Pulse Oximeter

The clip on a finger reads a colour, and it reads that colour less accurately through darker skin.¹

This was demonstrated in the laboratory years before it was treated as urgent, and the rule intended to fix it is still a draft.^{1, 3}

The device shines red light, at around 660 nanometres, through the finger.¹ Haemoglobin carrying oxygen absorbs that wavelength differently from haemoglobin that is not, and saturation is estimated from the comparison. The light has to cross the skin on the way, and melanin absorbs in the same red region as oxyhaemoglobin, so the two signals overlap.¹ The consequence is directional rather than random: readings tend to overestimate saturation in people with darker skin, and the error grows as true saturation falls, which is to say it grows as the patient gets sicker.¹ A laboratory study in 2005 found the largest bias in dark-skinned subjects at saturations between 60 and 70 per cent, and a smaller one above 80.¹

In December 2020 Michael Sjoding and colleagues published a comparison of oximeter readings against arterial blood gas measurements in Black and white patients, before and during the pandemic.² Occult hypoxaemia — a true saturation below 90 per cent while the device displays 92 per cent or higher — occurred about three times as often in Black patients.^{1, 2} A separate study at Johns Hopkins found Asian, Black and Hispanic patients more likely to receive inaccurate readings.² The finding landed during a period when that reading was deciding who received oxygen and who was sent home.

The regulator has moved deliberately. A safety communication followed in 2021, an advisory panel met in November 2022, a discussion paper was published in November 2023, and a further panel met in February 2024.³ Draft guidance issued on 7 January 2025 would require testing across a measured range of skin pigmentation, using the Monk Skin Tone scale alongside an objective measurement, with a minimum of 150 participants in place of the 10 required under the 2013 guidance.⁴ As of October 2026 it remains a draft, and the 2013 guidance it would replace is still the one in force.³