Apps aren’t a reliable way to measure blood oxygen levels

Doctors say one of the best ways to monitor patients with COVID-19 is to monitor their blood oxygen levels, namely show when they have dangerous breathing problems, even if they are not short of breath. But along with toilet paper and digital thermometers, devices that measure these levels, called pulse oximeters, are difficult to find. They are sold out or it takes weeks to ship to major retailers.

Now that the devices are out of reach, people are turning to dubious alternatives: the third most popular paid iPhone app last week claims to measure blood oxygen levels through the phone’s camera, despite a disclaimer that the app is not a medical device. On Reddit, some people are fighting COVID-19 say they use a health feature on some Samsung phone models to check their oxygen levels. Others say they use pulse oximetry capabilities on smartwatches.

That concerns doctors. Despite their accessibility, research shows that pulse oximetry apps don’t accurately measure blood oxygen levels, especially when they are low. And relying on apps can be dangerous, says Walter Schrading, director of the office of wilderness medicine at the University of Alabama at the Birmingham School of Medicine.

The apps are easy party tricks when you’re not sick: put your finger on the camera and get a normal oxygen reading. “You can see, I am a normal person, I breathe normal air,” he says. But when someone actually has a low oxygen level, they will likely still give that normal reading. “They don’t work well if you really need them to work properly, and your oxygen levels drop,” says Schrading.

Schrading and colleagues evaluated three iPhone pulse oximetry apps in a study published in 2019 and found they couldn’t reliably identify people who didn’t have enough oxygen. Their findings were consistent with other studies, which also found pulse oximetry apps to be inaccurate. A recent analysis from the Center for Evidence-Based Medicine at the University of Oxford, which reviewed research on apps related to the COVID-19 pandemic, also concluded that they are unreliable.

“Oxygen saturation levels obtained with such technologies should not be trusted,” the authors of the analysis wrote.

Apps don’t work well because most use a different mechanism to test blood oxygen levels than standard medical pulse oximetry devices. The devices send two different wavelengths of light – usually red and infrared – through a fingertip, which contains a lot of blood close to the skin’s surface. Hemoglobin, the protein that carries oxygen in the blood, absorbs more infrared light when it carries oxygen and more red light when it doesn’t. The device calculates the difference to determine how much oxygen is circulating.

Smartphones usually only have White lightso they can get a less accurate reading. Samsung phones have a red light feature, said the Center for Evidence-Based Medicine, but they only use one wavelength and would likely also be unreliable.

In addition, standard pulse oximetry devices control light wavelengths by finger and read the results of a sensor on the other side. Smartphones control and receive the light from the same place – they rely on the reflection of the wavelengths. This is usually the method less precise and can be skewed by light from the environment.

Some models of Fitbit and Garmin smartwatches also have pulse oximetry capabilities. Fitbit can monitor trends in oxygen levels during sleep, and Garmin can provide on-site measurements. Their watches do use red light, but they use the less accurate reflection method. They also take blood flow measurements at the wrist – which is not as strong as at the finger. Both companies note their websites that their devices should not be used for medical purposes.

Accurate blood oxygen measurements are important when people are trying to track a disease, but it is especially important for COVID-19. Most of the time, someone with low blood oxygen levels knows – they are struggling to catch their breath. If a person takes a reading on a lower-quality oximeter that says they’re low in oxygen, but they don’t have those symptoms, doctors can usually assume the sensor just isn’t working, James Hudspeth wrote, the COVID response to the hospital floor at Boston Medical Center, in an email to The edge.

However, people with COVID-19 sometimes feel good even when their oxygen levels drop. That’s why some doctors want wrist oximeters to be given to anyone who is sick with the virus: if they can come to the hospital as soon as their oxygen levels drop, but before they gasp for air, perhaps they are treated more effectively. But it also means that doctors would have less confidence in interpreting a pulse oximeter result that doesn’t match symptoms, Hudspeth said.

If apps cannot see when the oxygen level is really low, they can give a false sense of security to people who are feeling well too. “This thing can be read normally, but you can’t be normal,” says Schrading. It’s a big risk, he says. “It would be dangerous to trust them because they didn’t measure what to measure.”

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