Skin biopsy in adult-onset Still's disease: A specific, rapid, and widely available diagnostic test.

Adult-onset Still's disease (AOSD) is a diagnostic challenge, as its features overlap with many inflammatory syndromes and no single finding is diagnostic. Skin biopsies showing dyskeratotic keratinocytes (DKs) in the superficial epidermis, including the horny layer, are thought to be highly specific for AOSD but are not included in current diagnostic or classification criteria. We conducted a retrospective clinicopathological study of 16 adults with confirmed AOSD and skin biopsies from a Canadian rare diseases program, and developed a differential diagnosis for superficial DKs using expert consensus and literature review. Clinical morphology varied, but histopathology was similar: 12 of 16 patients had DKs or necrotic keratinocytes in the upper epidermis. Superficial DKs have been reported in conditions distinct from AOSD, but not in its major mimics. Skin biopsy is low-cost, minimally invasive, and widely available and should be performed early in suspected AOSD; superficial DKs should be added to future diagnostic and classification criteria.