Effect of hematocrit and added heparin on ionized calcium in capillary blood samples from neonates.

Sampling of capillary blood for determination of ionized calcium (Ca2+) in neonates requires that extra heparin be added to prevent clotting in the sampling tube and (or) in the Ca2+ analyzer. Because the additive dissolves in the plasma compartment, different hematocrit (erythrocyte volume fraction, EVF) values may cause different results for Ca2+. To study the effect of EVF and heparin additive, we repeatedly removed plasma, thereby increasing the EVF. These samples with different EVF's were aspirated into commercial capillary tubes containing heparin and, according to our routine procedure, an additional 10 microL (approximately 0.9 int. unit) of sodium heparin. We found a negative bias of 0.05-0.09 mmol/L in Ca2+, depending on the EVF. Adding saline instead of heparin gave the same effect, indicating that this bias was entirely due to dilution. We suggest compensating for this by adding 0.09 mmol/L to the actual value for ionized calcium when EVF exceeds 70%. The increase in Ca2+ in neonates on days 1 to 5 postpartum is physiological and not an effect of change in EVF.