UCSF Navigation Bar

UCSF Departments of Pathology & Laboratory Medicine Home Page

Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

Internal Resources

Click here for more information about laboratory operations and procedures

If you have additional questions regarding this test, please call: 415-353-1667

Search Our Test Menu

Methotrexate

Item Value
Approval req'd? Approval is required for testing patients who have received carboxypeptidase rescue therapy.
Available Stat? No
Test code MTX
Performed by Parnassus Chemistry
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
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.
LOINC code 14836-1
Last Updated 9/9/2014 10:26:18 AM
Entry Number 657
UCSF home page UCSF home page About UCSF Search UCSF UCSF Medical Center