Why You Shouldn’t Trust Your Phone’s Blood Oxygen Sensor

There’s a belief that you can use your smartphone to accurately measure your SpO2—that’s your blood oxygen level. And while it would certainly be handy right now to have easy access to that figure—the same one you get when your doctor sticks your finger in that little clamp during a checkup—I have the sad duty to report that whatever number your smartphone is delivering you is likely not accurate.

If you use an app, or even a built-in feature on your phone, to measure your blood oxygen levels, you’re wasting your time. To be clear: This is different than if you’re using a pulse oximeter that connects to your phone to output the data; in that case, a piece of hardware specifically designed for and dedicated to taking that measurement is doing the heavy lifting—not your smartphone’s “health sensor.”

Don’t take my word for it, though: As reported by Android Central, even Oxford University is getting in on the debunking. According to a recent post from its Centre for Evidence-Based Medicine:

“There is no evidence that any smartphone technology is accurate for the measurement of blood oxygen saturation. Furthermore, the scientific basis of such technologies is questionable. Oxygen saturation levels obtained from such technologies should not be trusted.”

Researchers tested both an app—which is no longer available on the App Store, so that’s good—and Samsung’s claims that its smartphones’ red light emitting diode can somehow detect your “stress,” which is somehow a component of your “oxygen saturation levels.” The Oxford University researchers strongly disagree, noting that, “Oxygen saturation level cannot be measured using a smartphone assessment of stress.”

What about other devices that can allegedly measure your blood-oxygen levels? Well, your newer FitBit can give you a reading, for example, but even the company notes in the fine print that you shouldn’t treat the number as gospel:

“Unless otherwise specified, Fitbit products and services are not a medical devices, and are not intended to diagnose, treat, cure, or prevent any disease. With regard to accuracy, Fitbit has developed products and services to track certain wellness information as accurately as reasonably possible. The accuracy of Fitbit’s products and services is not intended to be equivalent to medical devices or scientific measurement devices.”

Why is everyone talking about oximeters right now?

It’s simple. Knowing your blood-oxygen levels can help you better understand whether the symptoms you are experiencing are likely to be indicative of a COVID-19 infection. I say even that much with hesitation, as your blood-oxygen level just one data point—and not even the most important one, as Dr. Albert Rizzo, chief medical officer for the American Lung Association, recently told Wirecutter:

“Most people don’t need a pulse oximeter,” he said. “A drop in oxygen level in somebody with COVID-19 is one of the criteria to think about getting more care than staying at home […] monitoring your symptoms—cough, shortness of breath, chest discomfort—those are the main things.”

And as Ada Stewart, MD, FAAFP, recently told us knowing how to use a pulse oximeter correctly is also a big part of the accuracy issue. As we wrote:

“…if a patient doesn’t know how to correctly use a pulse oximeter, trying to do so can end up adding unnecessary anxiety to an already scary situation. For example, a bit of nail polish on your fingernail can result in an artificially low number.”

While it might be good to have a baseline knowledge of your blood-oxygen levels if you have actual cause for COVID-19 concern, it doesn’t do you much good to measure it with a scientifically inaccurate device. You’re likely to send yourself into a panic when your number drops five per cent over the course of a day or so, unaware that number might be within a device’s normal range of error.

I haven’t seen any studies directly comparing the quality of your Fitbit’s blood-oxygen reading to, say, what you’d get at the doctor’s office, so make sure to those results with more than a grain of salt. Pulse oximeters with FDA clearance in the United States have to have an accuracy of ±3 per cent. The oximeters included in various smart devices generally use a different technology for getting their readings than what you’d find on a fingertip device—and one that’s less accurate.

I can’t tell you if you need a solid way to measure your blood-oxygen levels at home, but if you do (or think you do), know that the quick and easy solution (or inexpensive) solution might not give you the accuracy you’re looking for. Worse, using that number as the basis for an at-home diagnosis of COVID-19 is probably going to do more harm to your mental health than good to your physical health. If you feel you truly need one of these devices to measure your blood-oxygen levels on a regular basis, run it by your doctor. Better yet, see if they have any recommendations for an accurate scanner.


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