SaO2 stands for arterial oxygen saturation, which is the percentage of available binding sites on hemoglobin that are bound with oxygen in arterial blood. It is determined by arterial blood gas (ABG) analysis, which is an invasive procedure that measures the percentage of oxygen on the hemoglobin molecules in the arterial blood. SaO2 is different from SpO2, which is the oxygen saturation level per the pulse oximeter reading. Typically, the discrepancy between SaO2 and SpO2 is about 2% to 3% . SaO2 is an important indicator to assess respiratory failure, and normal SaO2 levels range from 94% to 100% . Levels below 90% indicate hypoxia, which may be a result of underlying conditions such as anemia, acute or chronic kidney failure, uncontrolled diabetes, hemorrhage, shock, or heart failure. SaO2 is mainly determined by PaO2, which is the pressure exerted by oxygen on the arterial wall. The relationship between PaO2 and SaO2 is the familiar sigmoid-shaped oxygen dissociation curve.