ferritin, although not disease-specific, remains a quintessential indicator of systemic hyperinflammation in Still's disease and MAS, with concentrations frequently rising into the high-thousand to multi-ten-thousand ng/mL range. Glycosylated ferritin may offer added specificity, assisting in the differentiation of Still's disease from key mimics, including systemic infections, malignancies (such as lymphoma), and other systemic inflammatory disorders, though it remains underutilized in many clinical settings 10. S100 proteins (S100A8/A9 and S100A12) are also elevated in Still's disease and mirror innate immune activation. Their correlation with disease activity and flare risk makes them valuable assessment adjuncts 5. Meanwhile, the chemokine CXCL9, induced by IFN-γ, provides a more reliable proxy of IFN-γ tissue activity than serum IFN-γ levels themselves 10. Recent evidence has identified the neutrophil CD64 (nCD64) index as an early predictive biomarker for MAS in patients with AOSD. Reflecting IFN-γ and G-CSF-mediated activation, a nCD64 index threshold of > 1.81 demonstrates high sensitivity and specificity in distinguishing MAS from active AOSD without MAS, offering a crucial window for early therapeutic intervention 11. A synthesized appraisal of the current biomarker landscape, integrating mechanistic insights with diagnostic applicability and known limitations, is presented in Table 1. The convergence of IL-18, CXCL9, and S100 proteins creates a promising triad for clinical application. Yet, variability in assay methods and lack of standardized cut-offs limit widespread implementation. While traditional hematologic and biochemical markers, such as soluble IL-2 receptor (sIL-2R), triglycerides, fibrinogen, D-dimer, NK cell function, and soluble CD163 (sCD163), remain foundational in MAS clinical practice 12-14, integrating these emerging biomarkers into standardized clinical frameworks could significantly expand the diagnostic and monitoring toolkit, facilitating more precise phenotyping and personalized management of Still's disease and MAS. High sensitivity and specificity for Still's disease. Markedly elevated in active sJIA/AOSD. Predicts MAS risk and refractory phenotypes. Clinical assays predominantly measure total IL-18, including both free cytokine and IL-18 bound to IL-18BP. Free IL-18 assays are not standardized. Reflect systemic disease activity. Correlate with flare propensity. Useful adjuncts for early inflammatory assessment. Alerts to hyperinflammation and MAS. Levels often rise into the high-thousand to multi-ten-thousand ng/mL range in severe disease. Non-specific. Also elevated in infection, malignancy, and liver disease. IFN-γ-inducible chemokine. Downstream surrogate of IFN-γ activity. Tracks MAS activity and therapeutic response. More reliable indicator of IFN-γ bioactivity than serum IFN-γ. Underutilized. Limited availability across laboratories. In MAS, IFN-γ plays a pivotal pathogenic role. Emapalumab, a monoclonal antibody targeting IFN-γ, demonstrated efficacy in refractory MAS with rapid normalization of CXCL9 levels and ferritin, highlighting its value as both a therapeutic agent and biomarker validation tool 10. Interestingly, patients whose MAS was controlled with emapalumab but had persistent sJIA activity despite suppressed CXCL9 levels underscore the need to distinguish between Still's disease and MAS as overlapping but biologically distinct syndromes 10. Beyond IFN-γ blockade, clinicians must recognize that other targeted therapies can profoundly alter conventional inflammatory indices, complicating longitudinal monitoring. For instance, IL-6 pathway blockades (e.g., tocilizumab) pharmacologically suppress hepatic CRP synthesis, while IL-1 inhibition and glucocorticoids can mask clinical fever and neutrophilic leukocytosis. In these scenarios, routine markers may fail to reflect subclinical disease activity or emerging MAS, necessitating a greater reliance on CXCL9 and IL-18 kinetics to accurately assess the underlying pathogenic drive and guide therapeutic adjustments. To bridge the gap between biomarker research and clinical implementation, we propose a conceptual framework for biomarker-informed risk stratification and monitoring in Still's disease and macrophage activation syndrome (Figure 1). While cytokine-driven therapeutic algorithms currently lack robust randomized controlled trial (RCT)-level validation, the proposed framework operationalizes biomarkers into clinically interpretable signals that support risk stratification and monitoring. However, advancing the clinical utility of emerging biomarkers in Still's disease and MAS will require coordinated progress across several domains 15. Establishing standardized assay methodologies for IL-18, CXCL9, and S100 proteins remains a foundational priority, as current variability in analytic platforms and threshold definitions hampers cross-study comparability and limits the development of robust diagnostic algorithms. Clarifying the relative clinical relevance of free versus total IL-18 is similarly essential, particularly given evidence that free IL-18 may more accurately reflect biologically active inflammation. As precision-based approaches continue to evolve, biomarker-informed therapeutic strategies merit systematic evaluation. Prospective studies examining IL-18 concentrations or CXCL9 kinetics as triggers for treatment initiation, escalation, or de-escalation could enable earlier recognition of MAS and facilitate more individualized care. In particular, elevation of IL-18 and CXCL9 may identify an IL-18/IFN-γ-driven endotype associated with increased MAS risk and subclinical cytokine activation. Such a profile may inform therapeutic decision-making, particularly by identifying patients who may require earlier treatment escalation or, in selected cases, consideration of IFN-γ-targeted approaches. In parallel, multivariable MAS prediction models integrating these biomarkers may enhance risk stratification across pediatric and adult populations. Longitudinal studies will also be crucial to define biomarker trajectories during remission and to determine whether persistent abnormalities reflect subclinical inflammation or predict relapse. Finally, expanding validation efforts from sJIA to AOSD remains imperative. Biomarker thresholds and predictive values derived from pediatric cohorts may not directly translate to adults due to the more complex comorbidity spectrum in older patients, the higher prevalence of malignancy and infection mimics (e.g., solid tumors or chronic viral infections), and age-related variations in the baseline inflammatory milieu 13-15. To bridge this gap and ensure external validity, future research must adopt rigorous design elements, including the use of harmonized assay platforms, clear stratification by age group, and the implementation of standardized clinical endpoints for both disease activity and MAS. Still's disease and MAS exemplify cytokine-mediated autoinflammatory disorders in which innate immune dysregulation drives both disease expression and life-threatening escalation. An expanding body of evidence positions IL-18, CXCL9, S100 family proteins, and glycosylated ferritin as key components of an emerging biomarker framework with relevance for diagnosis, phenotypic delineation, therapeutic monitoring, and risk stratification. Despite this progress, significant barriers remain, including the absence of assay standardization, limited availability of free IL-18 measurement, and uncertain threshold definitions. Furthermore, practical implementation is often hindered by prolonged turnaround times, high diagnostic costs, and restricted accessibility in many clinical settings, factors that must be addressed to ensure the effective real-world translation of these biomarkers into actionable tools. Addressing these gaps will be essential for determining whether these biomarker-informed strategies ultimately improve the timeliness, precision, and effectiveness of care for patients with Still's disease and MAS. I-Chang Lai: conceptualization, writing – original draft, visualization. Chen-Ching Wu: conceptualization, writing – review and editing. Pei-Fen Liao: writing – review and editing, supervision, management. The authors have nothing to report. The authors have nothing to report. The authors have nothing to report. The authors declare no conflicts of interest. There are no new data associated with this article.