Many patients with severe cases of Covid-19 have been shown to have extremely low levels of oxygen in the blood. Others who have tested positive find themselves experiencing shortness of breath. Perhaps that’s why people are so interested lately in pulse oximeters, medical devices that measure the saturation of oxygen in red blood cells. Pulse oximeters were sold out on the websites of CVS and Walgreens, a search on Friday showed. US sales of pulse oximeters spiked on January 20 when the country’s first Covid-19 case was confirmed, and again in mid-February, according to Quartz. While the rate of increase has slowed, sales have continued to grow each week since, the outlet reported. Meanwhile, Google searches for “pulse oximeters” spiked in early April and reached a peak in recent days, after emergency room physician Richard Levitan published an op-ed in The New York Times suggesting that the devices could provide early warnings about a need for treatment against Covid-19. So, should you go out and buy one? Here’s what to know. What a pulse oximeter is A pulse oximeter is a medical device that measures the saturation of oxygen in a person’s red blood cells. It typically clips to your finger, though it can also be attached at the ear, forehead, nose or toes. The device works by shining a light through the skin, which is then analyzed to determine how much oxygen is being carried in the blood. Physicians and other medical professionals use pulse oximeters on patients experiencing shortness of breath or those who have a lung or heart conditions to determine if they’re getting enough oxygen. Health care professionals routinely use them in hospitals and clinics when checking vital signs. Pulse oximeters are occasionally used in home settings for people who have underlying health conditions. They can also be found on Amazon, in pharmacies and in medical supply stores, although prices and quality can vary widely. One doctor suggests widespread screening Levitan, who spent 10 days in New York treating pneumonia caused by the coronavirus, wrote in The New York Times that pulse oximeters could detect a form of oxygen deprivation in which patients don’t experience shortness of breath, despite low oxygen levels and pneumonia readings through chest X-rays. According to Levitan, the devices signaled to two emergency physicians he knew that they needed treatment early on, and both went to the hospital and recovered. “Widespread pulse oximetry screening for Covid pneumonia — whether people check themselves on home devices or go to clinics or doctors’ offices — could provide an early warning system for the kinds of breathing problems associated with Covid pneumonia,” he wrote. But you probably don’t need one, experts say Experts with the American Lung Association and the American Thoracic Society said that for most people, having a pulse oximeter at home wouldn’t be particularly helpful in detecting the virus. “If the question is, ‘Would it be a good early indicator if somebody has Covid-19 infection?’, I would say probably not,” said Dr. J. Randall Curtis, a professor of pulmonary and critical care medicine at the University of Washington. That’s because low oxygen levels are a relatively late indicator that a person has Covid-19, Curtis said. People who potentially have the illness will likely experience other symptoms, like fever, dry cough, body aches or fatigue, that would prompt them to seek medical attention days before they noticed a drop in their blood oxygen levels. It’s also possible that people using pulse oximeters at home could see inaccurate readings. Nail polish, artificial nails, cold hands and poor circulation are all things that can interfere with the light used by the devices and result in false numbers, says Dr. David Hill, a pulmonary and critical care physician and a spokesman for the American Lung Association. “One reason not to encourage everyone to go out and get one is there’s a higher likelihood of having falsely low readings in a normal population,” he said. “Then those people are going to be calling physicians or going into emergency rooms that are already busy for potentially something that’s nothing.” There are instances when it would make sense to use pulse oximetry at home, both Curtis and Hill said. People who have already tested positive for Covid-19 and are recovering at home might want to consult their doctor about using a pulse oximeter to monitor whether they need oxygen or more supportive care. But those who are healthy and aren’t experiencing symptoms can probably just save the $50 or so that they might otherwise spend on the device, Hill said. Some people are turning to apps Because of the shortage of the devices, some people are turning to oximeter apps instead. But a recent study suggests the apps don’t necessarily work as well – and could even be dangerous. An analysis from the University of Oxford looked at the use of oximeter apps in context of the Covid-19 pandemic and concluded that they weren’t reliable. “Oxygen saturation levels obtained from such technologies should not be trusted,” the authors wrote. Pulse oximeters measure the light transmitted or reflected through the skin at two different wavelengths (usually red and infrared) to determine the level of oxygen saturation in the blood, the authors explained. Smartphone apps, which typically claim to measure oxygen saturation through the flash light and camera, don’t generate as accurate of readings because the camera cannot measure the light reflection at two wavelengths. One app did claim to use a red light source, but the authors found that it only used a single wavelength measurement, making the reading inaccurate.