Source of error on A-aDO2 calculated from blood stored in plastic and glass syringes.

We studied the effects of time delay on blood gases, pH, and base excess in blood stored in glass and plastic syringes on ice and the effects of resulting errors on calculated alveolar-to-arterial PO2 difference (A-aDO2). Matched samples of dog whole blood were tonometered with gas mixtures of 5% CO2-12% )2-83% N2 (mixture A), 10% CO2-5% O2-85% N2 (mixture B), and 2.88% CO2-4% O2-93.12% N2 (mixture C). Tonometered blood samples were transferred to 5-ml glass (5G), 5-ml plastic (5P), and 3-ml plastic (3P) syringes and stored on ice. Blood gases were measured every 1 h up to 6 h. In 5G, PO2 progressively decreased in blood tonometered with mixture A but rose in blood tonometered with mixtures B and C. O2 saturation progressively fell in all cases. In 5G, blood PCO2 progressively rose regardless of which gas mixture was used, and pH as well as base excess progressively fell. The rise in PO2 was faster in plastic than in glass syringes, and O2 saturation always rose in plastic syringes. Differences between storage in plastic and glass syringes on PO2 change were greatest when initial blood PO2 was highest (mixture A). At the highest PO2, O2 exchange was faster in 3P than in 5P. The rise of PCo2 was just as fast in plastic as in glass syringes, but in both the rise in PCO2 was faster at a higher initial PCO2 (mixture B) than in lower initial PCO2 (mixtures B and C). Rates of PO2 and PCO2 change in matched samples were significantly faster in 3P than in 5P. Errors due to rises in PCO2 and PO2 cause additive errors in calculated A-aDO2, and when blood is stored in plastic syringes for > 1 h significant errors result. Errors are greater in normoxic blood, in which estimated A-aDO2 decreased by > 10 Torr after 6 h on ice in plastic syringes, than in hypoxic blood.