“While pulse oximeters may be useful for estimating blood oxygen levels, these devices have limitations that can result in inaccurate readings,” Dr. William Maisel, director of the Office of Product Evaluation and Quality in the FDA’s Center for Devices and Radiological Health, said in a statement.
The researchers wanted to know how often the pulse oximeter was showing a relatively normal oxygen level when it should have been registering something more concerning.
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 takeaway, Dr. Sjoding says, is that pulse oximeters were three times as likely to miss significantly low oxygen levels — or hypoxemia — in Black patients. The study suggests one in every 10 Black patients may be getting deceptive results.
Why the readings aren’t precise
There’s a relatively simple explanation for why. Pulse oximeters work by sending two types of red light through your finger. A sensor on the other side of the device picks up this light and uses it to detect the color of your blood; bright red blood is highly oxygenated, while blue or purplish blood is less. If the device isn’t calibrated for darker skin, the pigmentation could affect how the light is absorbed. Dark nail polish can cause a similar effect.
Experienced physicians don’t rely on the pulse oximeter alone to make a diagnosis or decide a patient’s treatment, says Dr. Michelle Ng Gong, chief of Critical Care Medicine and the 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 is a tool. And as a tool, we need to be able to use it properly in the context of other information.”
But, Dr. Gong says, during the pandemic, when hospitals are overwhelmed, and doctors who may not normally be in the emergency room are being brought in to see and triage patients, numbers from a pulse oximeter may hold more weight. An inaccurate reading could be particularly problematic if the patient’s oxygen level is borderline.
“The only way health disparities can be reduced,” Dr. Gong says, “is if we attack it from multiple ends, from both the larger contributions as well as our personal interactions.”
Amy Moran-Thomas, a professor of anthropology at MIT, started researching pulse oximeters last year when her husband was sent home with one.
The findings aren’t new
Moran-Thomas uncovered studies going back to the 1990s that suggested there was a problem with pulse oximeters in darker-skinned patients.
In 2005, a study done at the University of California, San Francisco’s Hypoxia Lab, 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.
Moran-Thomas wondered why the problem, identified decades ago, hadn’t already been fixed.
“I am a pulmonary and critical care physician,” says Dr. Sjoding, who began his study after reading Moran-Thomas’ article. “One of our coauthors is a prominent Black physician from the University of Michigan. None of us knew this. None of us knew about these studies from the mid 2000s. It was not a part of our training.”
“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 Wednesday. “Someone shouldn’t overly rely on a pulse oximeter reading, even if it’s the most accurate product.”
“Just make sure you’re aware of that when you’re making decisions,” says Dr. Sjoding. “The pulse oximeter reading could be a couple of points off. And if that is the case, would you care for this patient in a different way?”