Pulse oximeters, which spot falling oxygen levels, may work less well on darker skin, experts warn.
A device designed to spot early signs of dangerous falls in oxygen levels in Covid patients works less well in those with darker skin, experts are warning.
NHS England and medicines regulator, the MHRA, say pulse oximeters may sometimes overestimate oxygen levels.
The devices beam light through the blood, and skin pigmentation may affect how light is absorbed, they say.
Anyone with concerns is advised to look for changes over time rather than relying on a single reading.
NHS England is issuing updated guidance, advising patients from black, Asian and other ethnic minority groups to continue using pulse oximeters, but to seek advice from a healthcare professional.
An NHS Health and Race Observatory report published in March recommended that the MHRA should carry out an urgent review into the use of pulse oximeters.
The devices have been used increasingly during the coronavirus pandemic, both in hospital and within the community.
In people with Covid-19, oxygen levels in the blood can drop to dangerously low levels without them noticing – a condition known as “silent hypoxia”.