Intended for healthcare professionals

Letters Remote management of covid-19

Pulse oximetry gives artificially high readings in presence of carboxyhaemoglobin

BMJ 2021; 373 doi: (Published 06 May 2021) Cite this as: BMJ 2021;373:n1103
  1. John Moyle, retired chartered engineer and consultant anaesthetist
  1. Shipston on Stour CV36 4NR, UK
  1. drjmoyle{at}

Greenhalgh and colleagues omit a serious limitation of two wavelength pulse oximetry.1 They mention “falsely low readings with certain haemoglobinopathies,” but a much more important danger is that standard two wavelength pulse oximeters give artificially high readings in the presence of circulating carboxyhaemoglobin to the extent that the pulse measurement of oxygen saturation approximately equals arterial blood oxygen saturation (SaO2%) plus carboxyhaemoglobin (COHb%).

Common causes of raised carboxyhaemoglobin include smoking, cooking over BBQs, car exhaust, and conflagrations. A person who smokes heavily might have up to 8% COHb causing their oxygen saturation measured by pulse oximetry to be 8% higher. Actual SaO2 is only accurately measured in smokers with an in vitro multiwavelength carbon monoxide oximeter or a Masimo Rainbow multiwavelength pulse oximeter. Basic blood gas analysers that do not include spectrophotometers are not accurate indicators of SO2 either.

I may be retired, but I wrote a book called Principles and Practice of Pulse Oximetry, published by BMJ.2


  • Competing interests: None declared.

This article is made freely available for use in accordance with BMJ's website terms and conditions for the duration of the covid-19 pandemic or until otherwise determined by BMJ. You may use, download and print the article for any lawful, non-commercial purpose (including text and data mining) provided that all copyright notices and trade marks are retained.