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If you have additional questions regarding this test, please call: 415-353-1667
|Test Update Information||Effective October 2, 2014, methotrexate testing is performed on the Beckman DxC 800 analyzer using the ARK Diagnostics reagent kit. This test is now being offered routinely 24 hours per day.|
|Approval req'd?||Approval is required for testing patients who have received carboxypeptidase rescue therapy.|
|Performed by||Parnassus Chemistry|
|In House Availability||24 hours per day and 7 days per week|
|Method||Homogeneous Enzyme immunoassay – ARK Diagnostics. Performed on the Beckman DxC 800|
|Collection Instructions||Note: Contact laboratory if patient has received Carboxypeptidase rescue therapy|
|Container type||Gold top or Light Green top|
|Amount to Collect||1 mL blood|
|Sample type||Serum or plasma|
|Preferred volume||0.5 mL serum or plasma|
|Stability||Refrigerated 1 day, frozen at -20C 1 week|
|Turn around times||4 hours
Results on specimens received in the laboratory by 0900 will be available by 1030
|Additional information||Breakdown products of methotrexate due to Carboxypeptidase rescue therapy are still detected by this assay and therefore continued monitoring of methotrexate in this situation by the in-house assay is problematic. Testing with the in-house assay after carboxypeptidase therapy is only done with approval from a pathologist.
Specimens from patients who have received carboxypeptidase G2 (CPDG2) as a form of emergency methotrexate (MTX) rescue therapy can contain a hydrolysis fragment of MTX designated
4-[[2,4-diamino-6-(pteridinyl)methyl]- methylamino]-benzoic acid (DAMPA) that cross-reacts with the methotrexate antibody used in this immunoassay. Administration of CPDG2 results in a rapid fall in the methotrexate level, however, the magnitude of the fall may be somewhat underestimated by the methotrexate immunoassay due to cross reaction with the DAMPA hydrolysis fragment. 15 minutes after administration of CPDG2, methotrexate levels have been reported to fall by a median of 97% (range 73 – 99%) as judged by methotrexate measurements performed using a specific HPLC assay not subject to the DAMPA interference. In contrast, use of the less specific immunoassay in this circumstance underestimates the fall in methotrexate levels and shows a median decrease of 87% (range 70-99%) (Buchen et al, British Journal of Cancer 92:480, 2005).
Because this enzyme immunoassay interference can result in overestimation of serum MTX concentrations immediately after administration of CPDG2, it may be helpful to measure MTX once right after the CPDG2 dose using a more specific assay (e.g. LC-MS/MS) and then only if renal function deteriorates. Instructions for obtaining measurements of MTX by LC-MS/MS (Kumar VS et al, Methods Mol Biol. 2010;603:359-63) can be obtained by contacting the Lab Control section of the Department of Laboratory Medicine, Children's Hospital Boston, Boston, MA at 617-355-6351 (Dept. of Lab Medicine LAB CONTROL - Farley 7, Children's Hospital Boston, 300 Longwood Ave Boston, MA 02115). Minimum sample requirement is 200uL of EDTA or heparinized plasma or serum.
|Last Updated||9/30/2014 11:04:42 AM|