A RAPID METHOD FOR THE DETERMINATION OF ULTRAFILTERABLE CALCIUM IN SERUM

  • 1 January 1981
    • journal article
    • research article
    • Vol. 98  (5) , 691-696
Abstract
Alterations of Ca homeostasis may be expressed as hypercalcemia, hypocalcemia and normocalcemia with abnormal distribution of serum Ca fractions. The accurate diagnosis of disorders of Ca homeostasis requires the direct determination of free (ionized) serum Ca. The major problem in the direct measurement of ionized Ca with a Ca-sensitive electrode is the dependence on an anaerobic environment. The determination of ultrafilterable (unbound) Ca in serum was evaluated as an index of free Ca concentration. Ultrafilterable Ca measured in normal subjects by membrane binding analysis was (mean .+-. SEM [standard error of the mean]) 4.8 .+-. 0.14 mg/dl. The intra-assay coefficient of variation (CV) was 2.9% for ultrafilterable Ca and 3.0% for the calculated fraction bound to proteins. Storage at 0.degree. C had minimal effects: the interassay CV was 5.1% for ultrafilterable Ca and 8.5% for the protein-bound fraction. The same determinations in a serum pool standard showed an interassay CV of 4.8 and 4.2%, respectively. Freezing and thawing for up to 3 times did not affect ultrafilterable Ca concentration. In 56 subjects who were normocalcemic, hypercalcemic, or hypocalcemic, simultaneous determination of ionized Ca (by a Ca electrode) and ultrafilterable Ca showed a correlation coefficient of 0.91 (P < 0.001). These results show that membrane-binding analysis by ultrafiltration is a valid method, suitable for the routine determination of unbound (free) Ca in normal subjects and in patients with disorders with Ca homeostasis.

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