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Test Code MTHX Methotrexate, Serum

Useful For

Determining whether methotrexate is being cleared appropriately and verifying that a nontoxic concentration has been attained following therapy

Method Name

Enzyme Multiplied Immunoassay (EMIA)

Reporting Name

Methotrexate, S

Specimen Type

Serum


Ordering Guidance


For patients that have received glucarpidase (carboxypeptidase G2) as a high-dose methotrexate rescue therapy, order MTXSG / Methotrexate Post Glucarpidase, Serum.



Shipping Instructions


Ship specimen in amber vial to protect from light.



Specimen Required


Supplies: Amber Frosted Tube, 5 mL (T915)

Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Amber vial

Specimen Volume: 0.5 mL

Collection Instructions:

1. Methotrexate is sensitive to fluorescent light; avoid prolonged exposure of specimen to direct light.

2. Within 2 hours of collection, centrifuge the specimen.

3. For serum gel tubes, aliquot serum into a plastic vial within 2 hours of collection. Keep protected from light.

4. For red-top tubes, aliquot serum into a plastic vial immediately. Keep protected from light.


Specimen Minimum Volume

0.25 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 14 days LIGHT PROTECTED
  Frozen  28 days LIGHT PROTECTED
  Ambient  72 hours LIGHT PROTECTED

Reject Due To

Gross hemolysis Reject

Clinical Information

Methotrexate is an antineoplastic agent that inhibits DNA synthesis. The medication exerts its effects through competitive inhibition of the enzyme dihydrofolate reductase thus decreasing the concentrations of tetrahydrofolate essential to the methylation of pyrimidine nucleotides and consequently the rate of pyrimidine nucleotide and ultimately DNA synthesis.

 

Methotrexate is used in the treatment of certain neoplastic diseases, severe psoriasis, and adult rheumatoid arthritis. Methotrexate is effective against malignancies characterized by rapid cell proliferation. Intermediate to high doses of methotrexate with leucovorin (citrovorum-factor or folinic acid) rescue to salvage nontumor cells have been used with favorable results in the treatment of osteogenic sarcoma, leukemia, non-Hodgkin lymphoma, lung, and breast cancer.

 

Methotrexate has the potential for serious toxicity. Patients undergoing methotrexate therapy are closely monitored so that toxic effects are detected promptly.

Reference Values

Nontoxic drug concentration after 72 hours: <0.1 mcmol/L

Interpretation

Serum concentrations of methotrexate are commonly monitored during high-dose therapy (>50 mg/m[2]) to identify the time at which active intervention by leucovorin rescue should be initiated. Criteria for serum concentrations indicative of a potential for toxicity after single-bolus, high-dose therapy are as follows:

-Methotrexate >10 mcmol/L 24 hours after dose

-Methotrexate >1 mcmol/L 48 hours after dose

-Methotrexate >0.1 mcmol/L 72 hours after dose

Cautions

Important: Specimens from patients who have received glucarpidase (carboxypeptidase G2) as a high-dose methotrexate rescue therapy should not be tested with this immunoassay. Methotrexate concentrations should be performed using the liquid chromatography tandem mass spectrometry method to avoid the reporting of falsely elevated methotrexate values due to an interference that could confuse the efforts of the glucarpidase therapy. After glucarpidase therapy, it can take at least 5 to 7 days before accurate measurements of serum methotrexate can be obtained using an immunoassay.

Method Description

The ONLINE TDM MTX assay is a homogeneous enzyme multiplied immunoassay (EMIA). It is a two-reagent system used for the detection of methotrexate in serum and plasma. In this technology, drug hapten attached to the glucose-6-phosphate dehydrogenase (G6PD) enzyme serves as the binding partner to anti-methotrexate antibody. A competitive reaction to a limited amount of specific anti-methotrexate antibody takes place between the enzyme bound hapten and free methotrexate in the sample. Enzyme activity is reduced with bound antibody. Only active enzymes reduce nicotinamide adenine dinucleotide (NAD[+]) to NADH. The rate of NADH formation during the reaction correlates to the methotrexate concentration and is measured photometrically.(Package Insert: ONLINE TDM Methotrexate; Roche Diagnostics; 03/2025)

Report Available

Same day/1 day

Specimen Retention Time

1 week

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information

80204

LOINC Code Information

Test ID Test Order Name Order LOINC Value
MTHX Methotrexate, S 14836-1

 

Result ID Test Result Name Result LOINC Value
MTHX Methotrexate, S 14836-1

Day(s) Performed

Monday through Sunday

Forms

If not ordering electronically, complete, print, and send a Therapeutics Test Request (T831) with the specimen.