Simultaneous Measurement of Ionized and Total Calcium in Intensive Care Unit Patients

Purpose: This study was undertaken to determine the relationship between total magnesium and ionized magnesium in critically ill and injured patients. Methods: Eighty consecutive intensive care unit (ICU) admissions were evaluated and 34 patients were enrolled in the study. Patients were enrolled who had in- dwelling arterial catheters and were within 4 days of ICU admission. Six milliliters of blood was collected and assayed simultaneously for total and ionized magnesium, total and ionized calcium, and albumin level. An Acute Physiology and Chronic Health Evaluation (APACHE II) score was calculated at the time of blood collection. Results: The results of our study show a strong correlation between ionized and total magnesium (R =.903) that was not seen between ionized and total calcium (R = 0.748). We found total hypomagnesemia in 18% and ionized hypomagnesemia in 21% of ICU patients. We also found that 14.7% (5 of 34) of our patients had ionized hypermagnesemia whereas none displayed total hypermagnesemia. We did not find a correlation between APACHE II, sex, race, albumin level, and any electrolyte level. The mortality rate in the subjects studied was 21% (7 of 34). Conclusions: Based on our results we would recommend that intensivists directly measure ionized calcium whereas ionized magnesium can be inferred from total magnesium.