SpO2 measures what aspect of oxygen in the blood?

• A. Percentage of hemoglobin bound to oxygen
• B. Partial pressure of oxygen
• C. Oxygen content
• D. Oxygen delivery

Answer: (A)

SpO2 reflects the percentage of hemoglobin that is bound to oxygen. Pulse oximetry estimates how much of the available hemoglobin is carrying oxygen by comparing light absorption from oxyhemoglobin versus deoxyhemoglobin, giving a noninvasive measure of Hb saturation with oxygen.

This is not the same as the partial pressure of oxygen in plasma (PaO2), which indicates how much dissolved oxygen is present, nor is it the total oxygen content in blood, which includes both oxygen bound to hemoglobin and dissolved oxygen. It also does not directly measure oxygen delivery, which depends on both the saturation and the amount of hemoglobin as well as cardiac output. In short, SpO2 tells you how saturated hemoglobin is with oxygen.