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Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

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Item Value
Approval req'd? No
Available Stat? All requests treated as Stat
Test code MTX
Performed by? Chemistry
Sendout? No
Price range $$
In House Availability 24 hours/ 7 days
Method Competitive immunoassay
Container type Red or green top non gel tube. Lavender top tube acceptable.

The same specimen type should be used for all measurements from an individual patient.
Amount to Collect 1 mL
Collection Instructions The sampling time of methotrexate will be dependent on dose, duration of infusion, and clinical status of the patient. Consult specific treatment protocols for sampling time.

Protect from light during collection, storage and shipment. Serum or plasma need to be separated from cells within 2 hours of collection.

Specimens from patients who have received glucarpidase (carboxypeptidase G2) as a methotrexate rescue therapy should not be sent to the laboratory for testing with this method. This results in falsely elevated methotrexate concentrations due to interference by a metabolite of methotrexate that is produced when glucarpidase is administered; it remains in the circulation for 5-7 days.
Sample type Blood
Preferred volume 0.5 mL serum or plasma
Units µmol/L
Normal range Methotrexate (MTX) interpretation depends on dosing and draw times as well as the target level for the disease being treated. MTX levels in excess of 50 µmol/L at 24 hours, 10 µmol/L at 48 hours, and 0.5 µmol/L at 72 hours portend potential toxicity.

Non-toxic drug concentrations after 72 hours are usually less than 0.1 µmol/L.
Stability 24 hours at room temperature, two weeks refrigerated
Interferences Specimens from patients who have received carboxypeptidase G2 (CPDG2) for MTX rescue contain a hydrolysis fragment of MTX (DAMPA) that can cross-react with the antibody used in this assay resulting in a falsely elevated MTX measurement. After CPDG2 administration DAMPA may continue to circulate for at least 5-7 days before accurate measurements of serum MTX can be determined using this assay.

This assay crossreacts slightly with triamterene and trimethoprim, however these drugs may be contraindicated for MTX treatment due to additional adverse effects if co-administered.
Turn around times 2 hours
CPT coding 80299
Last Updated 5/25/2013 10:54:18 AM
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