A 6-year-old girl presents with arthritis of the knees and ankles for 8 weeks, daily spiking fever to 40°C with characteristic evanescent salmon-colored rash coinciding with fever spikes, hepatosplenomegaly, and lymphadenopathy. Ferritin is 15,000 ng/mL. Which JIA subtype does this presentation MOST closely represent, and what LABORATORY feature has the highest specificity for this diagnosis?

• A Polyarticular RF-negative JIA; elevated ESR
• B Enthesitis-related arthritis; HLA-B27 positivity
• C Oligoarticular JIA; ANA positivity with uveitis
• D Systemic JIA (sJIA); markedly elevated serum ferritin ✓

Explanation

Systemic JIA (Still's disease) is characterized by the triad of quotidian fever (daily spikes to >39°C), evanescent salmon-colored macular rash coinciding with fever, and arthritis, plus systemic features (hepatosplenomegaly, lymphadenopathy, serositis). Extremely elevated ferritin (>500 ng/mL, often >10,000 ng/mL) is the laboratory hallmark, reflecting macrophage activation. Ferritin >10,000 ng/mL has specificity approaching 80% for sJIA. Macrophage activation syndrome (MAS) is a life-threatening complication of sJIA requiring early recognition.

Reference: Ghai Essential Pediatrics, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.