Unless blood gas measurements are made immediately after sampling, in vitro glycolysis of the blood causes a:

• A. Rise in pH and PCO2
• B. Fall in pH and rise in PO2
• C. Rise in pH and fall in PO2
• D. Fall in pH and rise in PCO2

Answer: (D)

After a blood sample is drawn, the cells there keep metabolizing glucose through glycolysis. This ongoing glycolysis produces lactic acid, which releases hydrogen ions and lowers the pH of the sample. At the same time, CO2 is generated as part of cellular metabolism; in a sealed or poorly ventilated sample, that CO2 accumulates and increases the partial pressure of CO2 (PCO2). Oxygen is also consumed, so PO2 tends to fall, but the characteristic change described here is acidification with rising PCO2. Therefore, in vitro glycolysis causes a fall in pH and a rise in PCO2. To avoid these artifacts, blood gas samples should be analyzed promptly or kept cold to slow glycolysis.