What is oxygen saturation?

There are many different terms that are frequently (and sometimes incorrectly) referred to as ‘oxygen saturation.’ Here we define them:

• ‘Oxygen saturation’ – A quantification of the proportion of hemoglobin inside red blood cells that is bound by oxygen. This can refer to multiple measurements, but most commonly is used to refer to functional arterial oxygen saturation.  
• SO2 or ‘functional oxygen saturation’ – The fraction of effective hemoglobin (e.g. excluding Hb species like CO-Hb or met-Hb and only measures oxy- and deoxy-Hb). This is expressed as (HbO2/[Hb+HbO2]).
• SpO2 – Functional, arterial oxygen saturation measured by pulse oximetry.
• FO2Hb or ‘fractional oxygen saturation’ – The fraction of total hemoglobin (Hb) that is carrying oxygen. Total Hb includes not only oxy- and deoxy-Hb, but also other hemoglobin species like Met-Hb, CO-Hb, S-Hb. Fractional oxygen saturation can be measured by relatively few pulse oximeters, and usually requires blood gas analysis and a capable co-oximeter. This is expressed as (HbO2/[Hb+HbO2+MetHb + COHb + SHb]).
• SO2 or SaO2 – Arterial oxygen saturation measured by arterial blood gas. 
• SvO2 – Venous oxygen saturation. 
• ScvO2 – Central venous oxygen saturation. 

Keywords: oxygen saturation, SpO2, SaO2, SvO2, SO2