Covid-19 patients are arriving in hospital in a far worse state than they realised because of a condition called silent hypoxia, which makes treatment even more difficult.
A potentially life-saving solution in the form of a pulse oximeter, however, allows patients to monitor their oxygen levels at home.
At a cost of about £20 the oximeter will allow people to seek medical help immediately should oxygen levels in their blood drop too low.
Research suggests that even a slight reduction in oxygen levels can be a sign of Covid-19 complications.
A normal oxygen level in the blood is between 95 per cent and 100 per cent.
In an attempt to save lives, the NHS bought hundreds of thousands of oximeters (oxygen monitors) for patients with Covid-19 to use at home.
The Covid Oximetry at Home service was set up by NHS England as a way of making sure patients vulnerable to coronavirus are monitored at home and any deterioration picked up and treated quickly, increasing their chance of recovery.
Once a person tests positive for Covid-19, the test-and-trace service notifies their GP, which enables them to refer eligible people into the service – this includes people aged over 65, people under 65 who are clinically extremely vulnerable, people under 65 who are deemed high risk due to either ethnicity, a high BMI or those with medical problems.
These patients are given an oximeter to test their pulse, how much oxygen is in their blood and record the results.
People measure and record their oxygen levels with the device three times a day.
The results will show their GP if their health is improving or deteriorating – which means that they can intervene quickly if needed to get them treatment.
A pulse oximeter slips over the middle finger and shines a light into the body.
It measures how much of the light is absorbed to calculate oxygen levels in the blood.
If oxygen levels drop below 94 per cent, oximeter users are advised to speak to their GP or call 111. If they go below 92 per cent, people should go to A&E or call 999 for an ambulance.
Studies, which have not been reviewed by other scientists, suggest that even small dips below 95 per cent are linked to an increased risk of death.
Because of the way that Covid-19 attacks the lungs, doctors said everyone should have one.
Dr Matt Inada-Kim became the national clinical leader of the Covid Oximetry at Home project.
"With Covid, we were admitting patients with oxygen levels in the 70s or low or middle 80s," said Dr Inada-Kim, a consultant in acute medicine at Hampshire Hospitals.
He told BBC Radio 4's Inside Health: "The point of this whole strategy is to try to get in early to prevent people getting that sick by admitting patients at a more salvageable point in their illness."
Dr Inada-Kim said there is no definitive proof that the gadget saves lives and it could take until April to know for sure. However, the early signs are all positive.
"What we think we can see are the early seeds of a reduction in the length of stay after a hospital admission, an improvement in survival and a reduction in the pressures on the emergency services," he said.
He is so convinced of the role of oximeters that he said everyone should consider buying one.
"Personally I would, and I know a number of colleagues who have bought pulse oximeters to distribute to their loved ones," he said.
But the devices do have their downfalls.
Dr David Strain, a clinical academic at the University of Exeter Medical School and clinician at the Royal Devon and Exeter Hospital, said that the oximeters used in hospitals “have a very good track record”.
“Our devices in hospital cost several hundred pounds, if not thousands of pounds in some places, so clearly the level of technology will be different between them and the technology you might buy in chemists for £20,” he said.
The machines may also work less well monitoring oxygen levels in black people.
A study from the US showed the devices can provide misleading results in more than one in 10 black patients.
Researchers analysed pulse oximetry measures of oxygen saturation and measures of arterial oxygen saturation in arterial blood gas from 1,333 white patients and 276 black patients admitted to hospital in 2020.
They found pulse oximeters overestimated oxygen levels 3.6 per cent of the time in white patients, but got it wrong almost 12 per cent of the time in black patients.
Nail varnish, tattoos and false nails can also affect a person’s reading, according to the British Lung Foundation, as can medical conditions including anaemia and Raynaud’s syndrome.