Abstract 3678: An immunoassay for measurement of methotrexate on ARCHITECT i system

Methotrexate (MTX) is a cancer therapeutic drug for leukemia, osteosarcoma, non-Hodgkin's lymphoma and others. MTX levels in blood are monitored in patients to avoid toxic effects caused by the excessive dosage of this drug, and to determine when to intervene with the counter-acting ‘rescue’ therapy. An assay for serum and plasma MTX measurement on ARCHITECT i System (ARCHITECT Methotrexate) is being developed and the analytical feasibility of this assay is presented. METHODS: The ARCHITECT Methotrexate is a competitive immunoassay. First, the instrument mixes and incubates a sample (calibrators, controls, sera or plasma) with anti-methotrexate antibody-coated magnetic microparticles, biotinylated anti-methotrexate antibody and acridinium-conjugated MTX. Then, the instrument washes the microparticles and adds pre-trigger and trigger solutions to initiate the chemiluminescence reaction. Signals obtained as relative luminescent units are inversely proportional to the amount of MTX in the sample. RESULTS: The limit of quantitation was ≤ 0.040 µmol/L MTX. The 20-day imprecision study showed a total CV ≤ 7.1% within the range of 0.056 to 1705.433 µmol/L MTX with 6 controls and 5 panels on 4 instruments using 3 lots of reagents (n = 80). Dilution linearity showed the deviations from linearity were -4.1% to 7.2% within the range of 1.881 to 0.012 µmol/L MTX. Spike recovery revealed a mean recovery of 92% - 94% for samples at 0.045, 0.909 and 9.090 µmol/L of MTX. In the interference studies, the average levels of MTX in the individual interfering substance-spiked samples (10 endogenous substances including human anti-mouse antibody and rheumatoid factor, and 21 therapeutic drugs) were within 90% - 110% of that in the unspiked control samples. Method comparison of ARCHITECT Methotrexate to TDx/TDxFLx Methotrexate II (TDx) generated a Passing Bablok correlation as [ARCHITECT] = 0.00 + 1.01 [TDx] for samples within the range of 0.040 to 1.415 µmol/L (n = 92) and [ARCHITECT] = 0.00 + 1.04 [TDx] for samples within the range of 0.040 to 1624.760 µmol/L MTX , n = 142). Specimen storage study denoted that specimens could be stored at room temperature for 24 hours or at 2-8oC for 48 hours since the MTX values of the stored specimens described above were within 10% deviation from that of the baseline control specimens. Specimen Tube type study demonstrated that specimens collected in K2-EDTA, sodium heparin, and lithium heparin tubes was within ± 10% deviation from that col