Before the coronavirus pandemic, most Americans had heard about pulse oximeters only via TV shows where EMTs shout out a patient’s “pulse ox” — or measure of oxygen in the blood. But when multiple news reports mentioned pulse oximeters as a possible tool in the limited arsenal of weapons against covid-19, the low-cost medical devices started flying off shelves almost as quickly as toilet paper.

Typically clipped over a finger (though a toe or ear lobe works, too), a pulse oximeter transmits light from sensors on one side of the device through the body part to sensors on the other side. In about 15 seconds, it calculates how much oxygen is being transported through the person’s bloodstream and displays the results as a percentage (abbreviated SpO). Doctors consider a SpO reading of 94 percent and higher to be normal. Low levels of oxygen in the blood (hypoxemia) can lead to low levels of oxygen in the tissue and organs (hypoxia), which can lead to death.

Pulse oximeters are usually purchased for home use by people who want or need to monitor their oxygen supply, including patients with chronic lung conditions such as COPD, performance athletes or people living at high altitude. Shortly after the pandemic began, however, the easy-to-use devices started being touted as a means to detect serious complications from covid-19, the disease the virus causes.

“The reason there is such great interest in pulse oximeters now is that covid-19 has a fairly significant effect on the body’s ability to oxygenate, sometimes discordant with how the patient feels,” says Benjamin Seides, director of interventional pulmonology at Northwestern Medicine Central DuPage Hospital in Winfield, Ill., outside Chicago. Though patients aren’t especially short of breath, their low blood oxygen levels may indicate they are headed for a crisis.

Seides said an idea started to spread: Maybe if physicians told people to watch their blood oxygen levels, they could contact a doctor or go to the emergency room if their saturation levels dramatically dipped. That prompted a run on the devices.

Sales data provided to The Washington Post from Bloomreach, an e-commerce software firm, shows that U.S. sales of fingertip pulse oximeters increased by 506 percent over the previous week during the week of Feb. 17 and 168 percent over that week during the week of Feb. 24. Since that time, demand held fairly steady and then tapered off the first two weeks of April. But in recent weeks, sales again have started to climb, perhaps as consumers prepare for the next phase of living with the pandemic. In mid-May, the devices were still sold out, delayed or on back order in stores such as Walgreens, CVS and Target.

Medical-grade pulse oximeters cost $200 or more, but consumers can find them for about $50 at pharmacies, big-box stores and online. Even the lower-cost versions are considered a fast and reliable way to measure how well you are oxygenating.

Correct usage is key to a trustworthy reading, however, which can fluctuate based on a number of factors — what you are doing physically (sitting vs. running), proper device placement and the altitude you live at. Artificial nails and nail polish, especially gel-based, can produce inaccurate measurements of SpO, as they block the light transmission. Cold hands or poor circulation can also interfere with the light and produce false numbers, because pulse oximeters need a steady flow of blood beneath the sensor.

Pulmonologist Federico Cerrone, managing physician at Overlook Medical Center in Summit, N.J., says people with chronic health issues may want to get a pulse oximeter at this time because it can be a helpful tool for regularly checking in with a doctor via telemedicine. Patients with covid-19 who are discharged from a hospital also may be given a pulse oximeter so they can monitor their numbers to see if they are improving or getting worse.

“It can be a reassurance or a harbinger of issues indicating you may need to return to the hospital,” Cerrone says.

Unless you have an underlying condition or are recovering from covid-19, however, you probably don’t need to add one to your collection of home health supplies.

“It’s a convenience, not a necessity. If you are short of breath or have a cough, you can usually call your physician and have a drive-through check of your oxygenation,” Cerrone says.

He also cautions against the use of oximeter apps that purport to tell you various medical readings, because their performance has not been validated in scientific studies. A recent University of Oxford study found no evidence that any smartphone technology is accurate for the measurement of blood oxygen saturation. In fact, the study states, “oxygen saturation levels obtained from such technologies should not be trusted.”

There are potential drawbacks to widespread usage of pulse oximeters. Seides says that some patients who possess them become compulsive and check their saturation levels too often. “In the hands of nervous patients, the device creates anxiety and confusion without much probative value, because even with a small dip in readings, they call their physician. It’s important to recognize that a pulse oximeter is a single data point of a complete picture.”

Another concern is that those who use a pulse oximeter as a covid-19 litmus test may ignore other troubling signs.

“I worry that though their saturation numbers look okay, people with other symptoms, such as shortness of breath or persistent cough, won’t seek medical attention,” Seides says.

Physicians agree that during this pandemic, the important actions remain physical distancing, washing hands, wearing a mask, listening to public health officials and, if you’re not feeling well, calling your doctor.