Blood oxygen saturation (SpO2) is an important, easy to measure biomarker that everyone should be tracking to optimize health, sleep, and fitness. As I have previously written, a drop in blood oxygen saturation was identified as a key symptom during the coronavirus pandemic as lung function became compromised.

Hemoglobin molecules

What is Blood Oxygen Saturation (SpO2)?

When we breathe, oxygen enters the body and travels from the air into our blood. Hemoglobin (hB) is the protein molecule in red blood cells that carries oxygen from the lungs to the body’s tissues and returns carbon dioxide from the tissues back to the lungs. Each hemoglobin molecule has the capacity to carry four oxygen molecules. Oxygen saturation, also represented as SpO2 (peripheral capillary oxygen saturation) is a measurement that represents the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. [1]

What are Typical Blood Oxygen Saturation Values?

For healthy individuals at sea level, normal blood oxygen saturation values are usually between 95-100 percent, but may be lower at higher altitudes (at 1 mile/1,600 meters altitude, oxygen saturation should be above 92%). Values below 90 percent are considered low (also known as hypoxemia). [2]

How to Measure Blood Oxygen Saturation?

Blood oxygen saturation is commonly measured using a pulse oximeter, a device that indirectly senses levels of oxygen saturation in arterial blood using spectral analysis, along with pulse rate.  Pulse oximeters are generally accurate in measuring oxygen saturation between 70 and 100 percent. [3]

There are several types of pulse oximeters:

Fingertip pulse oximeter showing blood oxygen saturation and pulse rate
Fingertip pulse oximeter showing SpO2 of 98% and pulse rate of 53 bpm

Fingertip Pulse Oximeter

The most common type of pulse oximeters (and easiest way to get started tracking your own SpO2) are fingertip-based, used by placing the device over a finger. After a few seconds a digital readout will appear showing pulse rate as well as SpO2.

More advanced fingertip pulse oximeters, such as Masimo’s MightySat, allow for real-time data logging via bluetooth to an app (you can also export raw data in .csv format), and in addition to SpO2 and pulse rate provide additional metrics such as Perfusion Index (PI), Pleth Variability Index (PVI), and breath rate. [4]

Ear Clip Pulse Oximeter

Another way to measure SpO2 is by using a pulse oximeter with an ear clip probe. Ear clip sensors are most commonly used when someone has poor blood circulation in the fingers and/or toes or a condition such as Reynaud’s Syndrome. [5]

Wrist pulse oximeter
Wrist-based pulse oximeter

Wrist-based Pulse Oximeter

Most wrist worn pulse oximeters still use a finger-based sensor, which is connected to the device/display worn on the wrist. These are mainly used during overnight sleep studies or during daily activities where there is a lot of movement

Pulse oximetry with Biostrap
Biostrap overnight blood oxygen saturation report

Wearables that Measure Blood Oxygen Saturation

A number of wearable makers are now “unlocking” SpO2 functionality in their devices:

Biostrap – tracks overnight SpO2 and overnight low oxygen events will negatively affect sleep score

Fitbit – some of their newer devices like the Fitbit Ionic, Charge 3, and Versa now provide overnight SpO2 tracking, but only indicate low/high events and not actual percentage values

Garmin – many of their wearables, including the VivoSmart 4, Vivoactive 4, Forerunner 245/645/945, and Fenix 6X, Fenix 5X track blood oxygen saturation

Withings –  Withings Pulse HR and their upcoming ScanWatch track SpO2

Unfortunately, Apple Watch and Oura Ring do not currently track SpO2.

Factors That May Prevent Accurate Pulse Oximeter Readings

  • Finger selection – studies performed with readings from all fingers have discovered that pulse oximetry measurements are most accurate using index, ring, and middle fingers (the thumb and little finger aren’t recommended)
  • Poor perfusion – poor blood flow to the finger/extremity, due to vasoconstriction (narrowing of the blood vessels) from cold (one solution is to rub hands together or run under warm water for 15 seconds to increase blood flow) or low blood pressure
  • Nail polish – especially darker colors will prevent the sensor’s LED light from passing through the finger
  • Intense ambient light – excessive bright light can interfere with the LEDs and photoreceptor in the sensor
  • Excessive movement – sensor will produce a noisy signal, thus affecting accuracy of readings
  • Altitude – recent change from lower to higher altitude may result in lower SpO2 readings due to lower oxygen pressure in the air
  • Carbon monoxide – carbon monoxide binds to hemoglobin at sites that normally carry oxygen, and will overrepresent actual SpO2 readings [6] . Seen in heavy cigarette smokers or, in more extreme cases, carbon monoxide poisoning

Resources

Pulse Oximeters

Facelake FL400 Pulse Oximeter (Amazon)
Masimo MightySat Pulse Oximeter
Nonin Pulse Oximeters

SpO2-enabled Wearables

Fitbit Ionic, Charge 3, and Versa (Amazon)
Biostrap (use code quantifiedbob to save 10%!)
Garmin VivoSmart 4, Vivoactive 4, Forerunner 245/645/945, and Fenix 6X, Fenix 5X (Amazon)
Withings Pulse HR (Amazon)

References

[1] https://www.medicinenet.com/hemoglobin/article.htm
[2] https://www.mayoclinic.org/symptoms/hypoxemia/basics/definition/sym-20050930
[3] https://en.wikipedia.org/wiki/Oxygen_saturation
[4] https://www.masimo.com/technology/co-oximetry/pvi/
[5] https://en.wikipedia.org/wiki/Raynaud_syndrome
[6] https://journal.chestnet.org/article/S0012-3692(16)33003-3/pdf

About Bob

Bob Troia

Bob Troia is a technology entrepreneur and citizen scientist who is focused on the intersection of data-driven citizen science, health and wellness, human performance, longevity, and self-optimization. He has been featured on CBS News Sunday Morning, PBS NewsHour Weekend, National Geographic Explorer, CBC (Canada), SBS-TV (South Korea), Fast Company, Men's Fitness, Outside Magazine, and on many leading health and wellness podcasts.