Blood Oxygen Tests Can Be Inaccurate for Black Patients, Raising COVID Risks

The fingertip devices that hospitals use to monitor patients’ oxygen levels might be less accurate in people with dark skin, a new study suggests.

At issue are pulse oximeters — small medical devices that clip onto a fingertip and estimate how much oxygen is making it into the blood. They are routinely used in hospitals to help providers make treatment decisions.

And during the COVID-19 pandemic, pulse oximeters have increasingly made their way into the hands of lay people. Fairly cheap home devices are available, and people with milder cases of COVID have used them to monitor their oxygen levels.

But the new study suggests that Black patients are at greater risk of having a falsely reassuring reading on pulse oximetry than white patients are.

Researchers at the University of Michigan Hospital found that in cases where Black patients appeared to have adequate oxygen levels on pulse oximetry, their blood oxygen was actually low 12% of the time.

In contrast, the mismatch happened in white patients less than 4% of the time, the researchers report in the Dec. 17 issue of the New England Journal of Medicine.

“It really surprised us all,” said lead researcher Dr. Michael Sjoding, an assistant professor of internal medicine at Michigan Medicine.

The impetus for the study was, like so many things in medicine, the pandemic.

Early on, when Sjoding and his colleagues were seeing more and more COVID patients — many of whom were Black — they noticed a recurring discrepancy: Some patients’ pulse oximetry readings were not matching up with arterial blood gas tests, which sample patients’ blood to directly gauge oxygen saturation.

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Overall, the study found, discrepancies were nearly three times more common among Black patients than white patients.

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As for why skin tone would matter, Sjoding explained the basic theory: Pulse oximeters work by emitting certain wavelengths of light, and skin pigment may absorb some of that light.

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