The Elusive Challenge of Diagnosing Adult Onset Stills Disease: The Great Mimicker

Summary: This is a case of a 35-year-old male who presented with a 10-day history of generalized body aches, arthralgias, fevers, and a pruritic rash primarily affecting the chest and upper back. Despite multiple emergency department visits and treatments, his symptoms showed minimal improvement and progressively worsened, with the addition of palpitations and night sweats. He denied any recent travel, sick contacts, new exposures or symptoms suggestive of sexually transmitted diseases. On initial evaluation, laboratory work-up revealed neutrophilic leukocytosis, thrombocytosis, and mildly elevated liver transaminases. Imaging studies, including chest X-ray and CT scans of the chest, abdomen, and pelvis, were unremarkable for acute pathology. Blood cultures and a broad range of serologies, including tests for HIV, Cytomegalovirus, Epstein-Barr virus, Dengue, Chikungunya, mononucleosis, and a full STD panel, all returned negative. With infectious causes ruled out, the differential diagnosis expanded to inflammatory etiologies. Autoimmune tests, including ANA, rheumatoid factor, p-ANCA, and c-ANCA, were all negative. However, during hospitalization, a distinct clinical pattern emerged: the patient experienced two fever spikes per day, and his partner observed that the rash worsened during these episodes of fever. These clinical features, followed up with a markedly elevated ferritin (4670 ng/mL) and extensive negative work up, strongly suggested a diagnosis of AOSD.