Abstract
Total serum calcium and magnesium concentrations are frequently abnormal in critically ill patients despite the occurrence of normal ionized levels. Since ultrafilterable calcium and magnesium values approximate ionized levels, we evaluated the accuracy of ultrafilterable levels in critically ill patients. Ultrafilterable serum calcium concentrations were sensitive (95%) and specific (93%) in predicting ionized hypocalcemia. Total serum calcium values were sensitive (93%) but lacked specificity (50%) in predicting ionized hypocalcemia. Total serum magnesium concentrations were sensitive (100%) but not specific (73%) in predicting ultrafilterable hypomagnesemia. We conclude that ultrafilterable calcium concentrations offer an alternative to ionized calcium measurements when an ion-selective electrode is not available. In addition, ultrafilterable magnesium levels may reflect more accurately the true ionized magnesium concentrations than total serum magnesium values.
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