Co-oximetry has been asked about in Question 26.3  from the second paper of 2008, and in  Question 17.2 from the first paper of 2010. Additionally, Question 19.3  from the first paper of 2009 refers to the "oxygen saturation gap" in regards to the diagnosis of intoxication. Generally, the college like to ask about the reasons for a difference between pulse oximeter readings and co-oximeter readings. In brief, one can say that low co-oximeter readings always represent true hypoxia, and high co-oximeter readings always represent true normoxia, whereas this is not the case for the pulse oximeter. The co-oximeter is not confused by ambient light,  absence of pulsatile flow, tricuspid regurgitation, carboxyhaemoglobin  or methylene blue dye.

  • The principle is similar to that of the pulse oximeter, except:
    • There are several wavelengths of light
    • There is no need for pulsatile flow
  • The co-oximeter detects the following haemoglobin species:
    • Oxyhaemoglobin
    • Deoxyhaemoglobin
    • Carboxyhaemoglobin
    • Methaemoglobin

Low co-oximeter readings always represent true hypoxia.

Causes of HIGH Co-oximetery and LOW Pulse Oximetry Readings

(i.e, pulse oximeter fails to detect that the saturation is normal)

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

Causes of LOW Co-oximetry and HIGH Pulse Oximetry Readings

  • Carboxyhaemoglobin
  • Methaemoglobin
  • Radiofrequency interference
  • Leukocyte larceny (oxygen consumption by cells in the collection tube, may also occur in situations such as severe thrombocytosis)

"Oxygen saturation gap"

Question 19.3  from the first paper of 2009 refers to the "oxygen saturation gap". Though not a formally accepted term to describe this phenomenon, the "gap" is a well recognised feature of dyshemoglobinaemia. It develops when the pulse oximeter reads a certain saturation, and the ABG machine or CO-oximeter returns a different reading. The "gap" in saturation readings is vaguely representative of the concentration of the abnormal haemoglobin in the blood.

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.

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.