If a blood gas specimen is left exposed to air, which changes will occur?

• A. PO2 and pH increase; PCO2 decreases
• B. PO2 and pH decrease; PCO2 increases
• C. PO2 increases; pH and PCO2 decrease
• D. PO2 decreases; pH and PCO2 increase

Answer: (A)

When a blood gas specimen is exposed to air, gases diffuse between the blood and the surrounding air driven by differences in partial pressures. Ambient air has a high PO2 and essentially no CO2, so oxygen diffuses into the blood while carbon dioxide diffuses out. This raises the dissolved oxygen tension (PO2) in the sample and lowers the dissolved carbon dioxide tension (PCO2). Since CO2 contributes to the formation of carbonic acid, its reduction shifts the pH upward, making the sample more alkaline. So the changes you’d see are an increase in PO2, an increase in pH, and a decrease in PCO2. This artifact is why blood gas samples must be protected from air and analyzed promptly.