Inflammatory biomarkers in MAS
CXCL9: making IFNγ measurable
Interferon gamma (IFNγ) plays a central role in driving hyperinflammation in macrophage activation syndrome (MAS). However, IFNγ itself can be difficult to measure reliably in clinical practice.1
CXCL9 is a downstream chemokine induced almost exclusively by IFNγ, making it a useful surrogate marker of IFNγ activity. By providing a stable and measurable signal of IFNγ-driven immune activation in the blood, CXCL9 helps translate underlying disease biology into clinically meaningful information.1-3
IFNγ pathway activation → CXCL9 production → Clinical insight
CXCL9: A more biologically precise marker
Traditional inflammatory markers such as ferritin, CRP, and LDH reflect broad immune activation and can increase across a wide range of conditions. They may indicate general inflammation, infection, or tissue damage, but they have limited ability to distinguish underlying inflammatory drivers.4
CXCL9 adds a more biologically precise signal of IFNγ-driven pathology to existing clinical assessments.1
Differentiating MAS from disease flares
Distinguishing MAS from active Still’s disease flares can be challenging because many clinical features overlap and patients can already be experiencing systemic inflammation. CXCL9 testing can help reveal deeper insights into their disease status.3
Differentiation: CXCL9 levels are significantly higher in MAS compared to Still’s disease flares, reflecting a distinct IFNγ-driven inflammatory profile.3
Early identification: CXCL9 levels increase with emerging hyperinflammation and may help differentiate active disease from a transition to MAS.3
Clinical relevance: Testing supports deeper evaluation when clinical features overlap and early recognition is critical.3
Monitoring disease activity and hyperinflammation
Serial monitoring of CXCL9 may provide a dynamic view of inflammatory status and help clinicians assess changes in IFNγ-driven hyperinflammation over time.2
In patients with MAS, CXCL9 levels and their changes over time have been consistently associated with disease activity, severity, and clinical outcomes.2
However, there is currently no universally accepted CXCL9 cutoff for diagnosis, risk stratification, or management guidance. Results should always be interpreted within the broader clinical context and alongside standard laboratory markers.2
CXCL9 can help identify high-risk patients early by reflecting the intensity and trajectory of IFNγ-driven hyperinflammation.2
A retrospective study:
CXCL9 as a novel prognostic marker to identify high-risk adults with hemophagocytic lymphohistiocytosis
A retrospective, multicenter chart review was conducted to broadly evaluate potential clinical biomarkers, including CXCL9, to determine which combination of markers provides the greatest prognostic utility in a large cohort of individuals undergoing clinical HLH evaluation.
CXCL9 emerged as the only optimal predictor of inpatient mortality among all markers assessed, with risk increasing as CXCL9 levels rose.2
Key findings for clinical practice2
The authors reported that CXCL9, a surrogate marker for IFNγ, served as a novel and consistent clinical laboratory marker in the identification and prognosis of adults presenting with HLH.*
Elevated CXCL9 levels were associated with early mortality in HLH+ patients
Strength of association and mortality risk
In patients with HLH, CXCL9 emerged as the only optimal prognostic marker of inpatient mortality across all evaluated labs, with elevated levels significantly associated with early mortality.2
Increasing risk
Higher baseline CXCL9 levels were associated with progressively higher mortality risk, further supporting the strength of its prognostic utility.2
Decreased levels in survivors
Among patients with serial measurements, significant decreases in CXCL9 levels were observed more frequently in 90-day survivors.2
Study limitations: Retrospective design may be impacted by unrecognized confounders. Restricted to patients ≥15 years, limiting generalizability to younger populations. Variability in HLH recognition timing across patients and centers may introduce selection bias. Sensitivity analyses were conducted to mitigate bias.
Inclusion criteria: Hospitalized patients ≥15 years undergoing HLH evaluation with ≥1 clinically validated CXCL9 test from a Clinical Laboratory Improvement Amendments (CLIA)-certified lab were included. Patients were excluded if CXCL9 testing occurred after initiation of HLH-directed therapy.
Patient characteristics: 171 adult patients were included and categorized as HLH- or HLH+. HLH+ was defined as meeting ≥4/8 HLH-2004 criteria and/or an HScore ≥169. Of 126 HLH+ patients, 73 met ≥5/8 HLH-2004 criteria.*
*Patients in the HLH+ cohort met at least 4 out of 8 HLH-2004 criteria and/or had an HScore ≥169. A cutoff of 4 out of 8 HLH-2004 criteria was used to capture a broad population including early HLH presentations and/or similar IFNγ-driven hyperinflammatory syndromes of which HLH represents the most severe manifestation. This modified cutoff also helped control for bone marrow biopsies not being routinely performed and a recent study suggested similar sensitivity when compared to the original cutoff.
CXCL9 testing sites
cincinnatichildrens/Tests/723501
cxcl9-level
This is not an exhaustive list of labs offering CXCL9 testing, as additional labs continue to build new capabilities. Please check for the availability of this test within your own institution prior to contacting these sites.
Additional inflammatory biomarkers that may support MAS evaluation
Some inflammatory biomarkers can be measured to evaluate key disease pathways that can lead to cytokine storm syndromes, and their specificity may be helpful for identifying MAS.
These include:
Biomarker | Context |
CRP | CRP, ESR, and LDH are commonly available tests that may help assess the extent of hyperinflammation in patients with suspected MAS.5 Persistently high CRP levels and increasing D-dimers, in combination with a fall in ESR and platelet count may suggest early stages of MAS development in febrile patients with underlying rheumatic disease. LDH is a general marker of cellular death or injury that is commonly elevated in patients with MAS.4-6 |
Ferritin | Ferritin is an intracellular iron-storage protein that is widely recognized as a marker of systemic inflammation. Since hyperferritinemia is a common feature of MAS, ferritin is a useful and readily available test for the initial screening of patients with suspected disease.7,8 |
sCD25/sIL-2Rα | sCD25, also known as sIL-2Rα, is an inflammatory marker of T-cell activation that can be helpful for MAS diagnosis and as an indicator of treatment response.1,6 |
IL-18 | IL-18 testing can help measure inflammasome activation, which can lead to the development of MAS in certain contexts. Elevated IL-18 can be observed in patients with Still’s disease, but significant elevations above a patient’s baseline may suggest development of MAS.9,10 |
Neopterin | Neopterin and CD163 testing can indicate the activation and inflammatory status of macrophages, which are essential for MAS pathology.6,9,11 |
ESR=erythrocyte sedimentation rate; sIL-2Rα=soluble interleukin 2 receptor alpha.
Some patients may experience recurrent MAS episodes. However, there is limited information about biomarkers or clinical factors that can predict this in patients.12
Monitoring trending lab values over time can provide important insights for assessing patients with MAS.11
References: 1. De Benedetti F, Prencipe G, Bracaglia C, Marasco E, Grom AA. Targeting interferon-γ in hyperinflammation: opportunities and challenges. Nat Rev Rheumatol. 2021;17(11):678-691. doi:10.1038/s41584-021-00694-z 2. Rocco JM, Oved JH, Patel RJ, et al. CXCL9 as a novel prognostic marker to identify high-risk adults with hemophagocytic lymphohistiocytosis. Blood. 2026;147(9):960-972. doi:10.1182/blood.2025030976 3. De Matteis A, Pardeo M, Caiello I, et al. Traditional laboratory parameters and IFNγ-related biomarkers in the diagnosis and management of MAS. Semin Arthritis Rheum. 2025;74:152812. doi:10.1016/j.semarthrit.2025.152812 4. Shakoory B, Geerlinks A, Wilejto M, et al. The 2022 EULAR/ACR points to consider at the early stages of diagnosis and management of suspected haemophagocytic lymphohistiocytosis/macrophage activation syndrome (HLH/MAS). Arthritis Rheumatol. 2023;75(10):1714-1732. doi:10.1002/art.42636 5. Minoia F, Davì S, Horne A, et al. Clinical features, treatment, and outcome of macrophage activation syndrome complicating systemic juvenile idiopathic arthritis: a multinational, multicenter study of 362 patients. Arthritis Rheumatol. 2014;66(11):3160-3169. doi:10.1002/art.38802 6. Grom AA, Horne A, De Benedetti F. Macrophage activation syndrome in the era of biologic therapy. Nat Rev Rheumatol. 2016;12(5):259-268. doi:10.1038/nrrheum.2015.179 7. Recalcati S, Invernizzi P, Arosio P, Cairo G. New functions for an iron storage protein: the role of ferritin in immunity and autoimmunity. J Autoimmun. 2008;30(1-2):84-89. doi:10.1016/j.jaut.2007.11.003 8. Price B, Lines J, Lewis D, Holland N. Haemophagocytic lymphohistiocytosis: a fulminant syndrome associated with multiorgan failure and high mortality that frequently masquerades as sepsis and shock. S Afr Med J. 2014;104(6):401-406. doi:10.7196/samj.7810 9. Schulert GS, Grom AA. Macrophage activation syndrome and cytokine-directed therapies. Best Pract Res Clin Rheumatol. 2014;28(2):277-292. doi:10.1016/j.berh.2014.03.002 10. Crayne C, Cron RQ. Pediatric macrophage activation syndrome, recognizing the tip of the iceberg. Eur J Rheumatol. 2020;7(Suppl1):S13-S20. doi:10.5152/eurjrheum.2019.19150 11. Lerkvaleekul B, Vilaiyuk S. Macrophage activation syndrome: early diagnosis is key. Open Access Rheumatol. 2018;10:117-128. doi:10.2147/OARRR.S151013 12. Erkens R, Esteban Y, Towe C, Schulert G, Vastert S. Pathogenesis and treatment of refractory disease courses in systemic juvenile idiopathic arthritis: refractory arthritis, recurrent macrophage activation syndrome and chronic lung disease. Rheum Dis Clin North Am. 2021;47(4):585-606. doi:10.1016/j.rdc.2021.06.003
