17.2.   Examine the data provided from a co-oximeter and a simultaneous pulse oximeter recording from patient A and B.  List three (3) causes in each patient for the discrepancy between the two oximeters.

Patient A:

Co-oximeter Oxy Hb 85%

Pulse     oximeter      oxygen saturation 95%

Patient B:

Co-oximeter Oxy Hb 98%

Pulse     oximeter      oxygen saturation 88%

[Click here to toggle visibility of the answers]

College Answer

Patient A: 
Met Hb
Radiofrequency interference

Patient B: 
Tricuspid regurgitation
Ambient light
Poor peripheral perfusion Dyes- Methylene blue

Poor probe contact


The pulse oximeter is a dumb machine, whereas the co-oximeter will measure lots of different subtypes of haemoglobin simultaneously.

In general terms, the co-oximeter is correct, and the pulse oximeter is frequently confused.

Thus, in Patient A, the co-oximeter reads 85% (the true saturation of haemoglobin) while the pulse oximeter reads 95%. Clearly, there is some haemoglobin here which closely resembles normal oxygenated haemoglobin, but is in fact carrying no oxygen.

There do not seem to be causes for this apart form those suggested by the college:

  • Carboxyhaemoglobin
  • Methaemoglobin
  • Radiofrequency interference

In Patient B, the co-oximeter confirms a normal oxygen saturation of haemoglobin; however, something is confusing the pulse oximeter.

  • Poor peripheral perfusion
  • Ambient light
  • Poor probe contact
  • Dyes – methylene blue, indocyanine green
  • Tricuspid regurgitation


Here is the operations manual for an AVOXimeter 4000.


Barker, Steven J., et al. "Measurement of carboxyhemoglobin and methemoglobin by pulse oximetry: a human volunteer study." Anesthesiology105.5 (2006): 892-897.


Mathews Jr, P. J. "Co-oximetry." Respiratory care clinics of North America 1.1 (1995): 47-68.