More on the measurement of ionized magnesium in whole blood

Current technology has made it possible to measure ionized magnesium with user-friendly ion selective electrode technology. Although this technology has not reached a perfect status literature shows that it can be used in clinical routine especially in serum. This paper deals with issues concerning the measurement of ionized magnesium in whole blood. To measure in whole blood apart from an instrument with good general performance data an adequate sample treatment is especially critical. We tested a number of different sample containers and found that they interfere in different degrees with the magnesium determination. In some cases this is due to silicone giving falsely high ionized magnesium values which may be twice the real value and even higher. Another problem is the heparinization of the sample. We found that different heparins may alter the result in a different way. The well known complexing capability of heparin may lead to a reduction of the ionized magnesium within the sample and thus to false low results. But some heparins cause elevated results. The reason for this may be zinc. Test procedures to check the quality of anticoagulants are given. Due to these procedures sample containers could be found which do not interfere with the determination of ionized magnesium in whole blood - thus making the measurement possible. All these factors indicate that the measurement of ionized magnesium really needs a very well defined sample handling - otherwise false results may arise. This is not only true for the measurement of whole blood but also for plasma or even for "so called serum" as there might be a sample container within the chain from patient to the instrument which is unsuitable- and thus will alter the result.