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Test Code MTX METHOTREXATE

Methodology

Homogeneous enzyme immunoassay

Useful For

Following therapy, serum concentration is used to judge whether the drug is being cleared appropriately and verify that a nontoxic concentration has been attained.  Methotrexate has the potential for serious toxicity. Patients undergoing methotrexate therapy are closely monitored so that toxic effects are detected promptly.

Methotrexate (MTX) is an antineoplastic agent that inhibits DNA synthesis. High-dose MTX is used in the treatment of certain malignancies including leptomeningeal metastases, non-Hodgkin lymphoma, acute lymphoblastic leukemia and osteosarcoma, lung and breast cancer.  Low-dose MTX is commonly used in the treatment of various nonmalignant disorders including rheumatoid arthritis (RA), psoriasis and ectopic pregnancies.

Specimen Requirements

Preferred:

Specimen Type:          Serum

Container/Type:          Serum Gel Tube

Specimen Volume:      0.5 mL serum (minimum 0.25 mL)

 

Alternate:        Serum, Plain red-top tube

                        Lithium or Sodium Heparin Plasma, Green-top gel tube

                        EDTA plasma, Lavender-top tube

           

Collection Instructions:        

  1. The sampling time of methotrexate is dependent on dose and duration of infusion. High-dose methotrexate therapy is typically monitored at 24 hours, 48 hours, and 72 hours post dose.  Collect samples at times specified by the physician.
  2. Centrifuge specimen within 2 hours of draw.
  3. Protect specimen from light during transport and until analysis.

CAUTION:  Specimens from patients who have received glucarpidase (carboxypeptidase G2) as a high-dose rescue therapy should not be tested with this assay.  Methotrexate concentrations from patients receiving glucarpidase treatment should be performed using a LC-MS-MS method to avoid the reporting of falsely elevated methotrexate values.  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.

Day(s) Test Set Up

Monday through Sunday

Specimen Stability

Refrigerated

Reference Values

< 0.10 µmol/L  72 hours after dose

Performing Laboratory

Logan Health Medical Center Laboratory

Test Classification and CPT Coding

80299