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For decades, scientists have discovered that pulse oximeters, devices that assess blood oxygen saturation, can be affected by the color of a person’s skin. In 2021, the FDA issued a warning about this limitation of pulse oximeters. The agency indicates that it plans to organize a pulse oximeter meeting Later this year. Because low oxygen saturation, called hypoxemia, is a common symptom of COVID-19, low blood oxygen allows patients to receive certain medications. In the first study to examine this question in COVID-19 patients, published in JAMA internal medicinein May, researchers found that the inaccurate measurements led to “systemic failure,” delaying care for many black and Hispanic patients and, in some cases, preventing them from receiving the correct medications. The study adds a growing sense of urgency to a question raised decades ago.

“We found that among black and Hispanic patients, there was a significant delay in identifying severe COVID cases compared to white patients.”
—Dr. Ashraf Fawzy, Johns Hopkins University

Pulse oximeters work by passing light through a part of the body, usually a finger. These devices infer a patient’s blood oxygen saturation (i.e. the percentage of hemoglobin carrying oxygen) from the absorption of light by hemoglobin, the blood pigment that carries oxygen. In theory, pulse oximeters shouldn’t be affected by anything other than blood oxygen levels. But research has shown otherwise.

“If you have melanin, which is the pigment responsible for skin color…it could potentially affect the transmission of light passing through the skin,” said Govind Raoengineering professor and director of the Center for Advanced Sensor Technology at the University of Maryland, Baltimore County, who was not involved in the study.

To examine how patients with COVID-19 were affected by this flaw in pulse oximeters, researchers used data from more than 7,000 COVID-19 patients in the Johns Hopkins Hospital System, which includes five hospitals, between March 2020 and November 2021. In the first part of the study, researchers compared blood oxygen saturation for the 1,216 patients whose measurements were taken using a pulse oximeter and arterial blood gas analysis, which determines the same measurement using direct blood analysis. The researchers found that the pulse oximeter overestimated blood oxygen saturation by an average of 1.7% for Asian patients, 1.2% for black patients, and 1.1% for Hispanic patients.

Then the researchers used these results to create a statistical model to estimate what the arterial blood gas measurements would be for patients with only pulse oximeter measurements. Since arterial blood gases require inserting a needle into an artery to collect blood, most patients only have a pulse oximeter measurement.

To qualify for COVID-19 treatment with remdesiviran antiviral drug, and dexamethasone, a steroid, patients were required to have a blood oxygen saturation of 94% or less. Based on the researchers’ model, nearly 30% of 6,673 patients about whom they had enough information to predict their arterial blood gas measurements met this threshold. Many of these patients, most of whom were black or Hispanic, had their treatment delayed between 5 and 7 hours, with black patients being delayed an average of an hour longer than white patients.

“We found that in black and Hispanic patients, there was a significant delay in identifying severe COVID compared to white patients,” said Dr. Ashraf Fawzyassistant professor of medicine at Johns Hopkins University and author of the study.

There were 451 patients who never qualified for treatments but the researchers predicted likely; 55% were black, while 27% were Hispanic.

Study ‘shows how urgent it is to get away from the pulse [oximeters]said Rao, and to find other ways to measure blood oxygen saturation.

Studies showing that skin color can affect pulse oximeters go back until the 1980s. Despite knowing the problem, there are few ways to fix it. Wu says increased awareness helps, and doing more arterial blood gas testing can also be helpful.

A long-term solution will require changing technology, either using an entirely different method or having devices that can better adjust results to account for differences in skin color. A technological alternative is to have devices that measure the diffusion of oxygen through the skin, called transdermal measurement, which Rao’s lab is working to develop.

The researchers said one of the limitations of their study was how the patients self-identified, meaning that a wide range of skin pigmentation could be represented in each of the sample groups, based on how each patient identified themselves. Nor did the researchers measure the clinical impact of delaying or refusing treatment on patients, such as the likelihood of them dying, their health status, or the duration of their illness. Researchers are currently working on a study examining these additional questions and factors.

Although the problem of racial bias in pulse oximeters has no immediate solution, the researchers said, they are confident that the main obstacle is not technological.

“We believe the technology exists to solve this problem, and that would ultimately be the fairest solution for everyone,” Wu said.

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