Utility of skin tone on pulse oximetry in critically ill patients: a prospective cohort study

Abstract

Importance: Pulse oximetry, a ubiquitous vital sign in modern medicine, has inequitable accuracy that disproportionately affects Black and Hispanic patients, with associated increases in mortality, organ dysfunction, and oxygen therapy. Although the root cause of these clinical performance discrepancies is believed to be skin tone, previous retrospective studies used self-reported race or ethnicity as a surrogate for skin tone.

Objective: To determine the utility of objectively measured skin tone in explaining pulse oximetry discrepancies.

Design setting and participants: Admitted hospital patients at Duke University Hospital were eligible for this prospective cohort study if they had pulse oximetry recorded up to 5 minutes prior to arterial blood gas (ABG) measurements. Skin tone was measured across sixteen body locations using administered visual scales (Fitzpatrick Skin Type, Monk Skin Tone, and Von Luschan), reflectance colorimetry (Delfin SkinColorCatch [L*, individual typology angle {ITA}, Melanin Index {MI}]), and reflectance spectrophotometry (Konica Minolta CM-700D [L*], Variable Spectro 1 [L*]).

Main outcomes and measures: Mean directional bias, variability of bias, and accuracy root mean square (A_RMS), comparing pulse oximetry and ABG measurements. Linear mixed-effects models were fitted to estimate mean directional bias while accounting for clinical confounders.

Results: 128 patients (57 Black, 56 White) with 521 ABG-pulse oximetry pairs were recruited, none with hidden hypoxemia. Skin tone data was prospectively collected using 6 measurement methods, generating 8 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 to self-reported race, the Monk Scale had the best relationship with differences in pulse oximetry bias (point estimate: -2.40%; 95% CI: -4.32%, -0.48%; p=0.01) when comparing patients with lighter and dark skin tones.

Conclusions and relevance: 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.. Flow diagram

A total of 1,167 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 out 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 didn’t have complete skin measurement data, and four patients didn’t 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 didn’t have pulse oximetry locations specified, we presumed the measurement was done on the finger and imputed it using the four palm locations as well.

Figure 2.. Unadjusted error metrics in pulse oximetry across skin tone scale tertiles

Unadjusted error metrics of mean directional bias, standard deviation, and accuracy root mean square (also known as A_RMS or root mean square error), across tertiles. Tertiles are ordered from lightest to darkest, from the left to the right on the x-axis. Note that a pulse oximetry bias defined as SaO₂ - SpO₂ results in a negative bias reflecting that pulse oximetry overestimates true oxygenation values. Fitzpatrick and Monk appear to have a trend towards more negative bias (e.g., bias increasingly negative) from lighter to darker tertiles. A_RMS appears to be lower (that is, a lower root mean square error) in many darker tertiles than in lighter tertiles. Variable L* and Konica Minolta L* have fewer patients because there was more missingness. Some patients did not have these measurements either due to patient refusal (often due to feelings of being overwhelmed, stress, or experiencing pain) or interruptions by clinical workflow.