Methotrexate, Serum

Methotrexate, formerly known as amethopterin, is an antimetabolite and antifolate drug used in treatment of cancer and autoimmune diseases including ankylosing spondylitis, Crohn’s disease, psoriasis, psoriatic arthritis, and rheumatoid arthritis. It acts by inhibiting the metabolism of folic acid. Methotrexate is a weak dicarboxylic acid that is mostly ionized at physiologic pH. Oral absorption is saturatable and is dose-dependent. It is metabolized by intestinal bacteria which accounts for less than 5% loss of the oral dose. The sampling times will depend on the dose (high vs. low), duration of the infusion, and the clinical status of the patient. These are generally used to guide adjustment of folic acid or leucovorin rescue doses.

The measurement of Methotrexate level is performed to ensure concentrations are within therapeutic range and avoid drug toxicity. Possible side effects can include anemia, neutropenia, increased risk of bruising, nausea and vomiting, dermatitis and diarrhea. A small percentage of patients develop hepatitis, and there is an increased risk of pulmonary fibrosis. The higher doses of methotrexate can cause toxic effects to cells of bone marrow and gastrointestinal mucosa. Methotrexate is a highly teratogenic drug and categorized in pregnancy as a Category X by the US FDA. There is a risk of a severe adverse reaction if penicillin is prescribed alongside methotrexate. There have also been some reports of central nervous system reactions to methotrexate especially when given via the intrathecal route.

1 ml Serum Room Temperature

Usual sampling time depends on dosage and clinical status of patient. Steady state achieved 12-24 Hrs after chronic dosing.

24 Hrs after dose: 5.00 - 10.00 μmol/L 48 Hrs after dose: 0.50 - 1.00 μmol/L 72 Hrs after dose: < 0.20 μmol/L