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If you have additional questions regarding this test, please call: 415-353-1667
|Approval req'd?||Approval is required for requests to perform additional testing outside of the normal test schedule. Contact the laboratory 3-1667 and ask to speak to a laboratory resident or pathologist.
Approval is also required for testing patients who have received carboxypeptidase rescue therapy.
|Performed by||Parnassus Chemistry|
|In House Availability||Test run daily @ 9:00 AM. If additional testing is needed contact laboratory at 3-1667 and ask to speak to a laboratory resident or pathologist.|
|Method||Abbott TDx/Flx Fluorescent Polarization Immunoassay|
|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|
|Processing notes||Aliquot sample into brown light-protective tube.|
|Stability||Refrigerated 1 day, frozen at -20C 1 week|
|Turn around times||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 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||2/24/2014 9:56:24 AM|