A paramedic uses a pulse oximeter to check a patient's vital signs during an August home visit in the Bronx borough of New York. Angus Mordant/Bloomberg via Getty Images
The common fingertip devices that measures oxygen in the blood can sometimes give misleading readings in people with dark skin, according to a report Wednesday in The New England Journal of Medicine.
These devices, called pulse oximeters, are increasingly finding their way into people's homes, because of the COVID-19 pandemic. So, this is not just a concern for medical personnel using professional-grade devices.
Dr. Michael Sjoding and colleagues at the University of Michigan hospital in Ann Arbor came across this issue this year when they received an influx of COVID-19 patients from Detroit's overflowing hospitals. Many of these patients are Black. Sjoding noticed something odd about results from the fingertip device used throughout hospitals.
"It spits out this number called an oxygen saturation value, which gives us an understanding of how much oxygen is in the blood," Sjoding says.
But he noticed that oxygen reading was sometimes off when compared with a more sophisticated test that samples blood from a person's artery. So Sjoding and his colleagues started gathering data, comparing these readings in light-skinned people versus dark-skinned people. They measured how often a pulse oximeter reading — apparently in the normal range — actually came from someone with low oxygen as measured in an artery.
"We found this happened much more often in patients who were Black. Basically, about three times as often," he says.
His initial study was based on more than 10,000 measurements taken at his hospital – instances in which doctors had nearly simultaneous oxygen readings from a pulse oximeter and blood in a person's artery.
To validate that finding, he and his colleagues looked at another 37,000 measurements taken from 178 intensive care units.
About 12% of the time, apparently normal readings were actually misleading, the report says.
"It's not happening a lot, but if you think of how often these measurements are taken, if it's wrong 12% of the time, I worry that could be really impactful," Sjoding says.
And, though previous studies have flagged this issue, Sjoding says he doesn't think it's widely appreciated by physicians in ICUs. "When I ran these analyses, I was just really surprised." It's all the more important now that many people are buying pulse oximeters to monitor their own health at home.
He suspects the reason behind this is that the color of light used in the pulse oximeter can be absorbed by skin pigment.
Dr. Jewel Mullen, associate dean for health equity at the University of Texas Dell Medical School in Austin, agrees that this discrepancy is about skin color, not race. She's concerned about the impact the finding could have on people who may rely on consumer-grade pulse oximeter devices at home, sometimes in lieu of medical care. The devices are still a valuable tool, she says, but it's important to look at the results in context.
"If somebody has coronavirus and they're feeling really short of breath, but they're getting [an acceptable reading] — say an oxygen saturation of 92% to 96% or more — they should pay more attention to their shortness of breath, more than that oxygen that's being measured," Mullen says.
People should also keep an eye on trends, rather than absolute readings, she says. If a person's oxygen level drops significantly, that should prompt a call to a medical provider, even if the number is still in the "normal" zone.
Sjoding says pulse oximeters will remain important for hospitals as well, but he now thinks more carefully about how to interpret the readings.
You can contact NPR science correspondent Richard Harris at rharris@npr.org.
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