Diagnostic Use
Methotrexate (MTX) therapeutic drug monitoring (TDM) in the haematology/oncology setting, with bloods taken at 24 to 72hrs post high dose IV MTX infusion to assist leucovorin rescue titration, is quite well established.
In contrast, in the setting of low dose oral MTX regime for e.g. rheumatoid arthritis, psoriasis or inflammatory bowel diseases, MTX level has not been found to correlate well with disease activity, treatment efficacy or toxicity with its level rapidly drops off within 12-24hrs from intake. MTX TDM for these purposes thus is not recommended. Clinical monitoring on efficacy and toxicity coupled with laboratory investigations e.g. liver enzyme tests and full blood count to look for hepatotoxicity and myelosuppression can be useful. Serum P3NP monitoring for MTX related hepatic fibrosis is available from Waikato Hospital laboratory.
Reference Intervals
Interpretation of Methotrexate level depends on time of sampling with respect to methotrexate infusion. Suggest refer to chemotherapy chart and/or protocol in that regard or consult Haematology team for advice
Test Method
Principle: Homogeneous enzyme immunoassay
Analyser: Roche Diagnostics Cobas c703/c503
Reagent: ARK Methotrexate 2
Limitations / Interference
While normally DAMPA (2,4-Diamino-N-methylpteroic acid) is a minor metabolite of methotrexate, Glucarpidase (carboxypeptidase G2) therapy rapidly converts circulating MTX to DAMPA, significantly raising its circulating level. The MTX assay at CMDHB significantly cross reacts with DAMPA thus the assay should not be used during Glucarpidase therapy as a rebound phenomenon (from falsely raised serum level) can occur.
Uncertainty of Measurement
0.02 umol/L and 10% at 0.4 umol/L