EquiOx: A Prospective Study of Pulse Oximeter Bias and Skin Pigmentation in Critically-Ill Adults with Stable Oxygenation

Background Pulse oximeter performance may vary by skin pigmentation, but most data are retrospective with key limitations. Research Question Does pulse oximeter bias in critically-ill adults differ by skin pigmentation? Study Design and Methods In this prospective single-center study of 631 ICU patients (2022–2024) we directly observed simultaneous pulse oximeter oxygen saturation (SpO2) and arterial blood functional oxygen saturation (SaO2). Skin pigment was assessed using the subjective Monk Skin Tone Scale and objective spectrophotometry measurement (Individual Typology Angle [ITA]). We quantified pulse oximeter bias [mean difference between SpO2 and SaO2, and average root mean square error (ARMS) and estimated adjusted effects of skin pigment on bias or ARMS with targeted maximum likelihood estimation. Results Among 1,760 paired measurements, median SaO2 was 98% (IQR 96%, 99%) with 40 episodes of stable hypoxemia (SpO2<90%). SpO2 systematically underestimated SaO2 [median bias= -1.70 IQR (-2.84, -0.50)]. Bias was less in patients with darker skin (ITA <-30°) [-1.05 (-2.44, -0.10)] vs lighter skin (ITA >30°) [-2.01 (-3.34, -1.00)] and remained significantly different in adjusted analyses. ARMS was 3.87 (95% CI 3.25, 4.53) overall and 4.49 (95% CI 2.63, 7.07) in patients with darker skin. Interpretation In this large ICU cohort, hypoxemia was rare, FDA-cleared pulse oximeter SpO2 systematically underestimated SaO2, and performance varied by skin pigment. Bias was less in patients with darker skin. Pulse oximeter inaccuracies in ICU patients may be more substantial than clinicians recognize. Additional real-world studies with multiple oximeter brands and more stable hypoxemia are needed to further understand bias variation with skin pigment.