METHOTREXATE POST GLUCARPIDASE - (P)
Test Code: MISC
Test Indications: This test is ONLY for patients receiving glucarpidase rescue therapy. For monitoring Methotrexate please see (METHOTREXATE (P) LIM ENTRY)
Recommendations: Methotrexate is an antineoplastic agent that inhibits DNA synthesis. It is effective in the treatment of neoplastic diseases that have rapid cell proliferation. Methotrexate has the potential for serious toxicity. Testing is used to monitor patients undergoing methotrexate therapy and ensure the drug is being cleared appropriately to avoid toxicity.
Voraxaze (glucarpidase) is a carboxypeptidase enzyme indicated for the treatment of toxic methotrexate concentrations (>1 µmol/L) in patients with delayed methotrexate clearance due to impaired renal function.
However, measurement of methotrexate using immunoassays is unreliable for specimens collected within 48 hours following Voraxaze administration since it can result in falsely elevated results due to crossreactivity with the methotrexate metabolite. As a result, another methodology such as LC-MS/MS assay should be used to monitor methotrexate concentrations postglucarpidase therapy.
Must write on the requisition: "Alternative testing by LC-MS/MS assay (Glucarpidase therapy protocol)".
Methotrexate is light sensitive. Protect specimen from light by wrapping with foil or aliquot into a dark transfer vial.
Stability 14 days refrigerated, 1 month frozen
Samples must be transferred to an aliquot tube and stored frozen if analysis will not be complete within 48 hours (#110-10-05 Serum / Plasma Separation Procedure & Transport)
Target levels vary depending on the dosage protocol used.
Please refer to: http://wapps.sickkids.ca/paediatriclabmedicinems/test-catalogue/tdm/41588.html#SpecReq
Specify "Alternative testing by LC-MS/MS assay (Glucarpidase therapy protocol)".
The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642