Weak relationship between ionized and total magnesium in serum of patients requiring magnesium status

Measurement and monitoring of magnesium (Mg) are important to prevent the development of serious and potentially fatal complications in critically ill patients. Although ion-selective electrodes are available and earlier reports suggest that free ionized magnesium (iMg(2+)) is the most useful test to estimate Mg status, most clinical laboratories still only measure total Mg. To compare the relationship among iMg(2+), total Mg, and albumin in serum, samples were collected from 48 consecutive patients admitted to an intensive care unit or a primary health center. The mean serum level of iMg(2+) in 44 patients was 0.53 mmol/L, the total Mg was 0.96 mmol/L, and the albumin was 34.93 g/L. The correlation between iMg(2+) and total Mg in serum was r=0.585; the correlation between iMg(2+) and albumin in serum was r=378; and the correlation between total Mg and albumin in serum was r=0.340. The mean percent iMg(2+) in relation to total Mg in serum was calculated to be 55% in the patient samples. The important level of biologically active iMg(2+) was not reflected upon analysis of total Mg in 25% of consecutive patients. This report shows that the correlation of iMg(2+) and total Mg is weak, not only in critically ill patients but also in patients in whom Mg status is inquired as a whole.