Diagnosis: Systemic Juvenile Idiopathic Arthritis (Still's Disease)
The diagnosis is C - Systemic JIA (now termed Still's disease), based on the classic tetrad of daily high spiking fever, salmon-colored rash, arthritis/arthralgia, and markedly elevated inflammatory markers (ESR and CRP). 1
Clinical Reasoning
Key Diagnostic Features Present
The patient demonstrates all four cardinal operational definitions for Still's disease as defined by the 2024 EULAR/PReS guidelines 1:
- Fever: Daily high-spiking fever, typically ≥39°C (102.2°F) for at least 7 days 1
- Rash: Salmon-colored rash that is transient and often coincides with fever spikes, preferentially involving the trunk 1
- Musculoskeletal involvement: Arthritis and arthralgia are present (notably, arthritis is supportive but not mandatory for diagnosis) 1
- High inflammation: Elevated ESR and CRP, typically with neutrophilic leukocytosis 1, 2
Why Not the Other Options
Kawasaki disease (Option A) is excluded because:
- Kawasaki disease primarily affects children under 5 years old and presents with conjunctivitis, mucositis (strawberry tongue, cracked lips), cervical lymphadenopathy, and extremity changes (edema, erythema, desquamation) 3
- The salmon-colored evanescent rash is characteristic of Still's disease, not Kawasaki disease 1
- Kawasaki disease does not typically present with prominent arthritis as a primary feature 3
Acute rheumatic fever (Option B) is excluded because:
- Rheumatic fever requires evidence of preceding streptococcal infection (elevated ASO titers, positive throat culture) 3
- The rash in rheumatic fever is erythema marginatum (not salmon-colored), and Sydenham chorea or carditis are typical features 3
- The daily quotidian fever pattern is characteristic of Still's disease, not rheumatic fever 1
Diagnostic Approach
Confirm the Diagnosis
According to the 2024 EULAR/PReS guidelines, the following should be obtained 1:
- Complete blood count: Look for neutrophilic leukocytosis, thrombocytosis 1, 2
- Serum ferritin: Marked hyperferritinemia (>2,000 ng/mL) is highly supportive and seen in 57% of cases 2
- IL-18 and/or S100 proteins (calprotectin): Marked elevation strongly supports the diagnosis if available 1
- Liver function tests: Elevated transaminases are common 1, 2
Exclude Alternative Diagnoses
The 2024 guidelines emphasize that alternative diagnoses must be carefully excluded 1:
- Malignancies: Lymphoma, leukemia
- Infectious diseases: Sepsis, endocarditis, tuberculosis
- Other immune-mediated diseases: Systemic lupus erythematosus, vasculitis
- Monogenic autoinflammatory disorders: Familial Mediterranean fever, TRAPS
Classification Criteria
The Yamaguchi criteria have been validated in both children and adults with high sensitivity and notably do not require arthritis to be present 1. This is critical because arthritis often appears later, with a median delay of 1 month after disease onset (range 0 to several months) 1.
Critical Pitfalls to Avoid
- Delaying diagnosis while waiting for arthritis to develop: The 2024 guidelines strongly recommend that arthritis is NOT mandatory for diagnosis, as requiring it leads to unnecessary and potentially deleterious diagnostic delays 1
- Misdiagnosing as infection: Patients often receive multiple courses of antibiotics (mean 2.0±1.6 different antibiotics) before diagnosis due to prolonged fever and leukocytosis 2
- Missing macrophage activation syndrome (MAS): This life-threatening complication can occur at onset, during treatment, or even in remission, and requires prompt recognition with monitoring for persistent fever, splenomegaly, rising ferritin, falling cell counts, and abnormal liver function 1
Unified Disease Nomenclature
The 2024 EULAR/PReS guidelines establish that systemic JIA and adult-onset Still's disease are the same disease and should be designated by one name: Still's disease. 1 The arbitrary age threshold of 16 years separating the two entities was based on historical hospital department organization in the UK, not biological differences 1, 4.