Equity-Driven Sensing System for Measuring Skin Tone–Calibrated Peripheral Blood Oxygen Saturation (OptoBeat): Development, Design, and Evaluation Study

Abstract

Background Many commodity pulse oximeters are insufficiently calibrated for patients with darker skin. We demonstrate a quantitative measurement of this disparity in peripheral blood oxygen saturation (SpO2) with a controlled experiment. To mitigate this, we present OptoBeat, an ultra–low-cost smartphone-based optical sensing system that captures SpO2 and heart rate while calibrating for differences in skin tone. Our sensing system can be constructed from commodity components and 3D-printed clips for approximately US $1. In our experiments, we demonstrate the efficacy of the OptoBeat system, which can measure SpO2 within 1% of the ground truth in levels as low as 75%. Objective The objective of this work is to test the following hypotheses and implement an ultra–low-cost smartphone adapter to measure SpO2: skin tone has a significant effect on pulse oximeter measurements (hypothesis 1), images of skin tone can be used to calibrate pulse oximeter error (hypothesis 2), and SpO2 can be measured with a smartphone camera using the screen as a light source (hypothesis 3). Methods Synthetic skin with the same optical properties as human skin was used in ex vivo experiments. A skin tone scale was placed in images for calibration and ground truth. To achieve a wide range of SpO2 for measurement, we reoxygenated sheep blood and pumped it through synthetic arteries. A custom optical system was connected from the smartphone screen (flashing red and blue) to the analyte and into the phone’s camera for measurement. Results The 3 skin tones were accurately classified according to the Fitzpatrick scale as types 2, 3, and 5. Classification was performed using the Euclidean distance between the measured red, green, and blue values. Traditional pulse oximeter measurements (n=2000) showed significant differences between skin tones in both alternating current and direct current measurements using ANOVA (direct current: F2,5997=3.1170 × 105, P<.01; alternating current: F2,5997=8.07 × 106, P<.01). Continuous SpO2 measurements (n=400; 10-second samples, 67 minutes total) from 95% to 75% were captured using OptoBeat in an ex vivo experiment. The accuracy was measured to be within 1% of the ground truth via quadratic support vector machine regression and 10-fold cross-validation (R2=0.97, root mean square error=0.7, mean square error=0.49, and mean absolute error=0.5). In the human-participant proof-of-concept experiment (N=3; samples=3 × N, duration=20-30 seconds per sample), SpO2 measurements were accurate to within 0.5% of the ground truth, and pulse rate measurements were accurate to within 1.7% of the ground truth. Conclusions In this work, we demonstrate that skin tone has a significant effect on SpO2 measurements and the design and evaluation of OptoBeat. The ultra-low-cost OptoBeat system enables smartphones to classify skin tone for calibration, reliably measure SpO2 as low as 75%, and normalize to avoid skin tone–based bias.

Diversity, Equity, and Inclusion in Clinical Research: A Path Toward Precision Health for Everyone

“…There are other examples that reflect how the lack of representativeness in foundational data for the development of digital tools may ultimately lead to wrong practices 69 . Data captured by sensors can be hopelessly biased if calibrated with only one physical paradigm in mind, for instance, when skin pigmentation affects the accuracy of readouts, in a clear example of a representativeness failure; as a result, the evidence derived from unrepresentative data has led to wrong clinical practice patterns for specific segments within a diverse patient population 70 . The development of these technologies has to contemplate the diverse populations where they are to be deployed.…”

Effects of skin tone on photoacoustic imaging and oximetry

“…Measurement biases introduced by the differential absorption of melanin in darker skin tones have been identified in several technologies where light passes through the skin before making a quantitative measurement, including pulse oximeters, 8 – 18 bilirubinometers, 19 , 20 wearable technologies, 21 – 24 cerebral oximeters, 25 and optical reflectance measurements 26 . These biases can have significant implications for patient management, an issue that came to the fore in the management of COVID-19 patients, where the over-estimation of blood oxygenation by pulse oximetry in black patients may have led to under-diagnosis of hypoxemia compared to white patients 17 .…”

The effects of skin tone on photoacoustic imaging and oximetry

“…Measurement biases introduced by the differential absorption of melanin in darker skin tones have been identified in several technologies where light passes through the skin before making a quantitative measurement, including pulse oximeters, [8][9][10][11][12][13][14][15][16][17][18] bilirubinometers, 19,20 wearable technologies, [21][22][23][24] cerebral oximeters 25 and optical reflectance measurements. 26 These biases can have significant implications for patient management, an issue that came to the fore in the management of COVID-19 patients, where the over-estimation of blood oxygenation by pulse oximetry in black patients may have led to under-diagnosis of hypoxaemia compared to white patients.…”