The effect of draw-up volume on the accuracy of electrolyte measurements from neonatal arterial lines

Objectives: Contamination by infusate of blood samples withdrawn From arter ial lines has been recognized but nut well documented for neonates, The aim of this study was to investigate, using in vitro and in vivo studies, the effects of different draw-up volumes (withdrawn from the line prior to the sample being taken) on the concentration of sodium. Methods: In-vitro study: The tip of an umbilical artery catheter (dead spac e 0.6 mi), infused with half normal saline containing 1 unit/mL of heparin was placed in a beaker of normal saline. The line was flushed with 1 mi of, this infusate just before each sample was taken. Volumes from 0.5 mi to 2. 0 mt of infusate/normal saline were withdrawn in 0.1 mt increments from a t hree-way tap and discarded. A sample was then taken from the line into a bl ood gas syringe for analysis of the sodium concentration by the 860 Blood G as Analyzer (Chiron Diagnostics, Bayer, Scoresby). Control samples were tak en from the beaker. In-vivo study: A 22 gauge intravenous catheter was inse rted into a vein of an adult male volunteer. The dead space was also 0.6 mi . The line was flushed with 5 mi of half-normal saline immediately before s ampling. Draw-up volumes of 0.6. 0.9. 1.3, and 1.6 mt were withdrawn and di scarded. 10 mi was used as a control. A 0.5-mL blood sample was then taken and the electrolyte concentrations analysed immediately. Results: In-vitro: A minimum draw-up volume of 1.3 mi was required brf,rr t he sodium concentration was nut significantly different from the control sa mples. In-vivo: A minimum draw-up volume of 1.6 mi was required before the sodium concentration was not significantly different from the control sampl es, There were similar trends in the effect of draw-up volume for glucose, calcium, potassium, chloride and lactate. Conclusion: A minimum volume of 1.6 mi should be withdrawn From neonatal ar terial lines (dead space 0.6 mL) before taking blood for analysis.