Some advanced fitness watches like my Garmin Fenix 6x has built-in Pulse Ox (Oximeter) sensor. Since blood oxygen levels drop at high elevations, the Pulse Ox sensor gives athletes a way to monitor their body's altitude acclimation. But now some doctors say that Pulse Ox can be effective for early detection of COVID pneumonia.

Dr. Richard Levitan's article in the New York Times gives an in-depth look at how Pulse Oximeters can provide early warning of COVID-19:

Opinion | The Infection That's Silently Killing Coronavirus PatientsThis is what I learned during 10 days of treating Covid pneumonia at Bellevue Hospital.www.nytimes.com

From the article:

Pneumonia is an infection of the lungs in which the air sacs fill with fluid or pus. Normally, patients develop chest discomfort, pain with breathing and other breathing problems. But when Covid pneumonia first strikes, patients don’t feel short of breath, even as their oxygen levels fall. And by the time they do, they have alarmingly low oxygen levels and moderate-to-severe pneumonia (as seen on chest X-rays).

Normal oxygen saturation for most people at sea level is 94 to 100 percent. Dr. Levitan saw COVID pneumonia patients with oxygen saturations as low as 50 percent.

Even though I had the Pulse Oximeter built into my Garmin Fenix 6x watch on my wrist for the last six months, I rarely got to use it. Living and train at sea level, I've always had readings between 96%-100%.

After reading the article, I immediately tested my blood oxygen levels:

Reading came in at 100%, and I've decided to turn on the sensor and continuously monitor my blood oxygenation throughout the day. Turning on the Pulse Ox sensor for continuous monitoring dramatically affects battery life, though.

Pulse Ox sensor on the back of the watch uses a combination of red and infrared lights to illuminate the skin. Hemoglobin in the blood absorbs different amounts and wavelengths of light, allowing the sensor to determine the percentage of oxygenated blood (SpO2%) available in the bloodstream.

Garmin's manual states that Pulse Ox data should only be used for recreational purposes and not to be used to diagnose or treat diseases. That reads like a standard disclaimer to avoid potential lawsuits.

I'm also monitoring blood oxygen trends rather than relying on a single reading.

But most doctors only use finger based Pulse Oximeter:

What's a Pulse Oximeter, and Do I Really Need One at Home?A tiny fingertip device can give you valuable information about your health during a bout of Covid-19 or any respiratory illness.www.nytimes.com

How much should I trust Pulse Oximeter sensor in my watch to provide early COVID Pneumonia warning?

I read this too! I don't have a Garmin watch I did purchase a pulse Oximeter. Seems like a smart thing to have around and monitor during all illnesses

Pulse Oximeters are also useful in overnight monitoring of sleep apnea. It is probably easier to sleep with Pulse Ox sensor on the wrist rather than a finger.

Overnight pulse oximetry for obstructive sleep apnea screening among patients with snoring in primary care setting: Clinical case reportThis clinical case report illustrated a typical patient presented with snoring in the primary care. He was screened positive for obstructive sleep apnea (OSA) by overnight pulse oximetry and then referred to respiratory specialist care. With early confirmation ...www.ncbi.nlm.nih.gov

A similar discussion on the merits of wrist worn Pulse Ox is also happening on Slowtwitch.

Some notable comments from a member, who actually had COVID-19:

As someone wearing a Whoop and a 945 who is recovering now from a confirmed and tested positive case, they both showed these data changes. Garmin with pulse ox and respiration and whoop with lower HRV and higher respiration.

And another member:

Whilst my Fenix 6 didn't predict my Coronavirus, the stats made for interesting reading. Body Battery didn't go above (or, below strangely) 5 for around 2 weeks, my pulse Ox stats were around 82 at night going up to about 90 during the day.