= Carboxyhemoglobin
{title2=$\mathrm{COHb}$}
\b[<Carboxyhemoglobin>] is <hemoglobin> with <carbon monoxide> bound at a <haem> site. Carbon monoxide competes with <oxygen> for binding; a CO-occupied site is unavailable for oxygen while CO remains bound.<Hemoglobin by Radiometer>[Additional manufacturer parameter page, Carboxyhemoglobin, What is COHb?]{ref} This differs from <methemoglobin>, where the defining change is oxidation of haem iron. Binding carbon monoxide is not itself the same operation as converting an available ferrous site into a ferric site.
An original independent-site equilibrium model illustrates competition. Let oxygen and carbon-monoxide binding weights be $u=p_{\mathrm O_2}/K_{\mathrm O_2}$ and $v=p_{\mathrm{CO}}/K_{\mathrm{CO}}$, with $K$ values defined on the same pressure basis. For empty, oxygen-bound and CO-bound states,
$$
f_0=\frac1{1+u+v},\qquad
f_{\mathrm O_2}=\frac{u}{1+u+v},\qquad
f_{\mathrm{CO}}=\frac{v}{1+u+v}.
$$
With stipulated $u=9$ and no carbon monoxide, the oxygen-bound fraction is $9/10$. Adding a competing weight $v=9$ changes it to $9/19$ and gives CO the same fraction. These weights are hypothetical; this model omits the cooperative and conformational behavior of real hemoglobin.
The calculation explains why knowing the oxygen <partial pressure> alone does not identify the amount bound to <hemoglobin>. Available-site count, competing ligands and binding properties also enter. Even a high occupied fraction among the remaining oxygen-capable sites does not imply that all haem sites are oxygenated.
<Co-oximetry> separates <carboxyhemoglobin> from other derivatives using their spectral responses. The 1991 fetal/adult hemoglobin study quantified derivative absorptivities and emphasized that even small spectral differences affect multicomponent analysis.<Absorption spectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhemoglobin, and methemoglobin by W. G. Zijlstra and others>[Abstract]{ref} Identifying a derivative therefore requires a suitable measurement model; total hemoglobin concentration alone leaves its ligand composition unresolved.
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