What are the diagnostic criteria for adult-onset Still's disease?

Diagnostic Criteria for Adult-Onset Still's Disease

Use the operational definitions from the 2024 EULAR/PReS guidelines to diagnose Adult-Onset Still's Disease (now simply called "Still's disease"), which prioritize clinical recognition over waiting for arthritis to develop, and always exclude alternative diagnoses before confirming the diagnosis. 1

Core Diagnostic Features

The 2024 EULAR/PReS guidelines provide operational definitions that should guide your diagnostic approach 11:

Four Key Clinical Elements:

1. Fever: Spiking pattern with temperature ≥39°C (102.2°F) for at least 7 days

   • The fever pattern is characteristically intermittent with spikes

2. Rash: Transient, typically coinciding with fever spikes

   • Preferentially involves the trunk
   • Classically salmon-pink erythematous, but urticarial patterns are also consistent with diagnosis
   • May be challenging to identify on darker skin tones

3. Musculoskeletal involvement: Arthralgia or myalgia

   • Critical point: Overt arthritis is supportive but NOT mandatory for diagnosis 11
   • Arthritis often appears later (median 1 month after onset, range 0 to several months)
   • Requiring arthritis causes unnecessary and potentially harmful diagnostic delays

4. High inflammatory markers:

   • Neutrophilic leukocytosis
   • Elevated ESR, CRP, and ferritin
   • Also typically see increased platelet count, fibrinogen, and D-dimers

Supporting 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 yet universally available but provide strong diagnostic support when accessible.

Classification Criteria as Diagnostic Aids

While not developed specifically for diagnosis, the Yamaguchi criteria have been validated in both pediatric and adult populations with high sensitivity (78.57% sensitivity, 87.14% accuracy in validation studies) 12. Importantly, the Yamaguchi criteria do not require arthritis for diagnosis, aligning with current understanding that arthritis may develop later 1.

Other criteria sets exist (Calabro, Cush, Reginato), with Calabro showing highest specificity (100%) but lower sensitivity 3. The Yamaguchi criteria offer the best balance for clinical use 2.

Specific Laboratory Thresholds with Higher Diagnostic Value:

• Leukocytes ≥10,000/mm³ (better threshold than ≥15,000)
• PMN ≥80% (critical point, with ≥85% having higher specificity)
• Serum ferritin ≥5-fold the upper normal limit (more specific than 2-fold elevation) 2

Essential Exclusion Process

You must carefully exclude alternative diagnoses before confirming Still's disease 11:

• Malignancies (especially lymphomas and leukemias)
• Infectious diseases
• Other immune-mediated inflammatory diseases
• Monogenic autoinflammatory disorders

The diagnosis remains one of exclusion combined with characteristic clinical features—no single test is pathognomonic 4.

Common Pitfalls to Avoid

1. Waiting for arthritis to develop: This delays diagnosis unnecessarily. Arthralgia alone with other features is sufficient 11

2. Missing the rash: It's transient and may only appear with fever spikes. Examine patients during febrile episodes

3. Undervaluing ferritin levels: Extremely elevated ferritin (≥5x upper limit) with low glycosylated fraction is highly suggestive 24

4. Failing to monitor for MAS: Macrophage activation syndrome is a life-threatening complication that must be detected promptly 11

Practical Diagnostic Algorithm

1. Identify the four core features (fever ≥39°C for ≥7 days, transient rash, arthralgia/myalgia, high inflammatory markers)
2. Measure ferritin (look for ≥5x elevation), IL-18, and S100 proteins if available
3. Systematically exclude infections, malignancies, and other rheumatologic conditions
4. Apply Yamaguchi criteria to support clinical diagnosis
5. Do NOT wait for arthritis to develop before initiating treatment

The 2024 guidelines represent a paradigm shift: Still's disease (formerly AOSD/sJIA) is now recognized as a single disease entity across all ages, with diagnosis based on operational definitions rather than rigid classification criteria 11.