- During the COVID-19 pandemic, one measurement became more important than almost any other — blood oxygen saturation.
- It was the one concrete number that doctors could use to judge how severe a case of COVID-19.
- But pulse oximeters, the device most commonly used to measure blood oxygen levels, don’t work as well for patients of color.
“There’s no doubt in my mind that this has led to people not getting care, not getting timely care, or even being sent home or staying home to die from COVID-19,” says Noha Aboelata, a family practice physician at the Roots Community Health Center.
Inequity Kept In Dark
The inaccuracy of pulse oximeters is not a new problem. As early as 1976, scientists at Hewlett-Packard recognized that pulse oximeters needed to be calibrated to different skin tones. “Because skin pigmentation and other absorbers affect the measurement, the method is not capable of making absolute measurements,” two scientists wrote in the company’s journal at the time.
“It basically was born out of being tested on a white population,” says Usha Lee McFarling, a national science correspondent for STAT, who has covered pulse oximeters extensively. “They were tested in the eighties in a time when there was so little diversity in our clinical trials and testing.” Doctors were largely unaware that the device that was instrumental in their work was faulty. “This is just ubiquitous in my world. I wouldn’t think of taking a pulse or a blood pressure without taking a pulse oximetry in my daily practice,” says Dr. E. Jane Carter, a pulmonologist at Brown University.
Powered By Light
The pulse oximeter that most people use is a small device that clips onto a person’s finger. The device shines two wavelengths of light into the finger. One wavelength is absorbed by oxygenated blood, and the other is absorbed by deoxygenated blood. “Then we look at their relative absorption to take the ratio metric measure of how much one gets more absorbed than the other, and use that to estimate the amount of oxygen that is saturated in our blood,” Toussaint says.
The oximeter then calculates what percentage of a person’s blood has oxygen. But light is also absorbed by melanin, the protein that pigments skin. Because the devices were designed and calibrated using lighter skin tones, the melanin in people with darker skin causes the oximeter to overestimate how much oxygenated blood they have — and to underestimate how severe a case of COVID-19 is.
“We are trying to exploit the polarization of electric field properties of light,” Toussaint says. “What we’ve been exploring is whether or not we can use this to differentiate the deoxy- from oxyhemoglobin — preferably at the same wavelength of light to minimize some of the other challenges of going across the spectrum and having different responses to melanin.”
Exploring Solutions
Even if Toussaint’s device works, it will take time to be approved and reach patients’ fingers. In the meantime, doctors like Aboelata and Carter are using workarounds. Aboelata has patients use an oximeter when they are healthy, creating a baseline to compare against when they are sick. But in situations where precise blood oxygen levels are high stakes – like admission to the hospital for treatment or approval for home oxygen to treat emphysema, Carter and Aboelata are turning back to the more difficult arterial blood gas test.
The test requires both a more difficult type of blood draw than most clinicians are used to and for the blood sample to be put into ice and immediately taken to a lab. This means that many clinics like Aboelata can’t even run the test. So if her patients need it she has to send them elsewhere. But, doctors also have to change the way they think of oximeters, she says. Toussaint is hoping his polarization-based oximeter can be that test, so he can begin testing smaller models and companies can adopt his technology.
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Source: Npr
