Measurement of lonized magnesium in biological fluids

A method that provides for direct in vitro measurement at 37°C of ionized magnesium in biological fluids using the Abbot bichromatic analyzer, a fully automated instrument is described. The metallochromic dye Eriochrome blue SE was used to measure ionized magnesium levels in Mueller-Hinton agar fluids. Formulation of an appropriate blank was found necessary and was made by treating agar fluids with Chelex 100 to remove divalent cations. The influence of pH in the biological range was examined and poses a significant but manageable source of error. The method allowed an appropriate assessment of the role of ionized magnesium in the susceptibility testing of Pseudomonas aeruginosa on Mueller-Hinton agar with the aminoglycoside antibiotics.