Pseudohypophosphatemia associated with high-dose liposomal amphotericin B therapy

Hypophosphatemia is commonly observed in critically ill patients. Inorganic phosphorus is quantified by spectrophotometric measurement of a phosphomolybdate complex, a method with multiple documented interferents. Our clinical laboratory was contacted to investigate a case of asymptomatic hypophosphatemia in a patient receiving high-dose liposomal amphotericin B therapy (L-AMB). In vitro experiments were performed by spiking L-AMB into residual plasma specimens. Phosphate was measured on the Beckman Coulter AU and Ortho Diagnostics Vitros instruments. When measured on the AU, phosphate in plasma with approximately 250mcg/mL of L-AMB demonstrated a median negative bias of 3.45mg/dL relative to unspiked samples. In contrast, Vitros phosphate measurements demonstrated excellent agreement for specimens with and without L-AMB (median bias −0.2mg/dL). High L-AMB concentrations induced a significant negative bias on phosphate measured by the AU assay, but did not affect the Vitros assay. Laboratorians and clinicians should be aware of this phenomenon in patients receiving L-AMB who develop unexplained hypophosphatemia. •We document a previously unrecognized association between L-AMB therapy and pseudohypophosphatemia on the Beckman Coulter AU.•Phosphate measurements in plasma with 250mcg/mL L-AMB demonstrated a median bias of −3.45mg/dL compared to controls.•The Vitros method appears to be unaffected by high-dose L-AMB.•Clinicians and laboratorians should be aware of this phenomenon in order to avoid inappropriate phosphate repletion.