Chapter 3 64 Discussion Circulating magnesium (either plasma or serum) is still the most commonly used laboratory test for the assessment of magnesium status, despite only a small fraction of total body magnesium can be found in the circulation. Previous studies have shown that plasma magnesium concentrations correlate poorly with other magnesium tissue pools 2,8. In fact, the body can suffer from severe magnesium deficiency while plasma magnesium concentrations are within the normal range 25. Therefore, we believe that there is a need for a simple and rapid technique to measure intracellular magnesium concentrations. To the best of our knowledge, this is the first study to perform a method comparison between dIEM and iIEM in a large group of healthy participants. The cardinal finding of this study is a strong correlation between dIEM and iIEM, with only a small degree of systematic and proportional bias. These findings indicate that iIEM can be used as a cheaper, less labor intensive alternative for dIEM. In addition, we found that dIEM and iIEM correlated with plasma magnesium, but not with 24-h urinary magnesium excretion. Higher cholesterol and triglyceride concentrations were associated with higher dIEM, iIEM and plasma magnesium, while higher glucose and HbA1c were associated with lower plasma magnesium concentrations. dIEM requires extensive sample preparation, making it labor-intensive which is accompanied by additional costs, and is therefore not suitable for routine laboratory measurements. Issues regarding labor-intensity and high costs of the dIEM were also reported by Deuster et al.14. Particularly, obtaining erythrocytes by means of a washing procedure is time consuming and involves extra costs. In our case, it required two technicians for three hours to prepare fifty samples for dIEM, whereas for iIEM, the same amount of samples could be prepared by one technician in approximately twenty minutes, which makes the sample preparation of dIEM eighteen times more labor-intensive. Overall, personnel costs were four times higher for dIEM, whereas costs for material and additional measurements were comparable between the methods. Altogether, we estimate the total costs of dIEM to be almost twice as high as the total costs of iIEM (US$6.95 per sample vs. US$3.54 per sample, respectively). Thus, iIEM is a simple, rapid, and less expensive technique, which only requires measurement of hematocrit and magnesium in plasma and whole blood, and can therefore be embedded in current routine of clinical measurements. So far, only one study compared dIEM and iIEM 14. However, they included only a small group of men and women (n=10). The systematic bias that we observed can be attributable to the washing process. Removing all the supernatant PBS from the samples after centrifugation is difficult
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