Functional oxygen saturation ($s_{{\mathrm{O}}_2}$)

Functional oxygen saturation is the fraction of the sum of oxyhemoglobin and deoxyhemoglobin that is present as oxyhemoglobin:Here $H_O$ and $H_D$ are the corresponding concentrations in the same blood sample. Carboxyhemoglobin and methemoglobin are excluded from this denominator. Fractional oxyhemoglobin instead divides by the total concentration of measured hemoglobin species.^[ref]

A constructed example shows what the exclusion does. If a sample contains 90 concentration units of oxyhemoglobin, 10 of deoxyhemoglobin and 25 of carboxyhemoglobin, its functional oxygen saturation is $90/(90+10)=0.90$. Its fractional oxyhemoglobin is $90/125=0.72$. Both values describe the same mixture, with different reference populations.

If $d$ denotes the fraction of total hemoglobin excluded from the functional denominator, thenThus knowing only $s_{{\mathrm{O}}_2}$ does not determine the oxygenated fraction of total hemoglobin unless $d$ is also known or negligible. In the example, $d=25/125=0.20$ and $0.90(1-0.20)=0.72$.

The distinction has an experimental consequence for co-oximetry. Optical analysis must distinguish the absorbing hemoglobin derivatives before calculating the desired ratio; changing the denominator of the displayed result does not change the blood sample. Total oxygen content additionally requires hemoglobin concentration and the dissolved oxygen contribution. Hemoglobin-concentration and saturation separation therefore remains necessary even when functional oxygen saturation has been measured accurately.