Abstract
The use of heparin in a liquid form to measure ionized calcium (Ca+ +) in plasma or whole blood can induce preanalytical errors by dilution and by changing the original Ca+ + value by binding or by re-equilibration with calcium in the anticoagulant solution.
To quantify these errors, Ca+ + was measured on serum pools under different sampling conditions. Incomplete syringe filling and specimen volume/syringe nominal volume ratio effects were tested. Syringes were rinsed with saline to yield pure dilution effects, with sodium heparinate to study binding and with calcium-titrated heparinate to evaluate ‘calcium-distortion’. Detailed tables provide percentage error values for all sampling conditions. Dilution errors could reach − 5% and binding was always important (− 14 to − 50%). Distortion was minimal around 1·25 mmol/L but could reach − 4% for high and + 8% for low Ca+ + values. Errors increased when syringes were not filled to their nominal volume, especially with small-sized specimens.