Adsorption of serum calcium by plastic sample cups.

Sera left overnight in plastic AutoAnalyzer sample cups may give low calcium values; the effect is attributed to adsorption of calcium onto the walls of the vessel. The adsorption is brought about by a rise in the pH of the sera, and factors which promote the rise in pH increase the adsorption. This phenomenon is of practical importance because as much as 10% of the calcium in the serum may be adsorbed. Adsorption occurs particularly onto the walls of polystyrene cups, and when polypropylene cups were used the adsorption was reduced. The phenomenon cannot be evaluated or controlled by the use of control sera. In order to avoid the sampling error, serum for calcium analysis should be used fresh or stored at 4 degrees C under conditions such that any change in pH is minimal. Sera should not be left to stand in AutoAnalyzer cups at room temperature for longer than three hours before analysis.