How is adult-onset Still disease diagnosed?

How to Diagnose Adult-Onset Still's Disease

Diagnose adult-onset Still's disease using operational clinical criteria combined with laboratory findings, while systematically excluding alternative diagnoses—arthritis is NOT required for diagnosis. 1

Core Diagnostic Criteria

The 2024 EULAR/PReS guidelines provide specific operational definitions to facilitate rapid diagnosis 1:

Required Clinical Features

1. Spiking Fever: Temperature ≥39°C (102.2°F) for at least 7 days
2. Characteristic Rash: Transient, salmon-pink erythematous rash that typically coincides with fever spikes and preferentially involves the trunk (though urticarial variants may occur)
3. Musculoskeletal Involvement: Arthralgia or myalgia is sufficient—overt arthritis is supportive but NOT mandatory for diagnosis 1
4. Marked Inflammation: Neutrophilic leukocytosis, elevated ESR, CRP, and ferritin

Critical Laboratory Findings

• Neutrophilic leukocytosis with high neutrophil count
• Markedly elevated ferritin (often >1,000 ng/mL, can exceed 20,000 ng/mL) 2, 3
• Elevated CRP
• Additional supportive findings: thrombocytosis, elevated fibrinogen, elevated D-dimers 1

Enhanced Diagnostic Biomarkers

If available, measure serum IL-18 and/or S100 proteins (calprotectin), as marked elevation strongly supports the diagnosis 1. These biomarkers demonstrate high sensitivity and specificity for Still's disease, though validated cut-off values are still being established. They are currently used in tertiary centers to strengthen diagnostic confidence 1.

Essential Exclusion Process

Adult-onset Still's disease remains a diagnosis of exclusion—you must systematically rule out 1:

• Infections (bacterial, viral, fungal)
• Malignancies (lymphoma, leukemia, solid tumors)
• Other immune-mediated inflammatory diseases (systemic lupus erythematosus, vasculitis)
• Monogenic autoinflammatory disorders

This exclusion process is non-negotiable, as multiple conditions can mimic Still's disease 1, 4.

Classification Criteria as Diagnostic Aid

While not specifically designed for diagnosis, the Yamaguchi criteria have been validated in both pediatric and adult populations with high sensitivity and can help clinicians identify the disease 1. Importantly, the Yamaguchi criteria do not require arthritis for diagnosis, aligning with current understanding that arthritis often appears later (median 1 month after onset, range 0 to several months) 1.

Key Diagnostic Pitfalls to Avoid

Don't Wait for Arthritis

The most important pitfall is delaying diagnosis while waiting for arthritis to develop 1. The pathogenic mechanisms are similar whether arthritis is present or not, and requiring arthritis leads to unnecessary and potentially dangerous diagnostic delays. Arthralgia alone is sufficient if other criteria are met.

Watch for Diagnostic Confounders

• CPPD crystals may be found coincidentally and should not automatically redirect diagnosis away from Still's disease if the overall clinical picture fits 2
• Rash may be challenging to identify on darker skin 1
• Atypical presentations in older adults may include pulmonary manifestations 3

Monitor for Life-Threatening Complications

Once diagnosed, remain vigilant for macrophage activation syndrome (MAS), which can occur at any point—even during remission 1. MAS indicators include:

• Persistent fever with splenomegaly
• Rising or elevated ferritin with inappropriately low cell counts
• Abnormal liver function tests
• Elevated triglycerides
• Intravascular coagulation activation

Practical Diagnostic Algorithm

1. Identify the clinical syndrome: Spiking fever ≥39°C for ≥7 days + transient rash + arthralgia/myalgia + marked inflammation
2. Obtain key laboratory tests: CBC with differential, ESR, CRP, ferritin, comprehensive metabolic panel
3. Add supportive biomarkers if available: IL-18, S100 proteins (calprotectin)
4. Systematically exclude alternatives: Infectious workup, malignancy screening, autoimmune serologies, consider genetic testing if atypical features
5. Apply Yamaguchi criteria to support clinical diagnosis
6. Do NOT delay diagnosis waiting for arthritis to develop

The diagnosis is strengthened by the combination of characteristic clinical features, marked hyperferritinemia, and exclusion of mimicking conditions, with or without arthritis 1.