Certified Pulmonary Function Technologist (CPFT) Practice Exam

What impact does allowing an ABG sample to sit out without ice have on its values?

1. PO2 increase, PCO2 decrease, pH increase

2. PO2 decrease, PCO2 increase, pH decrease

3. PO2 decrease, PCO2 decrease, pH increase

4. PO2 increase, PCO2 increase, pH decrease

The correct answer is: PO2 decrease, PCO2 increase, pH decrease

When an arterial blood gas (ABG) sample is allowed to sit at room temperature without ice, several biochemical changes occur that directly affect its values. The partial pressure of oxygen (PO2) typically decreases over time as oxygen is consumed by cellular metabolism in the sample. This is because the sample, composed of living cells, will continue to metabolize, leading to a reduction in available oxygen. Simultaneously, the partial pressure of carbon dioxide (PCO2) usually increases. As cells metabolize glucose and produce carbon dioxide as a waste product, the CO2 accumulates in the sample while the oxygen is being utilized. This is a direct consequence of cellular respiration occurring even after the sample has been taken. The increase in PCO2 leads to a decrease in pH, reflecting the acid-base balance affected by the rising carbon dioxide levels. The production of carbonic acid from the increase in PCO2 decreases the pH, which represents an acidic shift in the sample. Therefore, allowing an ABG sample to sit out without ice results in a decrease in PO2, an increase in PCO2, and a decrease in pH, which corresponds with the values associated with a metabolic process continuing post-collection.