Apple's new Apple Watch Series 6 goes all-in on health care, including a new feature to measure blood oxygen level -- a vital sign that's been helpful monitoring COVID-19, among other things. Other gadgets that do this known as blood, or pulse, oximeters have been around for ages and have become something of a hot commodity during the coronavirus pandemic.
However, questions and controversy still surround the at-home use of pulse oximeters, especially when used to monitor for COVID-19. It's still not entirely clear if pulse oximeters can help detect a coronavirus infection or whether their widespread use can help curb the spread of COVID-19 overall.
Here's what you need to know about what pulse oximeters do, how they work, what the results mean and how accurate they might be.
Watch this: Apple Watch Series 6 adds blood oximeter
Pulse oximeters: Vital signs, at your fingertips
A pulse oximeter is a small medical device that measures heart rate and blood oxygen saturation. It's usually clipped to your finger, but it can also attach to your ear, nose, toe or forehead. On Apple Watches, it's the sensor on the underside of the watch, the part that goes against the top of your wrist, that takes the measurement.
Some pulse oximeters are battery powered and provide real-time results on a small LED display on the device itself. Others connect with a wire to a separate vital sign monitor that records even more precise information about your heart rhythm, body temperature and blood pressure using other sensors connected to your body.
How pulse oximeters measure heart rate and oxygen
A pulse oximeter measures your blood oxygen saturation and heart rate by shining a light through your skin and detecting both the color and movement of your blood cells. Oxygenated blood cells are bright red -- deoxygenated cells are dark red.
The pulse oximeter compares the number of bright red cells to dark red cells to calculate your oxygen saturation as a percentage. So, for example, a reading of 99% means only 1% of the blood cells in your bloodstream have been depleted of oxygen.
Watch this: How pulse oximeters measure your blood oxygen levels
Every time your heart beats, it pushes your blood through your body in a quick pulse (which is why "pulse" is another word for "heart rate"). A pulse oximeter, using light, detects this movement and calculates your heart rate in beats per minute, or BPM, basically the same way every Apple Watch since the first one does.
What's a healthy oxygen level and heart rate?
According to the Mayo Clinic, a normal pulse oximeter oxygen level reading is between 95% and 100%, and anything less than 90% is considered dangerously low, or hypoxic. Some doctors have reported COVID-19 patients entering the hospital with oxygen levels at 50% or below.
A normal resting heart rate is between 60 and 100 BPM. Typically, lower is better, as a slower heart rate is usually an indication of a strong cardiovascular system.
Can a pulse oximeter detect COVID-19?
Not exactly. Although many doctors report that patients with COVID-19 are presenting with dangerously low blood oxygen levels, COVID-19 isn't the only disease that can cause such a problem. Chronic lung diseases, like COPD, asthma and other non-COVID-19 lung infections can also result in a low oxygen count.
A low oxygen reading by itself is not enough to diagnose COVID-19, but your doctor would want to know about it, especially if you notice the level decreasing over time. And if you've been diagnosed with COVID-19, your doctor may want you to monitor your oxygen level to determine whether your condition is worsening or improving.
How accurate are over-the-counter pulse oximeters?
Like with any electronic equipment, not all pulse oximeters are created equal. A 2016 study of low-cost pulse oximeters concluded several inexpensive consumer-grade devices provided highly inaccurate readings.
Some pulse oximeters have been cleared by the FDA, which means they should meet FDA standards for accuracy. Note that there is a distinction between "FDA-approved" and "FDA-cleared," with "cleared" being the less rigorous of the two. That said, Class II medical devices like pulse oximeters are usually "cleared" rather than "approved."
You can look for pulse oximeters on the FDA-cleared list by visiting the FDA's Premarket Notification website and searching for "pulse oximeter" in the Device Name field, with or without a manufacturer's name.
How much should I spend on a standalone pulse oximeter?
In the 2016 study that found most low-cost pulse oximeters to be relatively inaccurate, "low-cost" was defined as costing less than $50. Pulse oximeters that have been cleared by the FDA tend to range in price from around $50 to $60 to well into the hundreds and even thousands of dollars.
Where can I buy a pulse oximeter if I don't get an Apple Watch 6?