= Co-oximetry
Co-oximetry determines hemoglobin composition using optical measurements at multiple <wavelengths>. In a laboratory blood analyzer, it can distinguish <oxyhemoglobin>, <deoxyhemoglobin>, <carboxyhemoglobin> and <methemoglobin> rather than infer all hemoglobin behavior from a single oxygen-related number. A manufacturer description explicitly identifies direct optical <spectrophotometry> as the measurement method for its analyzer's co-oximetry parameters.<Confidently manage critical moments in pediatric emergency care by Radiometer>[Confidence in results subsection, direct optical spectrophotometry]{ref}
An idealized mixture model makes the separating information explicit:
$$
A_j=\ell\sum_i\varepsilon_i(\lambda_j)c_i+b_j,
$$
where $A_j$ is measured <absorbance>, $\ell$ the path length, $c_i$ a species concentration, $\varepsilon_i$ its <molar absorption coefficient> and $b_j$ a background term. Estimating four unconstrained concentrations requires sufficient independent spectral observations and known response coefficients; merely having four wavelength labels does not guarantee independence. An <isosbestic point> shared by two species supplies information about their combined amount but cannot by itself separate their proportions.
The denominator of a reported fraction matters. In a simplified four-form example, stipulate $90\%$ oxyhemoglobin, $5\%$ deoxyhemoglobin, $4\%$ carboxyhemoglobin and $1\%$ methemoglobin, all expressed on the same total-hemoglobin basis. <Fractional oxyhemoglobin> is $0.90$, but the functional <oxygen saturation> is
$$
sO_2=\frac{c_{O_2Hb}}{c_{O_2Hb}+c_{HHb}}=\frac{90}{90+5}\approx0.9474.
$$
These differ because the second denominator excludes the other two forms. The manufacturer's parameter description distinguishes these definitions.<Oxyhemoglobin by Radiometer>[Fractional oxyhemoglobin versus functional oxygen saturation]{ref} The chosen fractions are an explanatory example, not clinical thresholds.
A blood-sample optical assay and a noninvasive pulsatile optical measurement use different observation models. Their output labels and denominators must be established from the specific method, rather than inferred from the common word oximetry. Likewise, oxygen <partial pressure> describes dissolved oxygen equilibrium and is not itself a measurement of all hemoglobin fractions.
An instrument test can compare reference mixtures with independently established composition, record full spectral residuals and vary a suspected interferent. Agreement of one derived saturation number alone does not validate every inferred species concentration. Background, spectral overlap and sample handling belong in the measurement model and its uncertainty.
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