Pulse oximetry validation studies rely on precise reference measurements to determine device accuracy. One of the most important concepts in these studies is understanding the difference between SpO₂ and SaO₂. Although the terms are closely related, they are not interchangeable. What Is SpO₂? SpO₂ refers to oxygen saturation estimated noninvasively by a pulse oximeter. Pulse oximeters use optical sensors and photoplethysmography (PPG) signals to estimate oxygen saturation