Pulse oximetry in the community – the debate

In his latest blog, Dr Jo Scott-Jones discusses the use of pulse oximetry as a tool for managing COVID-19 in the community.

When we are managing COVID-19 in our community a key tool to keep people away from health centres and hospitals will be pulse oximetry.

There has been much controversy and confusion as practices have started to plan for these scenarios.

How many? Which ones? Will “the system” provide them? What guidance is there for their use in the real world where people wear nail polish and put them on upside down? What about the effects of ethnicity – does skin colour effect the reading? What do we do with the result?

Let me dive headlong into the controversy, swim through the confusion and try to complete a width, if not the length of it.

(I knew that bronze medal in the Year 10 backstroke final would be useful one day.)

How many?

Prof Dee Mangin has shared great resources based on her experience in Canada.

2-3 per 1,000 patients were enough when their town (Ontario) of 500k was seeing 600+ cases a day.

Which ones?

The NZ standard to look for is: ISO 80601-2-61:2017. 

But on the basis of my research all the usual providers providing medical equipment meet these standards.

There are multiple brands – ask your supplier.

Will “the system” provide them?

My read is “unlikely” based on what I hear – the MOH will supply public health units (probably with super-dooper hospital grade equipment which patients won’t be able to use without a degree in astrophysics.)  

Real world guidance

Dee Mangin’s real world experience was that the $70-80 finger tip monitors we all have in our practices were sufficient and easy for patients to learn to use.

See this easy to follow instruction on how a patient can use a pulse oximeter (PDF) and a useful YouTube video on using pulse oximeter.

Impact of ethnicity

In the real world, ethnicity and accuracy of oxygen saturation/SpO2 is a “red herring” according to Dee Mangin’s experience and this is reiterated in this Association of Anaesthetists study. 

Of note they say:

“The US Food and Drug Administration sets out several standards for studies undertaken to test the accuracy of pulse oximeters including: ≥10 healthy subjects that vary in age and sex; ≥200 paired SpO2/SaO2 measurements; and a range of skin pigmentations, including at least two darkly pigmented subjects or 15% of the study population (whichever is larger) [1].” 

An FDA approved brand is going to have taken this issue into account. 

The Thoracic Society of Australia and New Zealand TSANZ guidelines (PDF) say:

“There is variable accuracy of pulse oximetry to predict SaO2 in acutely ill patients, with SpO2 measurements both over and under estimating SaO2, with wide limits of agreement.[4-9] The accuracy of SpO2 may worsen with progressive hypoxaemia.[8,9] Clinicians need to be aware of the variable accuracy of SpO2 in the utilisation of pulse oximetry in clinical practice. An SpO2 of >92% is a practical lower threshold to rule out hypoxaemia, defined as an SaO2.” 

And:

“There is variable accuracy of pulse oximetry to predict SaO2 in acutely ill patients, with SpO2 measurements both over and under estimating SaO2, with wide limits of agreement. [4-9] Clinicians need to be aware of the variable accuracy of SpO2 in the utlisation of pulse oximetry in clinical practice. Factors that might affect the accuracy of pulse oximetry include severe hypoxaemia, carboxyhaemoglobin and methaemoglobin levels, anaemia, dark skin, low perfusion, excessive ambient light and nail polish. [4,8,9]” 

And:

“At normal oxygen levels, skin colour does not affect SpO2 accuracy; however, at low oxygen saturations (particularly below 90%), very dark skin may potentially lead to SpO2 overestimating the true SaO2 by a small amount (around 2%).” [Feiner JR, Severinghaus JW, Bickler PE. Dark skin decreases the accuracy of pulse oximeters at low oxygen saturation: the effects of oximeter probe type and gender. Anesth. Analg. 2007; 105: S18–23, tables of contents]. 

Further advice and discussion on this topic can be found here ( attached paper pulse oximetry in the community.)

This is an area of much discussion and there are other studies out there but as the accuracy of the oximeters themselves is at best a 2-4% variance from actual O2 saturation.

The take home message is that SpO2 measurement is only part of an assessment, think about trends, symptoms, and risk stratification. 

What do we do with the results?

If previously healthy lungs or previously documented normal O2 saturation – a new consistent background reading of <92% is a red flag.

If underlying lung disease with documented low normal O2 saturation at baseline – a new reading of <90% is a red flag.

If the patient is on home oxygen normally and their O2 requirements increase with COVID-19 illness – this is a red flag.

If the person is on home oxygen – aim for SpO2 92-96%.

If there is a risk of CO2 retention aim for Sp02 88-92%.