Why Does the Device on Your Finger Struggle With Darker Skin?
A landmark NEJM study found Black patients had nearly three times the rate of dangerously low oxygen missed by a normal-looking pulse oximeter reading. The FDA has confirmed the accuracy gap in writing.
Why does the device on your finger struggle with darker skin? Because it was calibrated on a population that did not include enough of it, and the error is not small. A landmark 2020 analysis in the New England Journal of Medicine compared pulse oximeter readings against a direct blood gas measurement in two large patient cohorts and found that at pulse oximetry readings of 92% to 96%, true oxygen saturation had actually dropped below 88%, a dangerous level the device was missing entirely, in 17.0% of measurements from Black patients, against 6.2% from white patients.1 That is nearly three times the rate of undetected low oxygen in Black patients whose device readings looked reassuring.
A second, independent study found the same direction of error
This was not a one time result. A 2022 study of medical and surgical inpatients across the Veterans Health Administration from 2013 to 2019 measured the same gap and found that "unadjusted probabilities of occult hypoxemia were 15.6%... for white patients, and 19.6%... for black patients," a statistically significant difference confirmed at much larger scale, across a different health system and a different set of years.2 Two separate research groups, using two separate patient populations, measured the device making the same kind of mistake in the same direction.
The mechanism is about light, not about race as biology
A pulse oximeter works by shining light through the skin and measuring how much of it is absorbed by blood at different wavelengths. Melanin absorbs some of that same light, and the device's calibration has to account for how much. The devices in wide clinical use were developed and validated on populations that skewed toward lighter skin, so the correction built into the device's software was tuned to a light skin baseline, not because of anything different about the blood itself.
The regulator has confirmed the problem, without yet fixing it
The FDA has acknowledged this in writing. Its February 2021 safety communication found that "there are some accuracy differences in pulse oximeters between dark and light skin pigmentation," describing the difference as typically small at high oxygen saturations and larger once saturation drops, and noting that devices "tend to overestimate oxygen saturation levels in individuals with darker skin tones."3 The agency's advice in the meantime is to watch for other signs of low oxygen, shortness of breath, or bluish discoloration around the lips or nails, rather than to trust the number on the device alone.
Why this belongs in a dermatology conversation at all
A pulse oximeter is not a skin device, but the reason it fails is the same reason a visual exam can fail. An instrument calibrated on one range of skin tone is reading through that same tissue on a different one. The device on a fingertip in an emergency room and the eye of a clinician assessing a rash are both, in this specific sense, instruments trained on an incomplete population, and both currently give a systematically rosier picture than the reality for patients with more pigmented skin.
References
- Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial bias in pulse oximetry measurement. N Engl J Med. 2020, 383(25):2477-2478. PMID 33326721. doi.org/10.1056/NEJMc2029240
- Valbuena VSM, Seelye S, Sjoding MW, Valley TS, Dickson RP, Gay SE, Claar D, Prescott HC, Iwashyna TJ. Racial bias and reproducibility in pulse oximetry among medical and surgical inpatients in general care in the Veterans Health Administration 2013-19: multicenter, retrospective cohort study. BMJ. 2022, 378:e069775. doi.org/10.1136/bmj-2021-069775
- US Food and Drug Administration. Pulse Oximeters: accuracy, limitations, and the effect of skin pigmentation. FDA. fda.gov
