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Disclaimer: Due to health regulations and clearances, some ScanWatch features may not be available or may not have clinical validation in your region. Learn more.

What is SpO2?

SpO2 stands for peripheral capillary oxygen saturation, an estimate of the amount of oxygen in the blood. More specifically, it is the percentage of oxygenated hemoglobin (hemoglobin containing oxygen) compared to the total amount of hemoglobin in the blood (oxygenated and non-oxygenated hemoglobin).

It is often considered to be the fifth vital sign, along with pulse, respiratory rate, blood pressure, and body temperature.

Hemoglobin is a protein that carries oxygen in the blood to your organs. It is found inside red blood cells and gives them their red color. Once oxygen is in our organs, it acts as the fuel that allows our cells to work.

How is it measured?

SpO2 can be measured by pulse oximetry, an indirect, non-invasive method—meaning it does not involve the introduction of instruments into the body. It works by emitting and then absorbing a light wave reflecting the blood vessels, or capillaries, in the wrist.

  • Oxygen-rich blood absorbs infrared light better than oxygen-poor blood.
  • Oxygen-poor blood absorbs red light better than oxygen-rich blood. This is why venous blood is darker than arterial blood.

ScanWatch uses a multi-wavelength sensor called PPG (photoplethysmography) sensor that emits and measures red and infrared reflections on the blood vessels in the wrist. The algorithm then compares the reflection at each wavelength to compute the SpO2 value.


What is a normal SpO2 level?

A SpO2 value is represented by a percentage. If your ScanWatch says 98%, this means that each red blood cell is made up of 98% oxygenated and 2% non-oxygenated hemoglobin.

Good blood oxygenation is necessary to supply the energy your muscles need in order to function, which increases during a sports activity. If your SpO2 value is regularly below 90%, this could be a sign of poor blood oxygenation, also called hypoxemia.

SpO2 values generally vary between 90 and 100%:

  • 95% to 100%: Normal.
  • 90% to 94%: Below Average. This measurement detects that your blood oxygen level is below average but still normal. The results can vary based on a number of factors, including but not limited to your health profile (whether you are a smoker, if you have asthma, if you are very athletic or not, if you have tattoos, if you have known conditions such as hypotension, anemia, etc.), your environment (altitude, temperature), the way the measurement is done (standing/sitting position, wristband tightening, etc.). We suggest you check for best practices and train to improve your gesture.
  • Below 90%: Low. The value can be a possible sign of hypoxemia. The results can vary based on a number of factors including your health profile, your environment, and the way the measurement is done. We suggest you check for best practices and repeat the measurement. In case results in this zone are associated with symptoms, contact your doctor or a health professional who will help you make a diagnosis and take preventive actions. Symptoms include being short of breath after exertion, coughing, fast or slow heart rate, rapid breathing, sweating.

Note: The quality of SpO2 measurements is very dependent on the skin’s blood perfusion, which is naturally lower at the wrist than on the finger. As such, SpO2 measurements taken at finger tip can be more accurate than those taken at the wrist. This is especially true in situations where the measurement is taken while moving or under circumstances which contribute to low perfusion, such as low temperature or having an underlying health condition which causes poor circulation.

The measurement of SpO2 in the 70-100% range has been clinically validated on healthy adult volunteers, at rest, against a laboratory CO-oximeter. Our clinical study has shown that the ScanWatch's oximeter in accurate within 3 percentage points. For comparison, this figure is around 2 percentage points for most finger tip oximeters.

ScanWatch also allows you to follow trends on SpO2 averages over long periods by taking regular measurements as it is more practical, and comfortable, to wear the watch than a finger pulse oximeter. Furthermore, by wearing it at night and activating the Respiratory Scan feature, continuous SpO2 monitoring helps detect SpO2 drops corresponding to breathing disturbances.

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