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Methotrexate

Source Blood
Mnemonic METHO
    
Specimen Requirements

1 x 3 mL light green (mint) top PST tube.  Gold top SST tube is also acceptable.

0.2 mL plasma or serum

Minimum Whole Blood Volumes for Microcollections

Monitoring NOT required for low dose oral or IM therapy unless the patient has renal impairment or a previous history of toxicity.  However if necessary, collect 0.5 hours following low I.V. dose or 2 hours after low oral dose.

Intermediate doses 100-500 mg/m2 may require leucovorin rescue.  Doses >500 mg/m2 require leucovorin rescue.  Collect at 24 hours, 48 hours and 72 hours following high I.V. dose.

Specimen Handling Record time of last dose on requisition.
Additional Information

Methotrexate levels cannot be measured on patients who have received Carboxypeptidase-G2 rescue therapy due to significant metabolite cross-reactivity (varying from 26% to 59%). Contact Clinical Biochemist (Dr. Lawrence de Koning: 403-955-2277) or Pathologist on-call for suggestions of alternate laboratory and methodology for testing.

For additional information, see Half-life and Time to Steady State.  

Testing Location Chemistry-ACH
Testing Frequency Daily 
Alternate Name(s) Amethopterin, MTX 
Reference Interval

Therapeutic Level interpretation dependent on institutional protocols, type of therapy (intermediate vs high dose) and individual patient factors.

Suggested toxic methotrexate levels - from Wu, AHB. 2006. Tietz Clinical Guide to Laboratory Tests 4th Edition.

 Dosage  

Time

Toxic

Low

  After 1 week  

  ≥ 0.04 umol/L 

High

 After 24 hours 

≥ 5.0 umol/L

High

After 48 hours

≥ 0.5 umol/L

High

After 72 hours

≥ 0.05 umol/L