A blood gas analzyer typically only measures the PO2 and pH and calculates oxygen saturation by assuming a normal hemoglobin oxygen dissociation curve (although new technologies are emerging).
If you want to measure methemoglobin or carboxyhemoglobin (CO) then you need to use a co-oximeter which uses four different spectrophotometric wavelengths to directly measure oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin.
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