Methotrexate Crystals on Electron Microscopy of Kidney Biopsy for Acute Kidney Injury.

Acute kidney injury secondary to methotrexate therapy for hematologic malignancies is relatively uncommon. Methotrexate crystals in these cases are rarely seen on kidney biopsy, and in particular, their appearance in tissue prepared for transmission electron microscopy has not been described. A male patient with recurrent primary central nervous system lymphoma received high-dose methotrexate and rituximab for treatment. On day 2 of cycle 3, one day after the infusion of high-dose methotrexate, the patient was found to have high levels of serum methotrexate. Shortly after, he developed acute kidney injury. A kidney biopsy was performed, which showed methotrexate crystals only on tissue submitted for electron microscopy. To our knowledge, this is the first report to characterize methotrexate crystals on toluidine blue-stained thick sections and their ultrastructure on transmission electron microscopy.