ntil recently, unless you worked in healthcare, you’d probably never heard of a pulse oximeter. But on Sunday, emergency physician Richard Levitan made a compelling case for owning one in a New York Times op-ed, writing that “all persons with cough, fatigue and fevers should also have pulse oximeter monitoring even if they have not had virus testing”. An article in Quartz, meanwhile, reported that most of us would be fine without one. We spoke to experts to get to the bottom of it all. What is it? By very gently pinching the fleshy part of your fingertip and painlessly shining a light into it, the device detects your pulse rate and the percentage of your red blood cells carrying oxygen, or blood oxygenation, explains Alon Dagan, emergency medicine physician at Beth Israel Lahey Health. It’s useful for those who have or are suspected to have coronavirus. Per his hospital’s protocol, Dagan says patients who either test positive or are presumed positive for coronavirus but aren’t sick enough to be admitted to the hospital are sent home with a pulse oximeter and instructions to use it three times a day. For this population, the device can literally be a lifesaver. Usually, dangerously low blood oxygenation levels are associated with labored breathing to the point where it’s hard to speak in full sentences. With coronavirus, however, this is not necessarily the case. Covid-19 patients may be “breathing fairly comfortably but their oxygen saturations are much lower than we expected,” says Dagan. Do I need to get one? If you haven’t been diagnosed with Covid-19, or aren’t suspected to have it, home pulse oximeters “aren’t necessary”, says Shoshana Ungerleider, an internist at Crossover Health in San Francisco. This is particularly true if you’re not experiencing coronavirus symptoms, such as trouble breathing, severe cough or chest pain, and haven’t established a baseline. Without knowing what’s typical for you, it’s hard to know whether your numbers are concerning. While 95% to 100% is normal for most healthy people, Dagan points out that someone with chronic lung disease may be at 92% “on a good day”. Ungerleider suggests calling your doctor “if the number is downtrending over time and falls to 92% or lower, especially if you are exhibiting other respiratory symptoms”. Additionally, the reading isn’t particularly useful on its own. “It can’t replace a visit or call with the doctor who can assess all of your symptoms and how they relate to any underlying medical conditions you have,” explains Stephen Parodi, infectious disease physician and associate executive director of the Northern California Permanente Medical Group. And as Dagan points out, “any medical device has some error rate”. Nail polish, cold fingers, and poor circulation may interfere with the device’s accuracy. Among the downsides of using a pulse oximeter at home, says Ungerleider, is the potential for increased anxiety as a result of frequent checking. Worse, though, is the possibility that you’ll ignore physical symptoms that are accompanied by a normal-appearing oxygen level. Regardless of what your pulse oximeter shows, she says, “you should still call your healthcare professional if you have severe shortness of breath, chest pain, unrelenting cough or high fever”.