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.