If a blood gas specimen is sealed and left at room temperature for two hours or more, which changes are expected?

• A. PO2 increases, PCO2 increases, pH increases
• B. PO2 decreases, PCO2 decreases, pH decreases
• C. PO2 decreases, PCO2 increases, pH decreases
• D. PO2 increases, PCO2 decreases, pH decreases

Answer: (C)

When a blood gas specimen is sealed and left at room temperature, cellular metabolism within the sample continues. Because there is no gas exchange with the outside air, oxygen inside the sample is used up while carbon dioxide produced by metabolism accumulates. This causes the partial pressure of oxygen to fall (PO2 decreases) and the partial pressure of carbon dioxide to rise (PCO2 increases). The rising CO2 shifts the carbonic acid–bicarbonate equilibrium toward more hydrogen ions, lowering pH (pH decreases). So, after two hours or more, you’d expect decreased PO2, increased PCO2, and decreased pH.