Sign in →

Order Code MTHX Methotrexate, Serum

Additional Codes

MAYO CODE MTHX

Useful For

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

Method Name

Enzyme-Immunoassay (EIA)

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

Reference Values

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

Method Description

The Roche ONLINE TDM Methotrexate assay is a homogeneous enzyme immunoassay based on competition between drug in the specimen and methotrexate labeled with the enzyme glucose-6-phosphate dehydrogenase (G6PD) for binding to the antibody reagent. As the latter binds antibody, enzyme activity decreases. In the presence of drug from the specimen, enzyme activity increases and is directly proportional to the drug concentration. Active enzyme converts the coenzyme nicotinamide adenine dinucleotide (NAD) to NADH, which is measured spectrophotometrically as a rate of change in absorbance. Endogenous serum G6PD does not interfere with the results because the coenzyme NAD functions only with the bacterial enzyme used in the assay.(Package insert: MTX: ONLINE TDM Methotrexate. Roche Diagnostics; V 1.0, 03/2024)

Performing Laboratory

Mayo Clinic Laboratories in Rochester

CPT Code Information

80204

Day(s) Performed

Monday through Sunday

Forms

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