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Methotrexate

Item Value
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.
Available Stat? No
Test code MTX
Performed by Parnassus & Mission Bay 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
Units µmol/L
Normal range
Toxic > 0.1 µmol/L
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 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." Refer to the Mayo website for specimen requirements and interpretation of results
LOINC code 14836-1
Last Updated 4/7/2015 12:59:32 PM
Entry Number 657
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