In arterial blood gas sample handling, pH decreases by how much for every 20-minute delay?

• A. 0.01
• B. 0.1
• C. 1.0
• D. 0.001

Answer: (A)

pH drift in arterial blood gas samples during handling comes from ongoing cellular metabolism after collection. Cells in the blood continue to produce CO2 and lactic acid, which increases acidity and lowers pH. Because this process proceeds at a roughly steady rate at room temperature, the pH drops about 0.01 units for each 20-minute delay before analysis. That’s why prompt analysis or cooling the sample to slow metabolism is essential to preserve the true pH. The other options describe too large or too tiny a change for the typical preanalytic drift observed in practice.