Aim: Magnesium deficiency was investigated in critically ill patients, comparing measurements of plasma concentrations with the results obtained by the magnesium tolerance test.
Design and methods: 20 critically ill patients (5 females, 15 males) between the ages of 27 and 86 were investigated. Magnesium plasma concentrations were determined before the magnesium tolerance test according to Ryzen was performed. For this purpose, magnesium sulfate (0.1 mmol/kg) was infused intravenously over four hours. Renal magnesium excretion was measured in the 24 h urine beginning at the start of the infusion. Magnesium concentrations in plasma and urine were determined using atomic absorption spectrophotometry.
Main results: In 12 patients magnesium plasma concentrations were decreased to 0.58-0.79 mmol/l. 6 patients showed values within the normal range of 0.80 to 1.0 mmol/l. In 2 patients the plasma concentration was increased to 1.07 and 1.27 mmol/l. Parenteral magnesium tolerance testing revealed a considerable magnesium deficiency by retention of 65-100% of the loading dose in 14 of the 20 patients. The remaining 6 patients retained 23-48% of the loading dose, thus demonstrating a moderate magnesium deficiency.
Conclusion: Determination of magnesium plasma concentration appears suitable as an informative preliminary survey, since low values are reliable indicating a magnesium deficiency. However, this study confirms that normal magnesium plasma concentrations do not exclude a considerable magnesium deficiency, which is more sensitively determined by the magnesium tolerance test.