Quantifying the minimum discard sample required for accurate arterial blood gases

The purpose of this study was to determine the minimum amount of discard sample required to remove all heparinized normal saline from an indwelling arterial catheter to obtain an accurate arterial blood gas (ABG) sample. The pH and partial pressure of oxygen (PO2) and carbon dioxide (PCO2) were adjusted to known values prior to the onset of the study. The discard sample size ranged from zero to 5 cubic centimeters (cc), increasing at 0.5-cc increments. Twenty-nine sets of 11 consecutive discard/ABG samples, totaling 319 ABG specimens, were drawn over 20 minutes. Based on the data, a 2-cc discard is sufficient to guarantee accurate blood gases when withdrawing blood from an arterial catheter with a 1-cc dead space volume.