This spring, our daughter insisted that my husband and I, both in our 60s, get the latest weapon in the fight against the novel coronavirus: the pulse oximeter, a device that can measure the level of oxygen in the blood. Recent reports have shown that a lower-than-healthy level of oxygen saturation in the blood can be an early predictor of covid-related pneumonia, even in those with mild symptoms.

Joanna, in her third trimester of pregnancy at the time, lives in New York and had bought the device — about the size of a Matchbox car — for her and her husband so they could be fortified with information before they checked into the hospital for the arrival in early June of their daughter.

But I was game, too: I enjoy these kinds of tests because I am a sucker for ways to measure my fitness against anyone else’s.

Also, we had been in Indonesia and Japan in February, squeezing some travel before we realized the enormity of the coronavirus pandemic. After we returned from Asia, I drove to Upstate New York for a six-day visit with my 89-year-old mother. We hugged, we kissed, we went out to restaurants, shops, grocery stores. Back in Washington, D.C., I went swimming at Wilson High School, to yoga at the gym, got a haircut and color at my salon, and saw friends. Who knew?

As much of the world began quarantining I began to worry: Was I an asymptomatic carrier of the coronavirus? None of my contacts had gotten sick. But, there were a few days at the end of March when my eyes were bloodshot. That went away. And I did have a bad case of stomach upset in Asia, but figured that was due to my enthusiastic embrace of street food. But I didn’t know for sure, since it was almost impossible to get an actual coronavirus test without symptoms.

So I ordered the device, and it finally arrived in mid-May. I stuck my finger into the device, a small rectangular box with an opening for a finger, and waited.

My oxygen saturation was 91. Hmmm. Below that number was one that measures heart rate. Mine was 47. Did that seem right? I started consulting Dr. Google. “Values under 90 percent are considered low” for oxygen saturation, according to the Mayo Clinic’s online site. The normal range is 95 to 100. My super-low heart rate also seemed more like something an Olympic long-distance runner might boast.

I tried again. This time it was 90. Was I dying? Did I have an asymptomatic case of the coronavirus? I texted my daughter.

“Talk to your doctor,” she said. “That doesn’t sound right.”

Instead, I waited and consulted the Internet. I managed to get the readings up to 95 and then 100. My resting pulse was up to 50, which was still very low but I figured as a regular jogger that wasn’t too strange.

The next day, I went for a run in the brisk air, took a shower and had breakfast. I tried again. This time the reading said my oxygen saturation level was 71. Uh oh. I tried again. 78. I started feeling dizzy, a little queasy.

I took my oxygen level again — again and again. 93, 98, 98, 97. Then 81 at 10:30 a.m. and 83 at 11:30 a.m.

I called my doctor. He asked me to test my 34-year-old son, who has been staying with us for the quarantine. His reading was 95.

“Are your hands and feet blue?” he asked. No.

“Do you have a temperature?” No.

“Unless your feet and hands are blue, you’d never get a reading of 71,” my doctor said. “And if you had any kind of lung disease, the readings wouldn’t be all over like that. But if you consistently get a reading of 90 or below, that would require you to get tested,” he said. He told me to make sure my hands were warm and that I was using the device properly.

One of the oddities of covid-19 is that people with mild symptoms may think they can wait them out at home. If their oxygen readings are low, though, it could mean that even if they feel okay, they are headed in the wrong direction and might soon crash, doctors say. That’s why having that reading in their arsenal is another useful measure.

Here’s where I have to admit that I didn’t fully research how to use the pulse oximeter before I began testing myself. The instructions that came with the Zacurate Pro Series 500DL Fingertip Pulse Oxygen Saturation Monitor ($35.95, but seemingly out of stock on Amazon) mainly concerned how to put in the two AAA batteries.

It turned out that wearing nail polish or having cold hands could create a falsely low reading. I wasn’t wearing polish, but my hands were cold. I also needed to make sure that my finger was fully inserted after I turned on the device, not before. Some people suggested I use my middle finger, not pointer, and insert it palm side up to get a better reading. When I did all that, I had consistent readings in the high 90s. Turns out I wasn’t dying after all.

“If you are just checking and not feeling bad, and the number looks low, that’s probably not enough reason to be concerned,” says Sharon Chekijian, assistant professor of emergency medicine at Yale School of Medicine. She treats people who come into the hospital’s emergency room and often makes decisions about admittance based on how low their oxygen levels are.

Having a pulse oximeter at home is a good idea for people who suspect they might have the coronavirus. “We’re seeing a lot of people coming in [to the emergency room] feeling lousy, when their oxygen level is normal,” she says. That means extraneous trips to the hospital, and potential exposure to illnesses, including the coronavirus among people who are truly sick.

For people who are healthy but worried that they might get sick, constant testing with a pulse oximeter is not necessarily a great plan, she adds. “It can create so much anxiety. The right tool in the wrong hands can lead to a lot of anxiety.”

Panagis Galiatsatos, a pulmonary and critical care doctor at Johns Hopkins Hospital in Baltimore, also says that while having a pulse oximeter at home is a useful device for patients with chronic obstructive pulmonary disease, or other lung disease, it could be misread by those who are not using it properly or don’t know how to interpret it in the context of their medical condition.

“You don’t want to overreact and try to fix a number when the rest of the person is fine,” he says. If people are feeling unwell and need another data point with all their other symptoms, it could be useful. But taking an incorrect reading could send a person to the emergency room, and these days “we want to be mindful of who we send” to the ER, he says. You don’t want people “going in negative and coming out positive” for the coronavirus.

Patti Geary Kessel, a retired nurse who now lives in Florida, says that when she worked as a high school nurse, she used a pulse oximeter for the asthmatic students who came into her office. It helped her decide whether they needed to use an inhaler, get on a nebulizer for a stronger infusion of anti-inflammatory medicine or contact their doctor. And she has one at home because she had cared for her late father, who had congestive heart failure.

For healthy people, she says, it could be useful if they measure their oxygen level to get a baseline before they feel sick. “It should be in the high 90s,” she says. “If it starts dropping, something is probably going on.” At that point, a person should call their doctor. Healthy people who suddenly get a reading in the 80s would probably feel short of breath, she says.

As for me, I took my reading daily for a few weeks after I got the meter. Now, though, I only take a quick reading if I have other mild problems, like fatigue or a headache, to reassure myself once again that I’m not dying. My readings are consistently in the 90s. When my husband, 69, took his, he got a reading of 100. He thinks that makes him healthier than me. I think we’ve been quarantined together for way too long.