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13 Mar 2024
Independent review finds 'extensive evidence' that pulse oximeters over-estimate true oxygen levels in people with darker skin.
A panel of healthcare experts in the UK has urged makers of pulse oximeters to design “better, smarter” devices that are not biased by skin tone - possibly by developing multi-wavelength systems that are able to correct for the effects of skin pigmentation.
Chaired by healthcare equality expert Professor Dame Margaret Whitehead, the panel has just published its report entitled “Equity in Medical Devices: Independent Review”, which was instigated by concern during the Covid-19 pandemic about the potential bias of pulse oximeters.
Melanin complications
Pulse oximeters work by monitoring optical absorption at two wavelengths - one in the red, and one in the near-infrared - to determine blood oxygenation. However, the technique is complicated by the light absorption of melanin, which is present in higher levels in darker skin.
“Evidence of harm stemming from this poorer performance has been found in the US healthcare system, where there is a strong association between racial bias in the performance of the pulse oximeters and delayed recognition of disease, denied or delayed treatment, worse organ function and death in Black compared with White patients,” concluded the panel.
And although it did not find any direct evidence that this differential performance affected care in the UK’s National Health Service (NHS), the panel pointed out that “the potential for harm is clearly present”.
“This differential performance by skin tone would not necessarily be a problem for healthcare if the variation in performance had been recognised and appropriate adjustments made to calibrate the devices according to skin tone,” stated the report’s executive summary.
“However, no such recognition appears to have happened either in the case of pulse oximeters and in an unknown number of other optical devices, which are not adjusted for differential performance by skin tone.
“The problem has been compounded in pulse oximeters by the practice of testing the devices on participants with light skin tone, so that these readings are taken as the norm.
“We found extensive evidence of poorer performance of pulse oximeters for patients with darker skin tones. These devices over-estimate true oxygen levels in people with darker skin tones to a greater extent than with lighter skin.”
Panel recommendations
Those findings have prompted the panel to issue three recommendations for pulse oximetry that should now be implemented as a matter of priority.
The first of those is that regulators, developers, manufacturers, and healthcare professionals should take immediate mitigation actions to ensure existing pulse oximeter devices in the NHS can be used safely and equitably for all patient groups across the range of skin tones.
That includes improved guidance for patients and carers using pulse oximeters at home, who should look for changes in oximetry readings, rather than rely on a specific measurement, to identify when blood oxygen levels are decreasing and further medical assistance is needed.
The second recommendation is for approved bodies covering medical devices to strengthen the standards for approval of new pulse oximeter devices, such that they include sufficient clinical data to demonstrate accuracy overall and in groups with darker skin tones.
“Greater population representativeness in testing and calibration of devices should be stipulated,” stated the panel.
Thirdly, the panel suggests technological developments that would eliminate the skin-tone bias inherent to current devices. That could include improved algorithms that account for the effects of melanin, and the use of additional wavelengths that correct for skin pigmentation.
Aside from the specific actions on pulse oximeters, the report also makes a series of recommendations regarding the use of optical devices in healthcare more generally.
One is to carry out an “equity audit” of optical devices used in the NHS, starting with dermatological equipment, to identify those at particular risk of racial bias with potential for harm.
Another suggests that efforts should be made to increase skin tone diversity in medical imaging databanks used for developing and testing optical devices for dermatology, including in clinical trials, and to improve the tools for measuring skin tone incorporated into optical devices.
Photoacoustic bias
One emerging diagnostic technique that could also be prone to skin-tone bias is photoacoustics, something that has already been studied by researchers at Cancer Research UK’s Cambridge Institute, who concluded that photoacoustic imaging is confounded by skin color.
Reporting the findings during the recent Photonics West event, Thomas Else from Sarah Bohndiek’s University of Cambridge research group said that higher levels of melanin appeared to increase “acoustic clutter” in images generated by a multispectral optoacoustic tomography (MSOT) system.
The effect is thought to arise from the similar absorption profiles of melanin and hemoglobin in the near-infrared spectrum, with darker skin tones also generating imaging artifacts that could potentially confound a clinical diagnosis with MSOT - for example when looking for the presence of blood vessels resulting from a tumor.
Else also concluded that using longer wavelengths may help to solve the problem, and suggested that, at the very least, the six different skin-tone types defined by the “Fitzpatrick” scale should be incorporated into studies.
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