A rapid method for the determination of ultrafilterable calcium in serum

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.