The methotrexate polyglutamate assay supports the efficacy of methotrexate for severe inflammatory skin disease in children.

BACKGROUND The methotrexate (MTX) polyglutamate assay has been validated in adults with arthritis. OBJECTIVE We sought to assess clinical response rates and the value of the methotrexate polyglutamate assay in MTX-treated children with inflammatory skin diseases. METHODS In this retrospective review, 46 MTX-treated children with a diagnosis of atopic dermatitis, psoriasis, or psoriasis-eczema overlap were serially assessed with the methotrexate polyglutamate assay. RESULTS In all, 38 children (83%) achieved good to excellent response: 27 (59%) within 12 weeks and 11 (24%) after dose-adjustment. Good to excellent responses were highest for psoriasis/overlap: 15 of 16 (94%), compared with 23 of 30 (77%) with atopic dermatitis. Mean maximum polyglutamate levels were 31.5 nmol/L for responders versus 18.1 nmol/L for nonresponders (P = .035). This difference was also significant for the subset with atopic dermatitis, but not for those with psoriasis/overlap. After dose modification, late responders ultimately achieved a significantly higher mean maximum methotrexate polyglutamate assay (41.9 nmol/L) compared with nonresponders (P = .002). LIMITATIONS Retrospective design and small sample size were limitations. CONCLUSIONS MTX is an effective treatment for the majority of children with inflammatory skin diseases, but a subset requires dose modification to achieve good to excellent response. Methotrexate polyglutamate assay levels reflect response to treatment, but are most useful to support dose modification among children who fail to respond within 12 weeks.