“One of the mysteries of COVID-19 is why oxygen levels in the blood can drop to dangerously low levels without the patient noticing,” writes the BBC. This mystery in question is called ‘silent hypoxia’. Because it happens so quickly and without patients noticing, it leads many COVID-infected to arrive in hospital in far worse health than they realised and, in some cases, too late to treat effectively.

Now, a potentially life-saving solution, in the form of a pulse oximeter, allows patients to monitor their oxygen levels at home and costs about £20. The devices are currently being rolled out for high-risk COVID patients in the UK, and Doctor Matt Inada-Kim, who is now leading the scheme, thinks everyone should consider buying one.

A typical oxygen level in the blood is between 95 per cent and 100 per cent. As Doctor Inada-Kim told BBC Radio 4’s Inside Health, during the pandemic, hospitals started seeing more and more patients with oxygen levels in the 70s and low or middle 80s. “It was a really curious and scary presentation and really made us rethink what we were doing,” he added.

Shortly after that, Doctor Inada-Kim became the national clinical lead of the Covid Oximetry @home project, which involves the remote monitoring of patients with coronavirus symptoms. Patients use a pulse oximeter, a small monitor clipped to their finger, to measure their oxygen saturation levels three times a day in order to stay up to date with the oxygen level in their blood.

A pulse oximeter slips over your middle finger and shines a light into the body to measure how much of the light is absorbed in order to calculate oxygen levels in the blood. In England, they are being given to people with COVID who are over 65, younger but have a health problem, or anyone doctors are concerned about. Similar schemes are being rolled out across the UK, and overall, NHS England has supplied around 300,000 pulse oximeters for the home-monitoring scheme.

As mentioned previously, people need to measure and record their oxygen levels three times a day. If oxygen levels drop to 93 per cent or 94 per cent, then people need to speak to their GP straight away 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, have shown that even small drops below 95 per cent are linked to an increased risk of dying. Doctor Inada-Kim explained, “The point of this whole strategy is to try to get in early to prevent people from getting that sick, by admitting patients at a more salvageable point in their illness.”

Although Doctor Inada-Kim says there isn’t definitive proof that the gadget saves lives (as it could take until April to know for sure), 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 told the BBC.

He went to advise everyone to consider buying one, adding that “personally I would, and I know a number of colleagues who have bought pulse oximeters to distribute to their loved ones.”

If you’ve been convinced and are looking to get your own pulse oximeter (which you can easily find on Amazon, for example), Doctor Inada-Kim advised checking your new purchase as a CE Kitemark and to avoid apps on smartphones, which he said were not as reliable.