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Test Code Billings Clinic: 8205 Methotrexate, Serum

Important Note

Specimens from patients who have received glucarpidase (carboxypeptidase G2) as a high dose methotrexate rescue therapy should not be tested with this method.

Performing Laboratory

Billings Clinic Laboratory:  Chemistry

Useful For

Following therapy, serum concentration is used to judge whether the drug is being cleared appropriately and verify that a nontoxic level has been attained

Methodology

Homogeneous enzyme immunoassay

Specimen Requirements

Serum

  • Draw blood in a plain, red-top tube(s) or a serum gel tube(s). 
  • Spin down, separate serum from cells immediately, and send 2 mL of serum refrigerated.
  • Wrap specimen in foil to protect from light.
  • Indicate serum on request form.
  • Label specimen appropriately (serum).

Storage/Stability

Specimen Type Temperature Time
Serum Refrigerated (preferred) 14 days
  Frozen 1 month

 

Reject Due To

  • Specimens with greater than 3 freeze-thaw cycles
  • Specimens other than serum

  • Specimens from patients who have received glucarpidase (carboxypeptidase G2) as a high dose methotrexate rescue therapy

Reference Values

Therapeutic concentrations vary from patient to patient.

 

To avoid excess toxicity, see the following guidelines:

 

<10.00 µmol/L at 24 hours post-high dose infusion
<1.00 µmol/L at 48 hours post-high dose infusion
<0.20 µmol/L at 72 hours post-high dose infusion

Day(s) Test Set Up

Daily

Test Classification and CPT Coding

80299 - Quantitation of drug, not elsewhere specified

LOINC Code Information

Reporting Name LOINC Code
Methotrexate 14836-1