SaO2

SaO2, or arterial oxygen saturation, is the percentage of hemoglobin binding sites in arterial blood that are bound with oxygen. It is a direct estimate of how much oxygen the arterial blood is carrying to tissues and is typically obtained from an arterial blood gas analysis using co-oximetry. This is distinct from SpO2, the noninvasive estimate of oxygen saturation provided by pulse oximetry, which can be influenced by perfusion and other factors.

Measured SaO2 is reported as a percentage and is used in conjunction with the arterial partial pressure of oxygen (PaO2) and hemoglobin concentration to assess oxygenation and oxygen delivery. In healthy individuals, SaO2 is usually about 95–100%. In clinical settings, targets may vary, but values below approximately 92–94% often prompt investigation or intervention, depending on the patient’s condition.

Oxygen delivery to tissues depends on both SaO2 and hemoglobin level. A commonly cited relation is that arterial oxygen content increases with SaO2 and with hemoglobin concentration; dissolved oxygen in plasma (PaO2) contributes a small amount. Disturbances in the oxygen dissociation curve, such as changes in pH, temperature, or 2,3-DPG, can shift the relationship between PaO2 and SaO2. Additionally, dyshemoglobins (for example methemoglobin or carboxyhemoglobin) can alter SaO2 readings by affecting hemoglobin’s ability to bind or release oxygen.

SaO2 is used to evaluate respiratory function, guide oxygen therapy, and monitor patients with acute or chronic lung or cardiovascular disorders. Limitations include that SaO2 requires arterial sampling and co-oximetry, and that SpO2 readings may diverge in certain conditions or with dyshemoglobins.