Subtle shifts in the color of blood as it moves through a dense network of blood vessels can be a potent clue that a person has COVID-19, even if other signs of physical problems are absent.

That realization has set off a national buying spree for optical medical devices called pulse oximeters, which can clip onto a finger or earlobe and detect color variations to tell how much oxygen is in the blood. The devices are common in hospitals and doctor’s offices, though consumers are increasingly trying to buy them for home medical kits, alongside thermometers and bandages.

“The people who have been buying from us in the past are now trying to buy 10 times more. We can’t keep up,” said Phil Issacson, chief technology officer and executive chairman of Nonin Medical, the Plymouth-based firm that invented the fingertip pulse oximeter more than 20 years ago.

In most cases, COVID-19 causes mild to no symptoms, but as many as 5% of people who get it will require critical care in a hospital. Some of them will need invasive mechanical breathing machines called ventilators, which add oxygen and remove carbon dioxide through a breathing tube, after their lungs are badly compromised by COVID-related pneumonia.

But it turns out that some COVID-19 patients are far lower on oxygen than doctors would expect. That’s where pulse oximetry enters the picture, in the hospital and the home.

Typically, a person who can’t breathe normally falls into visible distress quickly.

Holding your breath for a minute or two leads to strong discomfort in most people, yet that would hardly cause a blip in oxygen levels.

Some patients with COVID-associated pneumonia have clinically significant oxygen deprivation, yet they are talking on their phones more or less normally and a short time later they have a breathing tube inserted for a ventilator, said Dr. Andrew Stiehm, a pulmonologist who has treated COVID-19 patients at Allina Health’s United Hospital in St. Paul.

The condition has been given names like “happy hypoxia” and “silent hypoxia.” “Some of these patients we are seeing, they don’t look as miserable as we would normally expect them to be … with their oxygen saturations,” Stiehm said.

For hospitalized patients, silent hypoxia has several implications.

First it underscores the need to get pulse oximetry on patients early, because low oxygen-saturation levels may be a sign that a severe case of COVID-19 is developing, even without other apparent symptoms — a point noted in a widely read opinion column in the New York Times this week about silent hypoxia.

Stiehm also said there’s growing debate in the medical community about whether COVID-19 patients are being intubated and ventilated too readily. Doctors have been conservatively intubating some patients before their oxygen levels drop to life-threatening levels, because of how fast their health might deteriorate in an emergency.

But it may be that people in silent hypoxia are more stable than they appear, and can tolerate lower oxygen without ventilation.

“We just lack good data on who they are right now,” Stiehm said.

“Accidentally putting someone on a ventilator even though they didn’t need one is a smaller sin than not putting someone on a ventilator who needed it.”

Pulse oximetry can also deliver important COVID-19 clues for people at home, though there are asterisks.

A typical person’s oxygen-saturation percentage level hovers between the mid-90s and 100. But readings taken at home showing 85% saturation and dropping can be an early warning sign of COVID-19.

Dave Hemink, CEO of Nonin Medical, said an early-warning signal like a low-oxygen saturation could cause people to call the hospital or clinic for help sooner, which is why the devices are becoming more common in first-aid kits at home — for those who can buy them.

(Nonin’s medical-grade devices are scarce online through the company’s distributors.)

Hemink also argued that earlier detection could help some patients avoid going on ventilators, though Stiehm was skeptical of that.

Both agreed, however, that any reading from a pulse-oximetry device is just one piece of data, and it can’t definitively diagnose a case.

An array of medical- and consumer-grade devices from various manufacturers is available online and in pharmacies. Consumer Reports said they cost between $25 and $100, and notes that devices sold for home use are not as accurate as hospital-grade models.

Besides device quality, other factors that can lead to erroneous readings from a pulse oximeter include opaque fingernail polish that interferes with the fingertip scanner, and cooler temperatures that give skin a bluish tint.