Pulse oximeters are one of the most commonly used tools in medicine. The small devices, which resemble a clothespin, measure blood oxygen when clipped onto a fingertip, and they can quickly indicate whether a patient needs urgent medical care.
Health providers use them when they take vital signs and when they evaluate patients for treatment. Ever since the pandemic started, doctors have encouraged patients with Covid to use them at home.
But in people with dark skin, the devices can provide misleading results in more than one in 10 people, according to a new study.
The findings, which were published last week as a letter to the editor of a top medical journal, sent ripples of dismay through the medical community, which relies heavily on the devices to decide whether to admit patients or send them home.
During the coronavirus outbreaks, the inexpensive devices have also become a widely sold item online, used by consumers to monitor their own oxygen levels at home when doctors have told them they’re not sick enough to be hospitalized.
The report also stirred concerns because the pandemic is taking a disproportionate toll on Black and Hispanic Americans, drawing attention to racial health disparities and prompting soul-searching among doctors about bias that permeates the practice of medicine. There have been several reports of acutely ill Black patients who sought medical care, only to be turned away, and studies have found that African-Americans were hospitalized at higher rates, suggesting delays in access to medical care.
The researchers who conducted the oximeter study said they were surprised by the findings. Though scientific reports of the inaccuracies have been published in the past, they did not receive widespread attention or get incorporated into medical training.
“I think most of the medical community has been operating on the assumption that pulse oximetry is quite accurate,” said Dr. Michael W. Sjoding, an assistant professor of internal medicine at the University of Michigan Medical School and lead author of the new report, which appeared in the New England Journal of Medicine. “I’m a trained pulmonologist and critical care physician, and I had no understanding that the pulse ox was potentially inaccurate — and that I was missing hypoxemia in a certain minority of patients.”
Dr. Utibe Essien, an assistant professor of medicine at the University of Pittsburgh School of Medicine who studies racial disparities in cardiovascular disease, noted that doctors practicing telemedicine have relied on reporting from these devices.
“If we cannot ensure that its definition of low oxygen in people, especially Black people, is accurate, there is a concern that it is increasing or driving disparities,” he said.
Pulse oximeters work by shining two wavelengths of light, a red light and an infrared light, that pass through the skin of a finger.
The device detects the color of blood, which differs depending on the amount of oxygen. Oxygenated blood is bright cherry red, and deoxygenated blood has a more purplish hue. Depending on the hue, different amounts of light from the device are absorbed, and the oximeter analyzes the proportions of the absorption and calculates the amount of oxygen. Researchers suspect that the inaccurate readings may be occurring because of the way the light is absorbed by darker skin pigments.
Dr. Philip Bickler, the director of the hypoxia research laboratory at University of California, San Francisco, which tests the performance of pulse oximeters, said the simplest way to explain the inaccuracies in patients with darker skin is that the pigment “scatters the light around, so the signal is reduced. It’s like adding static to your radio signal. You get more noise, less signal.” (Dark nail polish also reduces the accuracy, as do cold fingers.)
Generally health providers treating patients take many metrics into account, including imaging scans, inflammatory markers and other clinical signs, said Dr. Darshali Vyas, a resident physician at Massachusetts General Hospital who has done research on medical decision-making tools that incorporate race. But, she said, “pulse oximetry remains one of the mainstays.”
The new findings “help quantify the potential harm done by a ubiquitous medical tool that may normalize white skin as the default,” Dr. Vyas said in an email. She added that this could be “especially concerning” for doctors using the readings to adjust the amount of supplemental oxygen they give Covid patients, and to determine transfers to intensive care.
The study compared pulse oximeter measures with values obtained from a more invasive type of test, called an arterial blood gas test, carried out in the same patients at about the same time. Arterial blood gas tests are used more rarely, because they require drawing blood from an artery, which is a more invasive procedure than drawing blood from a vein.
The analysis, of 10,789 paired test results from 1,333 white patients and 276 Black patients hospitalized at the University of Michigan earlier this year, found that pulse oximetry overestimated oxygen levels 3.6 percent of the time in white patients, but got it wrong nearly 12 percent of the time, or more than three times more often, in Black patients.
In these patients, the pulse oximeter measures erroneously indicated the oxygen saturation level was between 92 and 96 percent, when it was actually as low as 88 percent (the results were adjusted for age, sex and cardiovascular disease).
The Coronavirus Outbreak ›
Words to Know About Testing
Confused by the terms about coronavirus testing? Let us help:
- Antibody: A protein produced by the immune system that can recognize and attach precisely to specific kinds of viruses, bacteria, or other invaders.
- Antibody test/serology test: A test that detects antibodies specific to the coronavirus. Antibodies begin to appear in the blood about a week after the coronavirus has infected the body. Because antibodies take so long to develop, an antibody test can’t reliably diagnose an ongoing infection. But it can identify people who have been exposed to the coronavirus in the past.
- Antigen test: This test detects bits of coronavirus proteins called antigens. Antigen tests are fast, taking as little as five minutes, but are less accurate than tests that detect genetic material from the virus.
- Coronavirus: Any virus that belongs to the Orthocoronavirinae family of viruses. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
- Covid-19: The disease caused by the new coronavirus. The name is short for coronavirus disease 2019.
- Isolation and quarantine: Isolation is the separation of people who know they are sick with a contagious disease from those who are not sick. Quarantine refers to restricting the movement of people who have been exposed to a virus.
- Nasopharyngeal swab: A long, flexible stick, tipped with a soft swab, that is inserted deep into the nose to get samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be collected with swabs that do not go as deep into the nose — sometimes called nasal swabs — or oral or throat swabs.
- Polymerase Chain Reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. Tests that use PCR enable researchers to detect the coronavirus even when it is scarce.
- Viral load: The amount of virus in a person’s body. In people infected by the coronavirus, the viral load may peak before they start to show symptoms, if symptoms appear at all.
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