Practical Aspects of a Measurement Technique for Calcium Ion Activity in Plasma

Abstract

Abstract We have evaluated the effects of the following on ion-specific electrode measurement of calcium ion activity in plasma: blood or plasma was stored in various ways for various periods of time, or exposed to air, or small amounts of heparin were added to the blood. Blood may be stored for as long as 120 min at room temperature before calcium ion activity changes measurably, but plasma stored for 20 min already has a slightly increased pH and decreased Ca ion activity. If plasma is frozen or refrigerated anaerobically after being collected, calcium ion activity is not measurably altered, but storage at room temperature is not suitable. If as much as 30 USP units of heparin is added per milliliter of blood and the blood is exposed to air for 1 min, calcium ion activity is unaltered, which allows rapidly processed samples to be used. Therefore, blood obtained by heel- or finger-prick may appropriately be used to measure calcium ion activity, particularly in newborns and infants.