methotrexate in serum/plasma

Reference interval

No precise relationship between methotrexate serum levels & antineoplastic efficacy has been established. Toxicicity is typically present in the form of myelosuppression, stomatitis, nausea, vomiting, convulsions, & liver & renal abnormalities. Anemia, leukopenia, thrombocytopenia, & skin & mucosa involvement with a fatal outcome have also been reported. Neurotoxicity & leukoencephalopathy are also reported as toxic effects involving methotrexate.

Principle

The TDX Methotrexate assay is a reagent system for the quantitative measurement of methotrexate, an antineoplastic drug, in serum or plasma. The measurements obtained are used in monitoring levels of methotrexate to ensure appropriate therapy. The Methotrexate assay utilizes Fluorescence Polarization Immunoassay technology as described in the TDX Procedure on page 1.

Clinical significance

Methotrexate is an antineoplastic drug used solely or in combination with other antineoplastic drugs for the treatment of leukemia & other diseases. Rheumatoid arthritis, severe psoriasis, sarcoidosis, & granulomatosis have been treated with methotrexate in relatively low doses. High dose methotrexate (> 20 mg/kg body weight) with citrovorum-factor rescue has been used with favorable results in the treatment of osteogenic sarcoma, leukemia, non-Hodgkin's lymphoma, lung cancer, head & neck cancer, & breast cancer. The efficacy of methotrexate in the treatment of other tumors such as prostatic cancer is being investigated.

Specimen

Collect blood samples by venipuncture following established good laboratory practices. If the sample is obtained through the infusion set, flush the line thoroughly with saline before taking the blood sample. With some exceptions, any anticoagulant may be used to collect plasma for analysis. Serum, as well as plasma, may be used for most assays. It is very important that the physician be informed of the times of sample collection & dose administration; this information should be supplied to the laboratory with each sample & reported with the results of each test. Samples derived from blood should be refrigerated upon collection & stored frozen (-20 degrees Celsius or colder) if not analyzed within 24 hours. Complete mixing of each thawed sample is required before analysis.

Urine samples must be collected in clean, previously unused containers. It is recommended that samples should be refrigerated upon collection & stored frozen if not analyzed within 48 hours.

CSF & amniotic fluid samples should be obtained using standard collection procedures.

Fibrin threads or large particles which could block the probe should no be pipetted or poured into the sample well. After sample transfer, assure there are no bubbles or foam present in the sample well. Remove bubbles or foam prior to running. Fluorescein is a constituent of all FPIA reagent systems Patient samples containing fluorescent compounds may interfere with these TDx methodologies resulting in high blank intensity readings & low net intensities. If patient samples cannot be diluted below the maximum background value an alternate methodology should be used. Automatic serum blank readings reduce optical interferences from grossly icteric, hemolyzed or lipemic samples. Serum blanks are automatically subtracted by the TDx analyzer before final results a re printed, when required by the specific assay mode. Minimum sample volume is 50 ul.

Notes

The concentration range of the Methotrexate assay (0-1000 uM) is adequate to accurately determine the methotrexate concentration in most of the patient samples encountered.

The lowest measurable level is defined as that concentration which can be distinguished from zero with 95% confidence; it was determined to be 0.01 uM.

More general terms

• therapeutic drug measurement

More specific terms

• methotrexate free in serum/plasma

Additional terms

• methotrexate (Rheumatrex, MTX)