Racial Disparities in Pulse Oximeter Device Inaccuracy and Estimated Clinical Impact on COVID-19 Treatment Course

Sylvia E. K. Sudat; Paul Wesson; Kim F. Rhoads; Stephanie Brown; Noha Aboelata; Alice R. Pressman; Aravind Mani; Kristen M. J. Azar

Disclosures

Am J Epidemiol. 2023;192(5):703-713.

Abstract and Introduction

Abstract

Arterial blood oxygen saturation as measured by pulse oximetry (peripheral oxygen saturation (SpO₂)) may be differentially less accurate for people with darker skin pigmentation, which could potentially affect the course of coronavirus disease 2019 (COVID-19) treatment. We analyzed pulse oximeter accuracy and its association with COVID-19 treatment outcomes using electronic health record data from Sutter Health, a large, mixed-payer, integrated health-care delivery system in Northern California. We analyzed 2 cohorts: 1) 43,753 non-Hispanic White (NHW) or non-Hispanic Black/African-American (NHB) adults with concurrent arterial blood gas oxygen saturation/SpO₂ measurements taken between January 2020 and February 2021; and 2) 8,735 adults who went to a hospital emergency department with COVID-19 between July 2020 and February 2021. Pulse oximetry systematically overestimated blood oxygenation by 1% more in NHB individuals than in NHW individuals. For people with COVID-19, this was associated with lower admission probability (−3.1 percentage points), dexamethasone treatment (−3.1 percentage points), and supplemental oxygen treatment (−4.5 percentage points), as well as increased time to treatment: 37.2 minutes before dexamethasone initiation and 278.5 minutes before initiation of supplemental oxygen. These results call for additional investigation of pulse oximeters and suggest that current guidelines for development, testing, and calibration of these devices should be revisited, investigated, and revised.

Introduction

In February 2021, both the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) issued public statements regarding possible suboptimal accuracy of pulse oximeters in individuals with darker skin pigmentation.^[1,2] The CDC noted the potential for underdetection of hypoxemia in darker-skinned people with coronavirus disease 2019 (COVID-19), citing data from several studies.^[3–9] Despite this caution, an oxygen saturation level of less than 94%, as measured by pulse oximetry, remains a key determinant of severe disease.^[10,11] Pulse oximetry therefore influences treatment decisions for people with COVID-19, such as whether to initiate hospital-based treatments (e.g., supplemental oxygen or intravenous dexamethasone).^[11,12]

Pulse oximeters are external monitoring devices that clip onto a part of the body, typically a fingertip or earlobe, and emit a light that passes through the tissue and blood that is then detected by a sensor.^[13] The amount of light not absorbed is used to estimate the blood's oxygen saturation.^[13] Pulse oximeter accuracy can be affected by multiple factors such as nail polish, skin features (thickness, pigmentation, temperature), and tobacco use.^[1] Several studies have also shown pulse oximeters to be less accurate at lower oxygen saturations among severely ill patients.^[14,15]

Studies typically measure pulse oximeter accuracy by comparing pulse oximetry with concurrent oxygen saturation measured by arterial blood gas (ABG), which is considered the gold standard for measuring arterial blood oxygen saturation.^[16] An ABG measurement requires a sample of arterial blood, usually taken from the radial artery. This is more invasive and uncomfortable than pulse oximetry and cannot be performed as frequently. ABG evaluates the partial pressures of gases (oxygen and carbon dioxide, among others) and blood pH (acid–base content) in the sample, more accurately assessing oxygenation.^[17] Historical studies have reported mixed results when comparing the accuracy of pulse oximetry with that of ABG. Some studies have found that pulse oximeters overestimate oxygenation, while other studies suggest the opposite.^{[4,7,14,18–20]} Overestimation of blood oxygenation by pulse oximetry in darker-skinned (as compared with lighter-skinned) individuals has been documented in the medical literature for more than 30 years; however, the clinical importance of this systematic measurement inaccuracy has not been clearly established.^[5,6,8,21]

A 2020 observational study of 2 large hospital cohorts found that people with COVID-19 who identified as Black were nearly 3 times more likely than those who identified as White to have occult hypoxemia not detected by pulse oximetry.^[9] The well-documented differences in pulse oximeter accuracy between darker- and lighter-skinned people could be a driver of these results. Given the central role of oxygen saturation in management of COVID-19 patients, even small systematic inaccuracies have the potential to affect access to treatment and patient outcomes.^[13,22] For example, in one study of COVID-19 patients in New York, New York, Lancet et al.^[23] estimated that each 1% decrease in prehospital oxygen saturation (as measured by pulse oximetry) was associated with 7% higher odds of death. Systematic overestimation of true blood oxygenation by pulse oximetry within Black patients with COVID-19—even by a modest amount—can therefore lead to underestimation of disease severity, potentially causing delays in care and contributing to health disparities. This is supported by a 2022 study that showed a link between predicted pulse oximeter–induced underestimation of blood oxygenation and delayed identification of treatment eligibility for supplemental oxygen in COVID-19 patients.^[24] However, the estimated associated delay in treatment (in minutes or hours) that could be directly attributable to differential pulse oximeter measurement accuracy, or its impact on other aspects of the COVID-19 treatment course, is not yet known.

To address this gap, we first investigated pulse oximeter/ABG discrepancies within non-Hispanic Black/African-American (NHB) and non-Hispanic White (NHW) patients of a large, integrated health-care system in Northern California. After establishing the existence of differential pulse oximeter accuracy, we analyzed possible impacts of these differences on COVID-19–related treatment and outcomes. We hypothesized that differential overestimation of oxygen saturation by pulse oximetry could negatively affect timely access to treatment for NHB individuals with COVID-19.

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Next Section

Abstract and Introduction
Methods
Results
Discussion
References

Table 1. Paired Arterial Oxygen Saturation and Peripheral Oxygen Saturation Measurements Taken Among Non-Hispanic Black/African-American and Non-Hispanic White Individuals in Northern California, January 2020–February 2021
Race/Ethnicity	No. of Paired Measurements^a	No. of Unique Patients	No. of Unique Encounters	SpO₂ Measurement,%^b		SaO₂ Measurement,%^c		Measurement Difference, PP		Proportion of Hypoxemia Not Detected by SpO₂, %^d	Proportion of SaO₂ Below 94% Not Detected by SpO₂, %
Race/Ethnicity	No. of Paired Measurements^a	No. of Unique Patients	No. of Unique Encounters	Median	Mean (SD)	Median	Mean (SD)	Median	Mean (SD)	Proportion of Hypoxemia Not Detected by SpO₂, %^d	Proportion of SaO₂ Below 94% Not Detected by SpO₂, %
NH Black	8,626	2,616	2,126	99	96.94 (5.55)	96	94.49 (6.42)	−2	−2.45 (6.91)	5.50	14.71
NH White	35,127	10,514	9,113	98	96.42 (4.76)	96	94.88 (5.77)	−1	−1.53 (5.52)	3.01	9.52
P for difference^f				<0.001		0.001		<0.001		<0.001	<0.001

Abbreviations: NH, non-Hispanic; PP, percentage points; SaO₂, arterial oxygen saturation; SD, standard deviation; SpO₂, peripheral oxygen saturation.
^aEach SaO₂ measurement was paired with the nearest recorded SpO₂ measurement for the same person, truncated at ±10 minutes from the SaO₂ specimen "taken time" or the SaO₂ "result time," whichever was earlier.
^bPercentage of blood oxygen saturation measured via pulse oximetry.
^cPercentage of blood oxygen saturation measured via arterial blood gas.
^dAn SaO₂ level below 90% paired with a concurrent SpO₂ measurement of 94% or above.
^eAn SaO₂ level below 94% paired with a concurrent SpO₂ measurement of 94% or above.
^fWilcoxon rank-sum test for quasicontinuous variables; χ² test for categorical variables.

Table 2. Baseline Characteristics ^a of COVID-19 Encounters Originating in a Hospital Emergency Department for Individuals Identifying as Non-Hispanic Black/African-American or Non-Hispanic White, Northern California, July 2020–February 2021
Variable	Race/Ethnicity		P for Difference^b
Variable	NH Black (n = 1,699)	NH White (n = 7,036)	P for Difference^b
Age, years			<0.001
Mean	51.3	58.8
Median	51.0	60.0
Female sex	52.2	52.3	0.919
Initial SpO₂ ^c level, %			<0.001
Mean	96.7	95.9
Median	98.0	97.0
Homelessness	3.1	1.8	0.001
Type of health insurance^d
Medicaid	41.8	22.3	<0.001
Medicare	30.6	45.0	<0.001
Uninsured or unknown	4.9	3.0	<0.001
Charity care	2.9	1.6	<0.001
Commercial or other insurance	24.7	31.0	<0.001
Mean CCI score	1.3	1.2	0.472
Comorbidity^e
Asthma	8.7	5.6	<0.001
Cancer	3.3	5.3	<0.001
Congestive heart failure	14.4	11.4	0.001
COPD	5.4	6.9	0.020
Cardiovascular disease	8.8	7.9	0.188
Type 2 diabetes	8.7	8.7	0.908
Depression	1.6	3.6	<0.001
Hypertension	19.7	21.0	0.225
Liver disease	2.7	2.2	0.228
Obesity	7.9	6.3	0.017
Renal disease	19.2	14.9	<0.001
Ever smoking tobacco	37.6	39.2	0.211

Abbreviations: CCI, Charlson comorbidity index; COPD, chronic obstructive pulmonary disease; COVID-19, coronavirus disease 2019; NH, non-Hispanic; SpO₂, peripheral oxygen saturation.
^aValues are percentages of patients unless otherwise indicated.
^bWilcoxon rank-sum test for quasicontinuous variables; χ² test for percentages.
^cPercentage of blood oxygen saturation measured via pulse oximetry.
^dValues may not total 100%, as a single encounter may have had more than 1 payer (e.g., Medicare/Medicaid).
^eComorbid conditions assessed as of emergency department presentation; does not include conditions diagnosed during the hospital visit.

Table 3. Differences in Observed COVID-19 Treatment Outcomes for Non-Hispanic Black/African-American Patients ( n = 1,699) Compared With Counterfactual Outcomes Derived Under the Assumption of No Racial Bias, Northern California, July 2020–February 2021
Variable	Mean Observed (O) Outcome	Mean Counterfactual (C₁) Outcome	Difference (O − C₁)	95% CI
Admitted to the hospital, %^a	34.5	32.1	2.4	0.2, 4.6
Treated with dexamethasone, %^a	22.5	21.2	1.3	−0.9, 3.5
Treated with supplemental oxygen, %^a	29.1	27.6	1.5	−0.6, 3.6
Returned to the hospital after discharge home, %^a
ED visits^b	37.0	28.7	8.3	4.6, 11.9
Admissions^c	43.2	34.5	8.7	0.7, 16.7
Measurement of SaO₂ ^d before admission/discharge decision, %^a,e	62.7	49.8	12.9	4.1, 21.7
Time to admission decision, minutes^f	176.6	192.5	−15.9	−31.5, −0.3
Time spent in the ED, minutes
Admitted^g	541.2	502.0	39.1	−5.0, 83.2
Discharged home from ED^b	250.6	227.3	23.4	−10.5, 57.2
Time to dexamethasone, minutes^h	1,069.8	961.4	108.4	−328.0, 544.8
Time to supplemental oxygen, minutesⁱ	718.5	1,030.1	−311.7	−715.2, 91.9

Abbreviations: CI, confidence interval; COVID-19, coronavirus disease 2019; ED, emergency department; SaO₂, arterial oxygen saturation.
^aValues are expressed as mean predicted probability, shown in percentage units.
^bPatients who were discharged home after the ED visit only (n = 997).
^cPatients who were discharged home after inpatient admission (n = 236).
^dPercentage of blood oxygen saturation measured via arterial blood gas.
^ePatients with SaO₂ level measured (n = 217).
^fPatients with a decision to admit (n = 641).
^gPatients who were admitted to the hospital (n = 586).
^h Patients who were given dexamethasone (n = 383).
ⁱPatients who were given supplemental oxygen (n = 484).

Table 4. Differences in Observed COVID-19 Treatment Outcomes for Non-Hispanic Black/African-American Patients ( n = 1,699) After Correction for Differential Peripheral Oxygen Saturation Measurement, Northern California, July 2020–February 2021
Variable	Mean Observed (O) Outcome	Mean Counterfactual (C₁) Outcome	Mean Counterfactual (C₂) Outcome (Corrected SpO₂ ^a)	Difference (O − C₂) (Corrected SpO₂)	95% CI	Difference BetweenCounterfactuals(C₁− C₂)	95% CI
Admitted to the hospital, %^b	34.5	32.1	35.2	−0.7	−2.9, 1.6	−3.1	−3.4, −2.8
Treated with dexamethasone, %^b	22.5	21.2	24.3	−1.8	−4.0, 0.5	−3.1	−3.4, −2.7
Treated with supplemental oxygen, %^b	29.1	27.6	32.1	−3.0	−5.2, −0.8	−4.5	−4.9, −4.2
Returned to the hospital after discharge home, %^b
ED visits^c	37.0	28.7	30.0	7.0	3.3, 10.7	−1.2	−1.9, −0.5
Admissions^d	43.2	34.5	33.5	9.7	1.7, 17.6	1.0	0.2, 1.7
Time to admission decision, minutes^e	176.6	192.5	186.5	−9.9	−24.2, 4.4	6.0	3.4, 8.6
Time spent in the ED, minutes
Admitted^f	541.2	502.0	498.5	42.7	−1.2, 86.5	3.6	1.3, 5.8
Discharged home from ED^c	250.6	227.3	236.4	14.3	−20.6, 49.1	−9.1	−12.2, −6.0
Time to dexamethasone, minutes^g	1,069.8	961.4	924.2	145.6	−287.1, 578.3	37.2	20.1, 54.3
Time to supplemental oxygen, minutes^h	718.5	1,030.1	751.7	−33.2	−370.3, 304.0	278.5	181.0, 376.0

Abbreviations: CI, confidence interval; COVID-19, coronavirus disease 2019; ED, emergency department; SpO₂, peripheral oxygen saturation.
^aPercentage of blood oxygen saturation measured via pulse oximetry.
^bValues are expressed as mean predicted probability, shown in percentage units.
^cPatients who were discharged home after the ED visit only (n = 997).
^dPatients who were discharged home after inpatient admission (n = 236).
^ePatients with a decision to admit (n = 641).
^fPatients who were admitted to the hospital (n = 586).
^gPatients who were given dexamethasone (n = 383).
^hPatients who were given supplemental oxygen (n = 484).