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How Apple Watch Series 6's Blood Oxygen Monitoring Function Compares to a Traditional Finger Pulse

Accuray is the Main Concern

Blood Oxygen Monitoring feature on Apple Watch Series 6

Apple has announced the inclusion of the blood oxygen monitoring feature in its Apple Watch Series 6. On the Apple.com support page, it describes the feature as the following:

"In Apple Watch Series 6, the optical heart sensor has been redesigned to add blood oxygen measurement capabilities. During a blood oxygen measurement, the back crystal shines red and green LEDs and infrared light onto your wrist. Photodiodes then measure the amount of light reflected back. Advanced algorithms use this data to calculate the color of your blood. The color determines your blood oxygen level: bright red blood has more oxygen, while dark red blood has less."

How does this new feature compare with the performance of the traditional FDA approved stand-alone finger pulse oximeters?


Transmission-mode vs. reflective-mode oximetry

The traditional finger pulse oximeter is a transmission mode device. This mode has been around the longest. Most doctors and health clinics use this method. This method has been studied more than other available methods. 

University of Iowa Health Care explains how transmission mode oximetry works at this LINK: it "utilizes an electronic processor and a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip." "Absorption of light at" 660nm (RED) and 940nm (Infrared) "differs significantly between blood loaded with oxygen and blood lacking oxygen." By monitoring the ratio of light in two wavelength reaching the photodiode, the finger pulse oximeter can determine the SpO2 level of the person under-test.

SpO2 (blood oxygen saturation levels) of a healthy person should fall in between 94% and 100%. Supplementary oxygen should be used if SpO2 level falls below 90%, which is unacceptable for a prolonged period of time.

The transmission mode is the most trusted method of monitoring the blood oxygen level.


Conversely, the method used by the Apple Watch Series 6 and Fitbits is the reflective mode.


Accuracy is the main concern

Reflective mode oximeter can monitor you. However, as this IEEE Spectrum article explains, taking measurement at wrist can pose many challenges:

The fingertip location has "an advantage over the back of the wrist because they have more capillaries, so provide a better signal to noise ratio." "The wrist is subject to a lot of motion, which is bad news for continuous measurement." The design is "an inherently less precise approach."

There are also other research papers pointing out the accuracy issue on reflective mode oximeters:

Research Paper Link #1

"Based on the performance of the oximeter, monitoring SpO2 at the wrist using the reflective mode presents challenges with regard to clinical use. Another limitation is that the reflected red and infrared pulses can only be used for specific areas, such as a radial artery; thus, most areas of the wrist are not available for monitoring. In addition, a slight position change at the measurement site significantly affects the performance of the oximeter."

Research Paper Link #2 (This research is not performed on the Apple Watch, but on the iPhone pule ox applications - iOx / Ox / POx.)

"While iOx has modest concordance with control, Ox and POx showed almost none. The iOx device was best in correctly identifying hypoxia patients, but almost 1/4 of patients were incorrectly classified. The three apps provided inaccurate SpO2 measurements and had limited to no ability to accurately detect hypoxia. These apps should not be relied upon to provide accurate SpO2 measurements in emergent, even austere conditions."



No FDA Clearance yet

FDA has established methods for regulating oximeter devices, and it looks at the accuracy of the measurement rigorously. This could be why Apple has not been able to get FDA clearance on the blood oxygen level monitoring function.
If you are looking to find a device to monitor the wellbeing of the body or to detect early signs of COVID-19, we strongly suggest to look for a traditional transmission mode, FDA approved oximeter. Please check out AmperorDirect’s oximeter category page for different solutions. You can also Contact us for any Oximeter related questions – we are located at Houston, TX and have both native English and Spanish speaking customer service representatives to help.