Wortsman J, Killam L I, Traycoff R B
J Lab Clin Med. 1981 Nov;98(5):691-6.
Alterations of calcium homeostasis may be expressed as hypercalcemia, hypocalcemia, and normocalcemia with abnormal distribution of serum calcium fractions. The accurate diagnosis of disorders of calcium homeostasis requires the direct determination of free (ionized) serum calcium. The major problem in the direct measurement of ionized calcium with a calcium-sensitive electrode is the dependence on an anaerobic environment. We evaluated the determination of ultrafilterable calcium in serum as an index of free calcium unbound concentration. Ultrafilterable calcium measured in normal subjects by membrane binding analysis was (mean +/- S.E.M.) 4.8 +/ 0.14 mg/dl. The intra-assay coefficient of variation was 2.9% for ultrafilterable calcium and 3.0% for the calculated fraction bound to proteins. Storage at 0 degrees C had minimal effects: the interassay coefficient of variation was 5.1% for ultrafilterable calcium and 8.5% for the protein-bound fraction. The same determinations in a serum pool standard showed an interassay coefficient of variation of 4.8% and 4.2%, respectively. Freezing and thawing for up to three times did not affect ultrafilterable calcium concentration. In 56 subjects who were normocalcemic, hypercalcemic, or hypocalcemic, simultaneous determination of ionized calcium (by a calcium electrode) and ultrafilterable calcium showed a correlation coefficient of 0.91 (p less than 0.001). These results show that membrane-binding analysis by ultrafiltration is a valid method, suitable for the routine determination of unbound (free) calcium in normal subjects and in patients with disorders with calcium homeostasis.
钙稳态的改变可能表现为高钙血症、低钙血症以及血清钙组分分布异常的正常钙血症。准确诊断钙稳态紊乱需要直接测定游离（离子化）血清钙。使用钙敏感电极直接测量离子化钙的主要问题是依赖厌氧环境。我们评估了血清中可超滤钙的测定作为未结合游离钙浓度的指标。通过膜结合分析在正常受试者中测得的可超滤钙为（均值±标准误）4.8±0.14mg/dl。可超滤钙的批内变异系数为2.9%，计算得出的与蛋白质结合部分的变异系数为3.0%。在0℃储存影响极小：可超滤钙的批间变异系数为5.1%，蛋白质结合部分为8.5%。在血清库标准品中进行的相同测定显示批间变异系数分别为4.8%和4.2%。冷冻和解冻多达三次不影响可超滤钙浓度。在56名血钙正常、高钙血症或低钙血症的受试者中，同时测定离子化钙（通过钙电极）和可超滤钙，相关系数为0.91（p<0.001）。这些结果表明，通过超滤进行膜结合分析是一种有效的方法，适用于正常受试者以及钙稳态紊乱患者中未结合（游离）钙的常规测定。
