Study Findings
A recent study, published in The BMJ, indicates that fingertip pulse oximeters, commonly used to detect low blood oxygen levels (hypoxaemia) at home, may provide higher readings for individuals with darker skin tones compared to those with lighter skin tones. This discrepancy suggests that low blood oxygen levels could be missed in patients with darker skin, potentially delaying necessary medical care, while patients with lighter skin tones might receive unnecessary treatment.
How Pulse Oximeters Work
Pulse oximeters utilize light to measure the oxygen saturation in the blood (SpO2). Normal readings typically range between 95% and 100%, with values below 90-92% generally considered low and warranting medical attention. Skin tone is a known factor that can influence these devices, as higher pigment levels in darker skin can absorb more light, potentially leading to an overestimation of oxygen levels. Previous research has not definitively established the impact of skin tone on diagnostic accuracy.
Research Methodology
To investigate this, researchers evaluated the measurement and diagnostic accuracy of five fingertip pulse oximeters provided by the NHS for its COVID oximetry @home scheme. The study involved data from 903 critically ill adults, with an average age of 56 years, across 24 NHS intensive care units in England from June 2022 to August 2024. Intensive care units were chosen as a test environment because patients there often have lower blood oxygen values and routinely undergo accurate blood oxygen measurements using hospital-grade equipment.
Skin tone was objectively assessed for each patient using a spectrophotometer. The pulse oximetry SpO2 values were then compared against "gold-standard" arterial blood gas measurements (SaO2). SpO2 values were analyzed at two critical thresholds: 94% or lower (indicating a need for medical help) and 92% or lower (indicating a need to attend the emergency department).
Key Results
Out of 11,018 paired SpO2-SaO2 measurements analyzed, all five pulse oximeters consistently showed higher SpO2 values for patients with darker skin tones than for those with lighter skin tones, irrespective of the actual SaO2 level. On average, SpO2 readings were 0.6 to 1.5 percentage points higher for patients with darker skin.
At both evaluated SpO2 thresholds, the rates of false negatives (where low oxygen was present but missed by the oximeter) increased with darker skin tones. Conversely, false positive rates (where low oxygen was indicated by the oximeter but absent) decreased with darker skin tone. Researchers noted that even small absolute differences in readings can lead to significantly higher rates of false negatives and lower rates of false positives in diagnosing hypoxaemia.
Limitations and Recommendations
The study acknowledges that, as an observational study conducted on critically ill patients, it cannot establish definitive cause and effect and its findings may have limited generalizability. However, the researchers emphasize its large scale and sophisticated statistical modeling.
They conclude that SpO2 readings should be interpreted in conjunction with other clinical information, and trends in SpO2 values should be prioritized over single readings, particularly for patients with darker skin tones. They recommend that healthcare systems develop specific guidance to inform practitioners, patients, and the public, especially in settings where advanced medical measurement devices are unavailable.
An accompanying editorial supports these findings, urging clinicians to acknowledge device limitations and interpret readings cautiously for patients with darker skin. It also stresses the need for regulatory frameworks to evolve with scientific understanding to prevent harm and ensure equitable access to effective technology.