A direct comparison of a pulse oximeter and the gold-standard measurement of oxygen saturation in arterial blood varied significantly between patients with darkly pigmented skin compared to lighter skin pigmentation, according to the EquiOx study presented during a Late-Breaking Clinical Trial session at ACC.25 in Chicago.

In the largest prospective real-world study to evaluate the relationship between skin pigmentation and pulse oximeter bias in critically ill patients, researchers enrolled 631 patients who received treatment in the intensive care unit at Zuckerberg San Francisco General Hospital, a level 1 trauma center between 2022 and 2024, to receive at least two oxygen saturation readings by both a pulse oximeter and blood gas analysis, performed simultaneously. Their mean age was 62 years, and in medical records 25% were identified as White, 21% as Black, 20% as Hispanic and 20% as Asian.

Skin pigmentation was measured using both the subjective Monk Skin Tone Scale and objective measurements of melanin content with a spectrophotometer. According to the objective measurements, 53% of patients were classified as having medium pigmentation, 33% as having light pigment and 14% as darkly pigmented.

Results showed that pulse oximeter readings, on average, underestimated blood oxygen levels. However, pulse oximeters also overestimated oxygen saturation, showing a positive bias in 20% of observations, and the proportion of observations with positive bias differed across skin pigment groups. Investigators determined that pulse oximeter performance was worse than current regulatory guidance standards.

"Although pulse oximeter bias on average was negative for all people, it was less negative in the darkly pigmented people than in the people with lighter pigment, meaning that pulse oximeters do not perform the same across different skin pigment categories," said Carolyn Hendrickson, MD, the study's first author. "We also found that the proportion of positive bias – the one that goes in the worrisome direction meaning that someone might have dangerously low oxygen saturation that is not detected with a non-invasive monitor – was higher in patients with dark skin pigment compared to those with medium and light skin pigment."

"Our study shows that the oximeters have a lot more uncertainty in the critically ill patients than they do in the healthy volunteers who participate in validation studies," she continued. "More discussion is needed between manufacturers, regulators and clinicians to draw attention to times when the oximeter is uncertain."

Hendrickson calls for additional large, prospective studies with a greater representation of patients with darker pigmentation and more observations in lower oxygen saturation ranges to further clarify patterns in pulse oximetry bias.

Keywords: ACC Annual Scientific Session, ACC25, Health Care Technology, Race and Ethnicity, Heart Rate