= Blood gas analysis
\b[Blood gas analysis] characterizes a specified <blood> specimen using gas tensions and acid-base measurements. Core measured quantities include <oxygen> and <carbon dioxide> <partial pressures> and <pH>; instruments can also report derived quantities. <Co-oximetry> supplies related information about <hemoglobin> species by optical measurement rather than inferring all of them from gas tensions.<AARC Clinical Practice Guideline: Blood Gas Analysis and Hemoximetry: 2013 by Michael D. Davis and others>[Section 1.0, Description, measured and derived variables]{ref}
The specimen's source is part of the result. An arterial sample addresses a different compartment from a peripheral venous sample, where tissues have already exchanged gases with the blood. The sample is analyzed outside the circulation, so collection time, inspired gas, temperature convention and sample handling affect interpretation. The guideline identifies contamination, delay and temperature conventions as distinct limitations.<AARC Clinical Practice Guideline: Blood Gas Analysis and Hemoximetry: 2013 by Michael D. Davis and others>[Sections 4.6 and 6.1.2–6.1.9]{ref}
A calculation can look like an additional measurement. For an illustrative bicarbonate equilibrium approximation at a stated temperature, the <Henderson-Hasselbalch equation> can be written
$$
\mathrm{pH}=6.1+\log_{10}\!\left(\frac{[\mathrm{HCO_3^-}]}{0.030\,P_{\mathrm{CO_2}}}\right),
$$
with <bicarbonate> concentration in mmol/L and carbon-dioxide <partial pressure> in mmHg. The numerical constants specify this approximation; they are not universal across all temperatures and media. Rearranging gives
$$
[\mathrm{HCO_3^-}]=0.030\,P_{\mathrm{CO_2}}10^{\mathrm{pH}-6.1}.
$$
For stipulated <pH> 7.40 and $P_{\mathrm{CO_2}}=40$ mmHg, the calculated value is about 23.94 mmol/L. Holding <pH> fixed and changing that <partial pressure> to 60 mmHg raises the calculation to about 35.91 mmol/L. Those derived concentrations do not constitute a third independent observation validating the two inputs.
A sample-specific gas <partial pressure> also differs from total gas content. Much of transported <oxygen> is bound to <hemoglobin>, while measured oxygen tension concerns dissolved <oxygen>. A panel should therefore identify which values were directly measured, which were computed and which specimen supplied them. Agreement among quantities calculated from one another is internal arithmetic consistency, not independent physiological confirmation.
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