The effect of draw-up volume on the accuracy of electrolyte measurements from neonatal arterial lines.
OBJECTIVE
Contamination by infusate of blood samples withdrawn from arterial lines has been recognized but not well documented for neonates. The aim of this study was to investigate, using in vitro and in vivo studies, the effects of different draw-up volumes (withdrawn from the line prior to the sample being taken) on the concentration of sodium.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 space 0.6 mL), 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 mL of this infusate just before each sample was taken. Volumes from 0.5 mL to 2.0 mL of infusate/normal saline were withdrawn in 0.1 mL increments from a three-way tap and discarded. A sample was then taken from the line into a blood gas syringe for analysis of the sodium concentration by the 860 Blood Gas Analyzer (Chiron Diagnostics, Bayer, Scoresby). Control samples were taken from the beaker. In-vivo study: A 22 gauge intravenous catheter was inserted into a vein of an adult male volunteer. The dead space was also 0.6 mL. The line was flushed with 5 mL of half-normal saline immediately before sampling. Draw-up volumes of 0.6, 0.9, 1.3, and 1.6 mL were withdrawn and discarded. 10 mL was used as a control.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 mL was required before the sodium concentration was not significantly different from the control samples. In-vivo: A minimum draw-up volume of 1.6 mL was required before the sodium concentration was not significantly different from the control samples. There were similar trends in the effect of draw-up volume for glucose, calcium, potassium, chloride and lactate.


CONCLUSIONS
: A minimum volume of 1.6 mL should be withdrawn from neonatal arterial lines (dead space 0.6 mL) before taking blood for analysis.
Davies
M W
MW
Division of Neonatal Services, Royal Women's Hospital, Carlton, Victoria, Australia.
Mehr
S
S
Morley
C J
CJ
D000328:Adult; D002408:Catheters, Indwelling; D004573:Electrolytes / Q000097:blood*; D006801:Humans; D066298:In Vitro Techniques; D007231:Infant, Newborn; D007234:Infant, Premature; D007261:Infusions, Intra-Arterial; D008297:Male; D012964:Sodium / Q000097:blood*; D014469:Umbilical Arteries
D004573:Electrolytes; D012964:Sodium