Method

Immunoassay

Interpretive Data: 

Specimens from patients who have received carboxypeptidase can contain a hydrolysis fragment of MTX (DAMPA) that cross-reacts with the methotrexate antibody used in this immunoassay.

Because enzyme immunoassay interference can result in overestimation of serum MTX concentrations immediately after administration of carboxypeptidase enzyme treatment (glucarpidase), it may be helpful to measure MTX after the glucarpidase dosing using a more specific assay (e.g. LC-MS/MS). The LC-MS/MS testing for MTX in these cases can be obtained by sendout to Mayo Medical Laboratories for "Methotrexate Post Glucarpidase, Serum."