Difficulties in the assay of liposomal amikacin (MiKasome) in serum

Antibiotic-free human serum was spiked with known concentrations of liposomal amikacin and assayed on the Abbott TDx System, using polarization fluoroimmuno assay (PFIA) kits from Abbott Laboratories, Oxis and Sigma. Although all three kits gave a linear response, the Abbott and Oxis kits showed very low recovery (<21%) with only the Sigma kit giving near 100% recovery. Heating samples at 56 degrees C for 30 min improved recovery with the Abbott and Oxis kits (75-80% of target value), but decreased recovery with the Sigma kit (85% of target value). The loss of amikacin from liposomal amikacin, as measured using the Sigma kit, was related to both temperature and duration of heating, reaching a maximal loss of 21% after 1 h at 60 degrees C.