Written by Vivian Lei
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In a controlled hypoxemia study, pulse oximetry was falsely elevated in subjects with darker skin pigmentation and low perfusion.
Pulse oximetry’s color bias
We have covered racial differences in pulse oximeter accuracy before. Researchers at the Hypoxia Research Laboratory at the University of California San Francisco prospectively measured the contributions of skin pigmentation, perfusion index, sex, and age on pulse oximeter readings in a controlled setting. Between 2020 and 2021, they recruited 146 healthy volunteers. Skin pigmentation was classified by the Fitzpatrick scale, of which 43 subjects were classified as skin type V or VI (dark), 78 were skin type III or IV (medium), and 25 were skin type I or II (light). Using 2 common pulse oximeters (Nellcor N-595 and Masimo Radical 7), they measured pulse oximeter readings (Spo2) and corresponding arterial oxygen saturation (Sao2) at controlled levels of hypoxemia to determine the “bias,” or error.
The pulse oximeter bias was found to be significantly associated with skin pigmentation, perfusion index, and degree of hypoxia, but not with sex or age. Errors were larger during low perfusion in darkly pigmented subjects. In low perfusion conditions, they found the frequency of missed diagnosis of hypoxemia (defined as pulse oximeter readings of 92%–96% when arterial oxygen saturation was <88%) was 1.1% for light, 8.2% for medium, and 21.1% for dark skin.
How will this change my practice?
It is disconcerting that disparities in the delivery of health care can arise from the seemingly objective diagnostic tools we use every day. Since many patients in the emergency department are critically ill and may have low perfusion states, it is important to remember that relying on pulse oximetry alone may lead to missed hypoxemia in patients with medium and dark skin tones.
Editor’s note: The Masimo device missed hypoxemia in 30.2% dark skin volunteers with low perfusion while the Nellcor missed only 7.9% (which is still a lot but much better). Miss rates were similar to light or medium skin tones for both devices at higher perfusion index levels. My take is that you can trust your pulse oximeter in large the majority of your patients; however, in those with dark skin and suspected low perfusion, there may be benefit to obtaining an arterial blood gas. ~Nick Zelt
Source
Low Perfusion and Missed Diagnosis of Hypoxemia by Pulse Oximetry in Darkly Pigmented Skin: A Prospective Study. Anesth Analg. 2024 Mar 1;138(3):552-561. doi: 10.1213/ANE.0000000000006755. Epub 2023 Dec 18. PMID: 38109495.
