Encountering and Solving the Negative Error of Calcium Values in Post-Hemodialysis Plasma During Storage

Abstract

The total calcium values of some post-hemodialysis plasmas measured by two different chelation procedures, the o-CPC colorimetric and the calcein fluorometric titration methods, decrease markedly when samples are stored in a refrigerator.After one day storage, the decrease was found for 9% of all plasmas examined (9 out of 100).Extention of storage time and increases in temperature and pH magnified the decrease.The extent of the decrease (ΔmEq/l) correlated well with the concentration of free fatty acids in the plasma (corrrelative coefficient r=0.81).Treatment of the plasmas whose calcium values had dropped during storage with an inorganic acid (volume ratio of 1: 20 for plasma to 50mmol/l HCl) solved this problem practically.These findings suggest that this phenomenon is due in part to complex formation between calcium and free fatty acids.