Pulse oximeters are essential in the fight against Covid. They may not work on dark skin

Pulse oximeters are essential in the fight against Covid. They may not work on dark skin

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Pulse oximeters, which increased in use during the pandemic, may yield inaccurate results, the U.S. Food and Drug Administration warned on Friday.
Earlier this week, the United States Centers for Disease Control and Prevention also updated their clinical guidelines on coronavirus to alert doctors and nurses that data from several studies suggest that skin pigmentation may affect the accuracy of devices.

“While pulse oximeters can be useful for estimating blood oxygen levels, these devices have limitations that can result in inaccurate readings,” said Dr. William Maisel, director of the Office of Product Quality and Evaluation at the Device Center and FDA Radiological Health in a statement.

Pulse oximeters are small tweezers that attach painlessly to the patient’s finger and constantly monitor the amount of oxygen in the blood. Covid-19 is a respiratory disease, which means that it attacks the lungs first, so low oxygen levels are a sign that the patient may be getting worse. CDC data shows that blacks, Latinos and Native Americans are four times more likely to be hospitalized with Covid-19 than others.
The recent caveats came after a study published in the New England Journal of Medicine in December. Dr. 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 gases – a much more accurate, but painful and invasive procedure.

The researchers wanted to know how often the pulse oximeter was showing a relatively normal oxygen level when it should be recording something more worrisome.

In white patients, the pulse oximeter gave a misleading number 3.6% of the time. In black patients, it was 11.7% of the time.

The conclusion, says Sjoding, is that pulse oximeters were three times more likely to lose significantly low levels of oxygen – or hypoxemia – in black patients. The study suggests that one in 10 black patients may be getting misleading results.

Why readings are not accurate

There is a relatively simple explanation as to why. Pulse oximeters work by sending two types of red light through the finger. A sensor on the other side of the device captures this light and uses it to detect the color of your blood; bright red blood is highly oxygenated, while blue or purple blood is less. If the device is not calibrated for darker skin, pigmentation can affect how light is absorbed. Dark enamel can have a similar effect.

Experienced doctors don’t just rely on the pulse oximeter to make a diagnosis or decide on a patient’s treatment, says Dr. Michelle Ng Gong, chief of intensive care and chief of pulmonary medicine at Montefiore Medical Center in New York.

“They would never tell a patient that I don’t care how badly you feel, as long as that number is OK, don’t worry about it,” says Dr. Gong. “It’s a tool. And as a tool, we need to be able to use it properly in the context of other information.”

But, says Gong, during the pandemic, when hospitals are overloaded and doctors who are not normally in the emergency room are brought in to see and screen patients, the numbers on a pulse oximeter may carry more weight. An inaccurate reading can be particularly problematic if the patient’s oxygen level is borderline.

Doctors also need to be aware of their own unconscious prejudice, says Dr. Gong. Studies show, for example, when African Americans complain of pain, they are less likely to receive as much (or any) pain medication as whites. If a patient says he is having difficulty breathing, doctors should not dismiss him just because the reading on the pulse oximeter is normal.

“The only way to reduce health disparities,” says Dr. Gong, “is to attack from multiple ends, both from major contributions and from our personal interactions.”

Doctors are also not the only ones using pulse oximeters. When hospitals reached their peak capacity during the pandemic, some Covid-19 patients with less severe symptoms were sent home to monitor their progress. According to market research, sales of domestic pulse oximeters have increased by 500% since the new coronavirus arrived in the United States.

Amy Moran-Thomas, professor of anthropology at MIT, started 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 that there was a problem with pulse oximeters in darker-skinned patients.

In 2005, a study at the Hypoxia Laboratory at the University of California, San Francisco, found that three different models of pulse oximeters overestimated oxygen levels in dark-skinned patients. They did a follow-up study in 2007 with similar results.

“A reading of 77 like my husband’s could hide a true saturation as low as 69 – even greater immediate danger. But paramedics or admission nurses may not be able to detect these discrepancies. The number seems objective and mixed race,” Moran- Thomas wrote in the Boston Review.

Moran-Thomas wondered why the problem, identified decades ago, had not yet been fixed.

“I am a pulmonary and intensive care physician,” said Dr. Sjoding, who started his study after reading the Moran-Thomas article. “One of our co-authors is a prominent black doctor at the University of Michigan. None of us knew that. None of us knew about these studies in the mid-2000s. It was not part of our training.”

Public attention seems to be increasing. On January 25, several US senators sent the FDA a letter urging them to examine the problem. The FDA is responsible for approving any medical pulse oximeter before entering the market. The agency currently requires that pulse oximeters be tested on a variety of skin tones, which means “at least 2 dark pigments (test subjects) or 15% of the subject pool, whichever is greater.”
The FDA says it is carefully reviewing the available data to determine whether additional guidance or studies from the agency are needed. She recently approved a smartphone pulse oximeter that claims to work on all skin tones, using 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 information to make a clinical diagnosis or treatment decision,” Dr. Maisel told CNN in an interview on Wednesday. “Someone should not rely too much on reading the pulse oximeter, even if it is the most accurate product.”

“Just make sure you are aware of this when making decisions,” says Dr. Sjoding. “The pulse oximeter reading may be a few points off. And if that is the case, would you care for this patient in a different way?”

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