Utility of IL ‐18 Immunohistochemistry in Distinguishing Dermatomyositis and Lupus Erythematosus in Skin Biopsies

BACKGROUND: Distinction of dermatomyositis (DM) and cutaneous lupus erythematosus (LE) in skin biopsies is often difficult due to significant histopathologic overlap. Prior transcriptional analysis has identified upregulation of IL-18 in DM compared with LE. We aimed to assess its utility as an immunohistochemical marker in this differential diagnosis. METHODS: Skin biopsies classified as DM (n = 35) or LE (n = 32) were immunostained with an IL-18 antibody. A positive IL-18 result was defined as presence of full-thickness epidermal staining of medium or strong intensity. Absent, weak, or partial-thickness staining was considered negative. Results were compared between groups by Fisher's exact test. RESULTS: IL-18 expression in keratinocytes, when present, was consistently cytoplasmic with or without nuclear positivity. Twenty-eight (80%) DM cases and 11 (34%) LE cases were positive for IL-18 (p = 0.0002). A positive result had a sensitivity of 80% and a specificity of 66% for DM. Specifically, strong full-thickness staining was only observed in DM (five cases; p = 0.05), whereas complete absence of epidermal staining was seen exclusively in LE (five cases; p = 0.02). CONCLUSIONS: IL-18 immunohistochemistry can be a useful diagnostic adjunct in distinguishing DM and LE in a small subset of skin biopsies when staining is strong or completely absent. Correlation with clinical and histopathologic findings remains imperative.