= Functional oxygen saturation
{title2=$s_{\mathrm O_2}$}
\b[Functional oxygen saturation] is the fraction of the sum of <oxyhemoglobin> and <deoxyhemoglobin> that is present as <oxyhemoglobin>:
$$
s_{\mathrm O_2}=\frac{H_O}{H_O+H_D}.
$$
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.<Oxyhemoglobin by Radiometer>[FO2Hb versus oxygen saturation (sO2)]{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, then
$$
F_{\mathrm O_2\mathrm{Hb}}=s_{\mathrm O_2}(1-d).
$$
Thus 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.
Back to article page
