(CNN) – In the fight against Covid-19, the pulse oximeter has been a fundamental tool for physicians and other medical professionals. But according to multiple studies and government health agencies, the tiny device that monitors oxygen levels may not work well for dark-skinned people.
Pulse oximeters, the use of which has increased during the pandemic, may give inaccurate results, the U.S. Food and Drug Administration warned Friday.
Earlier this week, the U.S. Centers for Disease Control and Prevention also updated their clinical guidelines on coronavirus to warn doctors and nurses that data from several studies suggests that skin pigmentation may affect the precision of the devices.
“While pulse oximeters can be useful in estimating blood oxygen levels, these devices have limitations that can lead to inaccurate readings,” says Dr. William Maisel, director of the Office of Product Evaluation and Quality at the FDA Center for Devices and Radiological Health, said in a statement.
Pulse oximeters are small, tweezer-shaped devices that are painlessly attached to the patient’s finger and constantly monitor the amount of oxygen in the blood. Covid-19 is a respiratory disease, which means it attacks the lungs first, so low oxygen levels are a sign that the patient may be getting worse. CDC data shows that black Americans, Latinos, and Native American Indians are four times more likely to be hospitalized with COVID-19 than others in the United States.
The recent warnings come after a study published in December in the New England Journal of MedicineDr. Michael Sjoding and several colleagues at the University of Michigan analyzed data from more than 10,000 patients. For each patient, they compared the oxygen levels recorded by a pulse oximeter with those measured by arterial blood gas, a much more accurate, but painful and invasive procedure.
The researchers wanted to know how often the pulse oximeter showed relatively normal oxygen levels when it should have registered something more concerning.
In white patients, the pulse oximeter gave a misleading figure in 3.6% of the cases. In black patients this was 11.7%.
The bottom line, says Dr. Sjoding, is that pulse oximeters were three times more likely to lose significantly low oxygen levels or hypoxemia in black patients. The study suggests that one in 10 black patients may get misleading results.
Why the measurements are not accurate
There is a relatively simple explanation as to why. Pulse oximeters work by sending two types of red light through your finger. A sensor on the other side of the device captures this light and uses it to detect the color of the blood; bright red blood is highly oxygenated, while blue or purplish blood is less. If the device is not calibrated for dark skin, pigmentation can affect how light is absorbed. Dark nail polish can have a similar effect.
Experienced doctors don’t rely solely on the pulse oximeter to diagnose or decide on a patient’s treatment, says Dr. Michelle Ng Gong, chief of Critical Care Medicine and chief of Pulmonary Medicine at Montefiore Medical Center in New York.
“They would never tell a patient that I don’t care how bad they feel, as long as that number is correct, don’t worry,” says Dr. Gong. “It’s a tool. And as a resource, we must be able to use it correctly in the context of other information.
But, says Dr. Gong, during the pandemic, when hospitals are overwhelmed and doctors who are not normally in the emergency room are brought in to see and classify patients, the numbers on a pulse oximeter can weigh more heavily. An inaccurate reading can be particularly annoying if the patient’s oxygen level is borderline.
Doctors also need to be aware of their own unconscious biases, says Dr. Gong. For example, studies show that when blacks complain of pain, they are less likely to (or not) receive painkillers than whites. If a patient says he is having trouble breathing, doctors shouldn’t rule this out just because the pulse oximeter reading is normal.
“The only way health inequalities can be reduced,” says Dr. Gong, “is if we attack them from multiple targets, both from the main contributions and from our personal interactions.”
Doctors aren’t the only ones who use pulse oximeters. As hospitals reached full capacity during the pandemic, some Covid-19 patients were sent home with less severe symptoms to monitor their progress. According to market research, sales of home pulse oximeters have increased 500% since the new coronavirus arrived in the United States.
Amy Moran-Thomas, a professor of anthropology at MIT, began researching pulse oximeters last year when her husband was sent home with one.
The findings are not new
Moran-Thomas discovered studies dating back to the 1990s that suggested there was a problem with pulse oximeters in dark-skinned patients.
In 2005, a study conducted in the Hypoxia Lab at the University of California, San Francisco, found that three different models of pulse oximeters overestimate oxygen levels in dark patients. They did a follow-up study in 2007 with similar results.
A reading of 77 like my husband’s could hide a true satiety as low as 69, an even greater immediate danger. But EMTs or admission nurses may not be able to see those discrepancies. The number seems objective and racially neutral, ”Moran-Thomas wrote in the Boston Review.
Moran-Thomas wondered why the problem, identified decades ago, had not yet been resolved.
“I am a pulmonologist and intensive care physician,” said Dr. Sjoding, who began his studies after reading Moran-Thomas’s article. “One of our co-authors is a prominent black physician from the University of Michigan. None of us knew this. None of us have known about these studies since the mid-2000s. It was not part of our training.
Public attention appears to be improving. On January 25, several US senators sent a letter to the FDA urging them to investigate the matter. The FDA is responsible for approving every medical pulse oximeter before it enters the market. The agency currently requires pulse oximeters to be tested on different skin tones, meaning that “at least 2 (subjects) are darkly pigmented or 15% of the subject group, whichever is greater.”
The FDA says it is carefully reviewing the available data to determine if additional agency guidelines or studies are needed. It recently adopted a smartphone pulse oximeter that claims to work on all skin tones, with an automatic calibration period of 10-20 seconds to accurately measure how light moves through the user’s skin.
“These devices are not intended to be the sole or primary use of the information to make a clinical diagnosis or treatment decision,” said Dr. Maisel Wednesday in an interview to CNN. “One shouldn’t put too much faith in a pulse oximeter measurement, even if it is the most accurate product.”
“Just make sure you are aware of that when making decisions,” says Dr. Sjoding. The pulse oximeter reading may be a few points away. And if so, would you care for this patient in a different way?