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Lab Dept:
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Chemistry
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Test Name:
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METHOTREXATE LEVEL
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General Information
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Lab Order Codes:
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MTX, TREX (Backup to Allina)
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Synonyms:
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Mexate, MTX
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CPT Codes:
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80299 – Quantitation of drug, not elsewhere specified
Backup to Allina:
83520 – Immunoassay for analyte other than infectious agent antibody or agent antigen, quantitative, not otherwise specified
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Test Includes:
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Methotrexate concentration measured in umol/L.
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Logistics
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Test Indications:
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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.
Methotrexate is an antineoplastic agent that inhibits the enzyme, dihydrofolate reductase. Such inhibition causes a stoppage of synthesis of tetrahydrofolate that, in turn, results in an inhibition of nucleotide and ultimately of DNA synthesis. Methotrexate is effective against malignancies with rapid cell proliferation such as acute lymphoblastic leukemia, choriocarcinoma, trophoblastic tumors in women, and carcinomas of the breast, tongue, pharynx, and testis.
Relative low doses of methotrexate (7.5 – 25 mg/week) have been used in the treatment of non-malignant diseases such as severe psoriasis, asthma, rheumatoid arthritis, sarcoidosis, and transplantation therapy.
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Lab Testing Sections:
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Chemistry
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Phone Numbers:
Minneapolis:
Saint Paul:
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612-813-6280
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651-220-6550
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Test Availability:
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Daily, 24 hours
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Turnaround Time:
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4 hours, testing performed daily 7 AM to 10 PM.
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Special Instructions:
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Wrap collected specimens in foil to protect from light during transport to the laboratory.
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Specimen
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Specimen Type:
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Blood
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Container:
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Green top (Li Heparin) tube preferred
Alternate tube: Red top (plain) tubes
Note: Gel barriers are not allowed.
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Draw Volume:
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1.5 mL blood
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Processed Volume:
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0.5 mL serum/plasma
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Collection:
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Routine venipuncture. Mix tubes containing anticoagulant by gentle inversion. Wrap the collected specimen in foil to protect from light during transport to the laboratory.
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Special Processing:
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Lab Staff: Centrifuge specimen, remove serum/plasma aliquot into plastic vial. Store at refrigerated temperatures. Wrap specimen in foil to protect from light.
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Patient Preparation:
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None
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Sample Rejection:
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Mislabeled or unlabeled specimen; specimens not protected from light; exposed to light for >2 hours
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Interpretive
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Reference Range:
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Dependant on clinical status
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Critical Values:
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>1 umol/L (>5 umol/L at 24 hours after high dose therapy.)
Backup to ANW:
>0.02 umol/L at 72 hours post infusion
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Limitations:
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Exposure to light degrades Methotrexate.
Specimens from patients who may be sensitized to mouse monoclonal antibodies should be tested by a different method.
4-[[2,4 – diamino-6-(pteridinyl) methyl] – methylamino] – benzoic acid (DAMPA) cross reacts with this method. DAMPA is a metabolite in patients who have received carboxypeptidase (G2) as a high dose methotrexate rescue therapy.
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Methodology:
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Fluorescence Polarization Immunoassay (FPIA)
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References:
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Tietz Textbook of Clinical Chemistry (1999) 3rd Edition, W.B. Saunders Company, pp. 896-897
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Methotrexate II package Insert (February 2006) E 7A12, Abbott Laboratories, Diagnostics Division, Abbott Park, IL 60064
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Allina Medical Laboratories Web Page February 2012 (Backup)
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TDxFLx System Operations Manual Abbott Laboratories (9/1994) Diagnostics Division, P.O.152020, Irving, Texas, 75015-2020, R-109
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Updates:
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9/11/2006: Testing method previously listed as Enzyme Immunoassay. Testing resumed in house.
9/18/2006: Turnaround time previously listed as 2 hours.
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