Csako G, Rehak N N, Elin R J
Clinical Pathology Department, W. G. Magnuson Clinical Center, National Institutes of Health, Bethesda, Maryland, USA.
Eur J Clin Chem Clin Biochem. 1997 Sep;35(9):701-9.
Changes in serum total and ionized magnesium (Mg and Mg2+) and calcium (Ca and Ca2+) were monitored in three patients who transiently developed severe (total Mg < 0.50 mmol/l) to profound hypomagnesemia (total Mg < 0.35 mmol/l) due to cisplatin or interleukin-2 therapies. Mg2+ and Ca2+ were measured with the Nova ion-selective electrodes at 37 degrees C and all results were normalized to pH 7.40. Independent of the etiology, the Mg2+ fraction (Mg2+/total Mg) increased as the concentration of the serum total Mg decreased in all three patients. When the total Mg was around or below 0.35 mmol/l the Mg2+ approached or exceeded total Mg, suggesting an error in the measurement of Mg2+. The findings were extended by including a group of 31 additional patients whose serum total Mg, Mg2+, total Ca, and Ca2+ concentrations varied from abnormally low to above normal. The serum total and ionized concentrations strongly correlated for both Mg (r2 = 0.88) and Ca (r2 = 0.92). The Mg2+ fraction rapidly increased with a fall in the total Mg concentration (r2 = 0.76) and total Mg/total Ca ratio (r2 = 0.71). In fact, with decreasing total Mg concentrations or total Mg/total Ca ratios, the Mg2+ fraction progressively increased to 93-128% of the total, confirming an error in the Mg2+ determinations. The Ca2+ fraction showed a slight and insignificant decrease with falling total Ca concentrations and total Mg/total Ca ratios. The Mg2+ concentration was directly related (r2 = 0.62), whereas the Ca2+ concentration showed a complex relationship to the total Mg/total Ca ratio. Whether this latter relationship represents a technical artifact or a true biological phenomenon requires further study. The apparent overestimation of Mg2+ at very low total Mg concentrations, and in the presence of a very low total Mg/total Ca ratio, could be due to improper chemometric correction of the Ca effect on the Mg electrode, non-linearity, and inadequate calibration. Whatever the mechanism, the failure of this method to correctly measure very low serum Mg2+ concentrations in the sera of patients with severe hypomagnesemia, or likely in any patient with an unusually low total Mg/total Ca ratio, erodes its diagnostic usefulness.
在3例因顺铂或白细胞介素-2治疗而短暂出现严重（总镁<0.50 mmol/L）至极重度低镁血症（总镁<0.35 mmol/L）的患者中，监测了血清总镁和离子镁（Mg和Mg2+）以及钙（Ca和Ca2+）的变化。使用Nova离子选择电极在37℃下测量Mg2+和Ca2+，所有结果均校正至pH 7.40。无论病因如何，在所有3例患者中，随着血清总镁浓度降低，Mg2+分数（Mg2+/总镁）均升高。当总镁在0.35 mmol/L左右或低于此值时，Mg2+接近或超过总镁，提示镁测量存在误差。通过纳入另外31例患者扩大了研究结果，这些患者的血清总镁、Mg2+、总钙和Ca2+浓度从异常低到高于正常不等。血清总镁和离子镁浓度对于镁（r2 = 0.88）和钙（r2 = 0.92）均高度相关。随着总镁浓度（r2 = 0.76）和总镁/总钙比值（r2 = 0.71）下降，Mg2+分数迅速升高。实际上，随着总镁浓度或总镁/总钙比值降低，Mg2+分数逐渐升高至总镁的93 - 128%，证实了Mg2+测定存在误差。随着总钙浓度和总镁/总钙比值下降，Ca2+分数略有下降但无统计学意义。Mg2+浓度呈直接相关（r2 = 0.62），而Ca2+浓度与总镁/总钙比值呈复杂关系。后一种关系是技术假象还是真正的生物学现象需要进一步研究。在总镁浓度极低且总镁/总钙比值极低的情况下，Mg2+明显高估可能是由于对钙对镁电极的影响进行化学计量校正不当、非线性以及校准不足。无论机制如何，该方法无法正确测量重度低镁血症患者血清中极低的Mg2+浓度，或者可能无法测量任何总镁/总钙比值异常低的患者的Mg2+浓度，这削弱了其诊断价值。
