Methotrexate interferes with determinations of conjugated bilirubin with the Kodak Ektachem 400

Immediately after intravenous infusion of a high dose (concentration in serum greater than 1000 mumol/L) of methotrexate, the apparent conjugated bilirubin (Bc) concentrations in serum of two osteosarcoma patients, as measured by the Kodak Ektachem 400 analyzer, were greater than the corresponding total bilirubin concentrations, but decreased as the concentrations of methotrexate in serum decreased. In an interference study we found that methotrexate added to sera containing a wide range of basal Bc concentrations increased the measured Bc concentration in a linear and dose-related fashion. Methotrexate also interfered negatively with measurements of unconjugated bilirubin (Bu). The source of the interference appears to be an overlap in the absorption spectrum of methotrexate with Bc and Bu at 400 nm.