Racial disparities in pulse oximeter readings: Research roundup

by Naseem S. Miller| July 20, 2022 |

A series of recent studies has found that pulse oximeters are more likely to give inaccurate readings for people with darker skin, especially Black people, by overestimating their blood oxygen levels. This is particularly important for critically ill patients, where doctors constantly rely on pulse oximeter readings to decide how much oxygen to give patients.

Pulse oximeters are small medical devices that clip on the fingertips and estimate the amount of oxygen in the blood. They are ubiquitous in doctors’ offices and hospitals and became popular during the pandemic as a way for people battling COVID-19 to monitor their oxygen levels at home.

Developed in the 1970s, pulse oximetry has been integrated into the routine monitoring of hospitalized patients and has become a fundamental tool in diagnosis and treatment of patients, especially those with respiratory illnesses. The devices have taken an even more significant role in hospitals since the COVID-19 pandemic began.

“For COVID, we became so dependent on pulse oximeters for [patient] triage,” says Dr. Theodore Iwashyna, an intensive care unit physician and senior author of a recent study on pulse oximeters in The BMJ. In addition, the readings from devices became critical in helping doctors decide which hospital patients were eligible for COVID-19 medication like remdesivir.

But a series of recent studies have found that pulse oximeters are more likely to give inaccurate readings for people with darker skin, especially Black people, by overestimating their blood oxygen levels. This is particularly important for critically ill patients, where doctors constantly rely on pulse oximeter readings to decide how much oxygen to give patients.

Unrecognized low blood oxygen levels can damage organs, most importantly the brain and the kidneys. Some studies that include data from COVID-19 patients suggest that inaccurate readings by pulse oximeters may be associated with worse outcomes for Black and Hispanic patients.

This is not the first time researchers have brought up the issue. Problems with pulse oximeters have been recorded since the late 1980s but those problems resurfaced as the pandemic spurred more use of the device and highlighted existing racial disparities in health care.

Researchers say today’s pulse oximeters aren’t calibrated to account for darker skin, because they’re built on data from mostly light-skinned, healthy study populations whose diversity doesn’t reflect that of the overall U.S. population.

Some manufacturers of pulse oximeters have asserted that their devices are accurate. But if their data doesn’t show a problem, “then how do [they] explain why this problem keeps showing up in critically ill patients?” asks Iwashyna, professor of medicine and public health at Johns Hopkins University. “There’s a role for journalists to keep asking the companies to comment on this and explain it.”

Dr. Eric Gottlieb, lecturer at the Laboratory for Computational Physiology at MIT and lead author of another study on pulse oximetry, also encourages journalists “to spread the word and raise awareness about” disparities in pulse oximetry readings.

During his research, Gottlieb was asked frequently what patients and physicians should know about the findings and whether they should consider them in their medical decision making, he says.

“And my response has been yes, I want them to know about it and be aware, but I think it’s really difficult to put the onus on either the patient or the physicians,” says Gottlieb. “I think this really puts the onus on the manufacturers, institutions and the government to really ensure that these devices are used equitably.”

In a safety communication in February 2021, the U.S. Food and Drug Administration referenced a 2020 study and said the pulse oximeters’ accuracy may be affected by several factors including skin pigmentation. In June, the agency announced in June plans to convene a public meeting to discuss the available evidence on the accuracy of pulse oximeters.

How pulse oximeters work

Blood oxygen levels are calculated in percentages. Generally, in healthy people, a blood oxygen level between 95% and 100% are considered normal, but the level varies for people who have chronic lung conditions such as asthma. In most people, blood oxygen levels below 88% require medical attention.

“Once you get below 88%, we start freaking out very quickly because it has a big impact on how much oxygen is getting to your tissues,” says Iwashyna.

There are two ways to measure the level of oxygen in the blood. The most accurate method is through a blood draw from an artery, usually in the wrist, which can be painful. The other method is pulse oximetry.

Medical professionals in hospitals depend on pulse oximeter readings because they are done in seconds, while it can take up to an hour for the results of an arterial blood sample to become available.

Pulse oximeters work by shining infrared and red lights through the skin. The devices then calculate the amount of light absorbed by hemoglobin — proteins inside red blood cells that carry oxygen from the lungs to tissues and organs.

“So it’s taking advantage of the fact that the light gets absorbed differently when the hemoglobin has oxygen on it versus when it doesn’t,” says Iwashyna.

Researchers haven’t exactly pinpointed why pulse oximeters are more likely to give inaccurate readings in people with darker skin, but one hypothesis is that some of the device’s light might be absorbed by melanin in the skin. Melanin is the substance that produces skin, eye and hair pigmentation. People with darker skin have more melanin.

Although skin tone is a predominant suspected factor affecting pulse oximetry readings, other factors may also play a role.

“I think it’s important to not make assumptions and really explore blood chemistry and other factors like differences by age and gender that can have effects as well,” said Gottlieb.

Errors in pulse oximeter readings can also be caused by motions such as shivering, interfering substances in the blood, misplacement, and low blood circulation. Even nail polish can affect the device’s readings.

“They mostly work,” says Iwashyna. “They just don’t work a lot and the times that they don’t work a lot happens more in Black patients.”

Below, we have gathered several studies that show the problem of racial bias in pulse oximetry. In most instances, researchers compare pulse oximetry readings with the results of arterial blood draw, usually done within 10 minutes of each other, to find out how accurate the devices’ readings are. And the most common condition they look for is hidden hypoxemia — also called occult hypoxemia — which is typically when a pulse oximeter shows a reading of 92% to 96% while arterial blood oxygen saturation is less than 88%.