Effect of Carbon Dioxide on Concentration of Calcium in an Ultrafiltrate of Serum Obtained by Centrifugation

¹ From the Medical Service, Veterans Administration Hospital and the Department of Internal Medicine, University of Minnesota, Minneapolis, Minnesota

A simple method is described for obtaining ultrafiltrate of serum or other biological fluids, using an ordinary centrifuge. Ten milliliters of serum yield 2.5–3.0 ml of protein free ultrafiltrate in 75 minutes. The ultrafiltrable calcium of serum from healthy human subjects was found to be 45–57% of the total calcium, when the serum was handled anaerobically and the final ph was in the range of 7.3–7.7. The result of experiments carried out at 37°C were the same as those carried out at room temperature. When a mixture of 5% CO₂ in 95% O₂ was bubbled through the serum, an initial lowering of ph and increase in CO₂ content resulted, and the range of ultrafiltrable calcium in healthy human subjects was found to be 57–76%. Therefore, an artefact is produced by equilibrating separated serum with 5% CO₂ in 95% O₂, thus accounting for higher values of ultrafiltrable calcium in normals as reported by some other authors.

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