Listed below are the co-oximetry data for two patients A & B. What would the pulse oximetry measured saturation be for patient A and patient B?  Give reasons for your answer.
.

Patient A

Patient B

Oxyhaemoglobin (%)

70

50

Reduced  haemoglobin (%)

10

10

Carboxyhaemoglobin (%)

20

40

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College Answer

Patient A – 90%

Patient B -  90%

Pulse oximetry only uses 2 wavelengths  and COHb  is measured as OxyHb.

Discussion

Well, this is a flaw of pulse oximetry. Both the oxygenated hemoglobin and the "carboxygenated" COHb will be measured as 100% saturated. Thus, in both patients, the oxygen saturation will read 90%.

References

Pulse oximetry is discussed at lengths in a fine article:

Mendelson, Yitzhak. "Pulse oximetry: theory and applications for noninvasive monitoring." Clinical chemistry 38.9 (1992): 1601-1607.

Co-oximetry is a fine thing indeed. LITFL has a nice summary.