Utility of Skin Tone on Pulse Oximetry in Critically Ill Patients: A Prospective Cohort Study

Abstract

Objective: Pulse oximetry, a ubiquitous vital sign in modern medicine, has inequitable accuracy that disproportionately affects minority Black and Hispanic patients, with associated increases in mortality, organ dysfunction, and oxygen therapy. Previous retrospective studies used self-reported race or ethnicity as a surrogate for skin tone which is believed to be the root cause of the disparity. Our objective was to determine the utility of skin tone in explaining pulse oximetry discrepancies.

Design: Prospective cohort study.

Setting: Patients were eligible if they had pulse oximetry recorded up to 5 minutes before arterial blood gas (ABG) measurements. Skin tone was measured using administered visual scales, reflectance colorimetry, and reflectance spectrophotometry.

Participants: Admitted hospital patients at Duke University Hospital.

Interventions: None.

Measurements and main results: Sao₂-Spo₂ bias, variation of bias, and accuracy root mean square, comparing pulse oximetry, and ABG measurements. Linear mixed-effects models were fitted to estimate Sao₂-Spo₂ bias while accounting for clinical confounders.One hundred twenty-eight patients (57 Black, 56 White) with 521 ABG-pulse oximetry pairs were recruited. Skin tone data were prospectively collected using six measurement methods, generating eight measurements. The collected skin tone measurements were shown to yield differences among each other and overlap with self-reported racial groups, suggesting that skin tone could potentially provide information beyond self-reported race. Among the eight skin tone measurements in this study, and compared with self-reported race, the Monk Scale had the best relationship with differences in pulse oximetry bias (point estimate: -2.40%; 95% CI, -4.32% to -0.48%; p = 0.01) when comparing patients with lighter and dark skin tones.

Conclusions: We found clinical performance differences in pulse oximetry, especially in darker skin tones. Additional studies are needed to determine the relative contributions of skin tone measures and other potential factors on pulse oximetry discrepancies.

Figure 1.

Bias and accuracy root mean square (A_RMS) across Skin Tone Scale tertiles. Unadjusted error metrics of Sao₂–Spo₂ bias and A_RMS across skin tone tertiles. Tertiles are ordered from lightest to darkest, from the left to the right, under each metric. For example, for the Monk Skin Tone scale, the bias is: lightest tertile: –1.371; 95% CI, –1.646 to –1.113); mid tertile: –1.643; 95% CI, –1.890 to –1.340; darkest tertile –1.767; 95% CI, –2.038 to –1.515. Other Skin Tone Scales show similar trends, whereby the lightest tertiles present a lower Sao₂–Spo₂ bias. For precision and A_RMS, the trends are reversed with darker tertiles presenting lower precision and A_RMS. Visual representations and sensitivity analysis are shown in Supplement Figure 6 (http://links.lww.com/CCX/B379). A, Bias and A_RMS in Administered Visual Scales: Monk Skin Tone, Fitzpatrick Skin Type, Von Luschan. B, Bias and A_RMS in Color Measurement Tools. ITA = individual typology angle.

Figure 2.

Flow diagram. A total of 1167 patients were screened. Exclusion criteria included unremovable fingernail polish, admission for a vascular complication (e.g., grafting or stenting), amputation, and large areas of skin discoloration where the accuracy of skin tone measurements could be affected due to arterial insufficiency or cytopenias. Pairs containing either a Sao₂ or a Spo₂ measurement of the 70–100% range were excluded. Of these, 301 patients qualified for this prospective study and were approached. Among the 134 patients who signed consent forms, one patient later withdrew, one patient did not have complete skin measurement data, and four patients did not have skin measurements. For patients who had pulse oximetry measurements done on the finger, we used the average of four palm locations (left ventral, right ventral, left dorsal, right dorsal). For patients who did not have pulse oximetry locations specified, we presumed the measurement was done on the finger and imputed it using the four palm locations as well.