Anticoagulant-induced preanalytical errors in ionized calcium determination on blood

When anticoagulated blood is necessary for ionized calcium (Ca2+) measurements especially in urgent circumstances, the type (sodium or 'calcium-titrated' heparinate) as well as the form (aqueous or dry) of anticoagulant induce preanalytical errors. To quantify these modifications Ca2+ was measured in three aqueous solutions and in three serum pools in different 'sampling' conditions. Incomplete syringe filling and specimen volume/syringe nominal volume ratio effects were tested. Syringes were rinsed (i) with saline to yield 'pure' dilution effect ('solution-dilution'); (ii) with sodium heparinate to study binding; (iii) with calcium-titrated heparinate to evaluate 'calcium-distortion'. All types of errors increased when syringes were not filled to their nominal volume, especially on small-sized specimens. Detailed tables provide percentage error values for all sampling conditions. Thus, 'solution-dilution' can reach -5%; binding is always important (-15 to -50%). 'Calcium-distortion' is minimal, around 1.25 mmol/l Ca2+, but can reach -7% for high Ca2+ and +10% for low Ca2+.