The tests take place in the background – you don’t notice them – over a month. Using data from the watch’s optical heart sensor, the algorithm looks for chronic high blood pressure by analysing how your blood vessels respond to every beat of your heart.

The algorithm, which uses machine learning (a form of artificial intelligence), works over rolling 30-day periods and sends you an alert if it sees patterns indicating possible hypertension.

If it does find indications of the condition, it recommends taking blood pressure measurements with a traditional cuff for a week, logging the results in Apple’s health app then discussing them with your doctor.

It also provides an easily digestible summary of its data that you can show your GP. Apple pitches it as a “conversation starter” rather than a diagnostic tool and cautions that it won’t catch all instances of hypertension.

Hypertension alerts are the latest in a series of health tools on the Apple Watch that include measures of your heart rate, blood oxygen level and an ECG for heart rhythm and sleep aponea detection.

But Phillips said it could have the biggest impact on public health, if used for life-long self-screening.

He said an estimated 1.3 billion worldwide suffer from hypertension. Apple expects its new watch feature to notify one million undiagnosed people in its first year.

Cardiovascular disease is the world’s biggest killer accounting for around one third of deaths – and hypertension is its leading cause.

“We know it’s under-diagnosed. We know it’s under-controlled,” Phillips said. But hypertension is also relatively easy to manage, if detected early, before it causes organ damage.

“We are excited about is the potential to partner or support research that uses this feature to see how it might positively impact public health and look at longer-term outcomes for how this might impact people.”

But privacy is also paramount. No personalised data are shared, unless you choose to do so. Not even Apple gets to see the data. “It stays on your device or Private Cloud Compute,” Phillips said.

Phillips says the hypertension feature – cleared by the US Food and Drug Administration (FDA) and other regulators around the world – was developed with photoplethysmography (PPG) data from more than 100,000 people. PPG measures blood volume changes through the skin.

(If you watch back Apple’s September 9 event, you’ll hear a reference to FDA clearance for hypertension alerts being expected soon; the agency granted approval on September 12. All up, the feature is approved for 150 countries.)

Clinical validation came through a study of 1863 people of mixed ages and ethnicities – but they all had one thing in common: none had a hypertension diagnosis. The trial found 585 participants exhibited hypertension.

It also found “a significant portion of people alerted without hypertension actually had elevated blood pressure”.

Those could be termed false positives, but Phillips told the Herald: “What we saw was that about half of those people actually had elevated blood pressure – so they were in the blood pressure range that wasn’t quite hypertension. And those people can really benefit from a conversation with their physician, because that’s a good time to intervene to prevent progression to hypertension.”

Would help clogged health system

Auckland Unviersity associate professor Dr Amy Chan is in the middle of a study on how wearables can help asthma suffers by detecting subtle symptoms over time and has emerged as an general advocate for greater use of wearables, including smart watches.

“With blood pressure monitors, the problem often comes down to patient engagement and cost,” Chan said.

“Even when patients go to health practitioners often, it’s only a single-point-in time-assessment. They can’t capture abnormalities that may only occur, you know, infrequently, and so again, wearables allow that ongoing monitoring.”

Chan, who is respiratory lead of the Medicines Intelligence group at the university’s School of Pharmacy and an advisor to the Auckland Medical Research Foundation, among other bodies – and who is now in touch with Apple’s medical team – said: “With an ageing population and increased workforce pressures. there really needs to be a lot more in terms of solutions for patient self-management and monitoring their own health conditions.”

“We’ve seen some pockets of excellence in primary care, where there is potential for people to submit their wearable records to their GPs or bring it along to the GP consultations or pharmacy or nurse appointments.

“But I think we do need to see a lot more awareness and data literacy, a lot more capability-building in the years to come.”

What you need

Hypertension alerts were first demonstrated with Apple’s latest watches in October, but the feature will also work on earlier models.

The feature will run on an Apple Watch Series 9 or later or Apple Watch Ultra 2 or later, as long as it is upgraded to watchOS 26 and paired with an iPhone 11 or later running iOS26 (you can upgrade watchOS on your watch or via the Watch App on your iPhone).

Full set-up instructions are here.

Chris Keall is an Auckland-based member of the Herald’s business team. He joined the Herald in 2018 and is the technology editor and a senior business writer.