Measurement of ultrafiltrable calcium in serum with use of the 'Worthington Ultrafree Anticonvulsant Drug Filter'

We describe a method for ultrafiltrable calcium involving the use of Worthington Ultrafree Anticonvulsant Drug Filters. When measured at 37°C, with mineral oil covering the sample to prevent loss of CO₂, values for ultrafiltrable calcium correlated reasonably well (r = 0.91) with those for ionized calcium as measured with an ion-selective electrode. All patients' samples with significantly high or low values for ionized calcium were identified by the ultrafiltration method, including one specimen for which the McLean-Hastings protein correction could not explain the discrepancy between ionized and total calcium. The method requires about 2 mL of serum, yields about 100 μL of protein-free ultrafiltrate, and with it any laboratory with a semi-micro calcium method can measure ultrafiltrable calcium.