Diagnostic Complexity in Systemic Inflammation: Adult-Onset Still's Disease.

Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder classically associated with fever, rash, arthritis, and marked inflammatory laboratory abnormalities. Diagnosis is clinical and typically requires exclusion of infection, malignancy, and other rheumatologic diseases. We describe a 33-year-old previously healthy Nigerian male with several weeks of daily fevers by history, diffuse polyarthralgia with synovitis, sore throat, and a transient non-pruritic rash initially treated as an allergic reaction. His symptoms progressed despite antibiotics. Outpatient testing showed ferritin >20,000 ng/mL and CRP 328 mg/L. During admission, he had persistent leukocytosis (29-30×10⁹/L) and hyperferritinemia (5,770-14,000 ng/mL). Knee aspiration revealed calcium pyrophosphate crystals consistent with calcium pyrophosphate dihydrate (CPPD), which added diagnostic uncertainty regarding whether a crystal arthropathy was driving his syndrome or represented a coincident finding. He received empiric broad-spectrum antimicrobials early in the hospitalization for possible severe infection, but clinical improvement occurred only after high-dose corticosteroids. Given profound hyperferritinemia, hemophagocytic lymphohistiocytosis and macrophage activation syndrome were considered, although cytopenias were absent and bone marrow biopsy showed no hemophagocytosis. Rheumatology diagnosed AOSD using classification criteria applied in a clinical context after exclusion of alternative etiologies. He improved with corticosteroids and later transitioned from anakinra to canakinumab with methotrexate due to suboptimal response and pruritus, with marked clinical improvement. This case highlights how CPPD crystal detection can complicate the interpretation of inflammatory arthritis in a patient whose overall presentation suggests systemic autoimmune inflammation and the importance of reassessing the unifying diagnosis when clinical features do not align.