Diagnostic Criteria for Adult-Onset Still's Disease
Use the operational definitions from the 2024 EULAR/PReS guidelines to diagnose Still's disease, recognizing that the Yamaguchi criteria remain the most validated classification tool with 96.2% sensitivity, though diagnosis is ultimately clinical and requires exclusion of alternative conditions. 1
Core Operational Definitions for Diagnosis
The 2024 EULAR/PReS recommendations provide specific operational definitions that should guide your diagnostic approach 1:
Four Key Clinical Features:
Fever: Spiking temperature ≥39°C (102.2°F) for at least 7 days 1
Rash: Transient, often coinciding with fever spikes, preferentially involving the trunk; typically salmon-pink erythematous, though urticarial variants are consistent with diagnosis 1
Musculoskeletal involvement: Arthralgia or myalgia is usually present; importantly, overt arthritis is supportive but NOT mandatory for diagnosis and may appear later (median delay of 1 month, range 0 to several months) 1
High inflammation markers: Neutrophilic leukocytosis, elevated ESR, CRP, and ferritin 1
Critical Diagnostic Caveat
The 2024 guidelines strongly emphasize that requiring arthritis for diagnosis leads to unnecessary and potentially deleterious diagnostic delays, as the underlying pathogenic mechanisms are similar in patients with or without arthritis 1. This represents a paradigm shift from older thinking.
Yamaguchi Classification Criteria (Most Validated)
The Yamaguchi criteria demonstrate 96.2% sensitivity and 92.1% specificity, making them the most accurate tool in clinical practice 2. They have been validated in both systemic juvenile idiopathic arthritis and adult-onset Still's disease 1, 3.
Major Criteria:
- Fever ≥39°C for ≥1 week
- Arthralgia ≥2 weeks
- Typical rash (salmon-pink, evanescent)
- Leukocytosis ≥10,000/mm³ with ≥80% neutrophils 2, 3
Minor Criteria:
- Sore throat
- Lymphadenopathy and/or splenomegaly
- Liver dysfunction
- Negative rheumatoid factor
- Negative antinuclear antibody 2
Diagnosis requires ≥5 criteria including ≥2 major criteria 2
Supportive Biomarkers
Marked elevation of serum IL-18 and/or S100 proteins (e.g., calprotectin) strongly supports the diagnosis and should be measured if available 1. These are not required but add diagnostic confidence 1.
Additional supportive findings include:
- Serum ferritin ≥5-fold upper normal limit (critical threshold for diagnosis) 3
- Ferritin ≥2-fold upper normal has 90% sensitivity 3
- Elevated platelet count, fibrinogen, and D-dimers 1
Essential Exclusion Process
Alternative diagnoses must be carefully excluded, including malignancies, infectious diseases, other immune-mediated inflammatory diseases, and monogenic autoinflammatory disorders 1. This is critical because Still's disease is a diagnosis of exclusion, often identified while investigating fever of unknown origin 4.
Practical Diagnostic Algorithm
Identify the clinical tetrad: High spiking fever (≥39°C for ≥7 days), transient rash with fever spikes, arthralgia/myalgia (arthritis not required), and marked systemic inflammation 1
Obtain laboratory confirmation: Neutrophilic leukocytosis (≥10,000/mm³ with ≥80% PMN), elevated CRP/ESR, ferritin ≥5-fold upper normal, negative RF and ANA 3, 2
Measure IL-18 and S100 proteins if available to strengthen diagnostic confidence 1
Systematically exclude: Infections (especially endocarditis, tuberculosis), malignancies (lymphoma, leukemia), other rheumatologic diseases, and autoinflammatory syndromes 1, 4
Apply Yamaguchi criteria for classification certainty (≥5 criteria with ≥2 major) 2, 3
Important Clinical Pitfalls
- Do not wait for arthritis to develop before making the diagnosis—this delays treatment and worsens outcomes 1
- Rash may be challenging to identify on darker skin, requiring careful examination during fever spikes 1
- Remain vigilant for macrophage activation syndrome (MAS), the most severe life-threatening complication occurring in up to 23% of patients, which can present at onset, during treatment, or even during remission 1, 4
- The Reginato criteria have highest specificity (99.29%) but lower sensitivity, while Yamaguchi offers best balance with 78.57% sensitivity and 87.14% accuracy 3