Clinical Pearl at #ACR21 from Dr. J. Stone: ⭐️Ferritin:ESR ratio helps distinguishes systemic JIA flares from macrophage activation syndrome (MAS). ⭐️During MAS: *️⃣Ferritin rapidly rises as an acute phase reactant *️⃣ESR declines as fibrinogen is consumed. #ACRBest @RheumNow