Ultrafiltrable calcium and magnesium in ultrafiltrates of serum prepared with the Amicon MPS-1 system.

We evaluated the Amicon micropartition system (MPS-1) for preparing ultrafiltrates of serum for use in evaluating ultrafiltrable Ca and Mg. We found no adsorption of either to the filter and 99.6% retention of serum proteins on the membrane. Ultrafiltrate volumes recovered (100-450 microL) varied with centrifugation time (10-30 min) and temperature. Centrifugation time did not affect the measured concentration of ultrafiltrable calcium and magnesium, and pH change in the 1-mL serum specimen during a 30-min centrifugation at room temperature was negligible. There was an inverse relationship between temperature and ultrafiltrable Ca and Mg concentrations. The precision (CV) between filters ranged from 1.2 to 5.1% for ultrafiltrable Ca and 1.5 to 2.7% for ultrafiltrable Mg. The correlation between ultrafiltrable Ca (y) and ionized Ca (x) in samples from 115 patients with calcium-related metabolic disorders was good (y = 1.04x + 0.18; r = 0.9128). We find the MPS-1 to be a simple and convenient tool for the rapid production of serum ultrafiltrates.