Methotrexate

1 x 5 mL light green (mint) top PST tube.  Gold top SST tube is also acceptable.

0.2 mL heparinized plasma or serum

Minimum Blood Volume for Patients less than 45 kg (100 lb) Including Neonates and Pediatrics

Protect from light by wrapping entire tube in foil.

Monitoring NOT required for low dose oral or IM therapy unless the patient has renal impairment or a previous history of toxicity.  However if necessary, collect 0.5 h following low I.V. dose or 2 h after low oral dose.

RRL & DSC: Send STAT to FMC. 

RECORD time of last dose on requisition.

Intermediate doses 100-500 mg/m² may require leucovorin rescue.  Doses >500 mg/m² require leucovorin rescue.  Collect at 24 h, 48 h and 72 h following high I.V. dose.
Separate as soon as possible. 

Methotrexate levels cannot be measured on patients who have received Carboxypepidase-G2 rescue therapy due to significant metabolite cross-reactivity (varying from 26% to 59%).
Contact Pathologost-on-call or Clinical Biochemist for suggestions of alternate laboratory and methodology for testing.

For additional information, see Half-life and time to Steady State.