Question 17.2

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%

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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.

The causes of that could be:

  • Carboxyhaemoglobin
  • Methaemoglobinaemia
  • Radiofrequency interference

Interestingly, only mild methaemoglobinaemia should be on that list. Pulse oximetry measurement of mixed normal haemoglobin and methaemoglobin will usually be depressed. When one's methaemoglobin level is in excess of 35%, the pulse oximeter will usually read 85% (Barker et al, 1989). The pulse oximeter will then continue to read this value, whether the oxygenation deteriorates or improves, i.e. at 60% methaemoglobin concentration and a fractional oxygen saturation of 30% or 100%, it will still give you an SpO2 of 85%. This is why highly respected resources report that "methemoglobinemia typically causes the pulse oximeter to report a [pulse oximeter] saturation of ~82-86% (even if the PaO2 is very high)." 

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.

Watcha, Mehernoor F., Michael T. Connor, and Anne V. Hing. "Pulse oximetry in methemoglobinemia." American Journal of Diseases of Children 143.7 (1989): 845-847.

Barker, Steven J., Kevin K. Tremper, and John Hyatt. "Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry." The Journal of the American Society of Anesthesiologists 70.1 (1989): 112-117.