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 degrees 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 microL of protein-free ultrafiltrate, and with it any laboratory with a semi-micro calcium method can measure ultrafiltrable calcium.