Can a smart ring replace a blood test?

What a ring actually reads

• Light. An optical sensor sees the small changes in blood volume with each heartbeat. Software turns that into heart rate, heart rate variability and an estimate of blood oxygen.
• Movement. A motion sensor tells sleep from waking and rest from activity.
• Temperature. A skin thermometer follows changes from one night to the next.

None of these is a sample of your blood. They are signals read at the surface and turned into estimates, which is why a ring is good at trends and why its numbers come with limits: are smart rings accurate?

The warning about blood sugar

In February 2024 the US Food and Drug Administration warned consumers, patients and doctors about smartwatches and smart rings that claim to measure blood glucose without piercing the skin. It has authorized, cleared or approved none that measures or estimates glucose on its own. For people with diabetes, a wrong reading can lead to the wrong dose of insulin or other medicines that lower blood sugar, and a dose that is too high can bring on dangerously low blood sugar within hours. Its advice is not to buy or use them, and the warning covers every such device, whatever the brand.

What changed in 2026

In January 2026 the FDA replaced its guidance on general wellness products. It may now treat a wearable that estimates blood glucose, blood pressure, oxygen saturation or heart rate variability without piercing the skin as a wellness product, provided the numbers are for wellness alone: no named diseases, no alerts that call for medical action, no claim of clinical accuracy, and values that look clinical validated by the maker. The FDA does not intend to examine such products. In its one glucose example, a wearable for understanding how food affects you and marked as not for people with diabetes or prediabetes, the claim counts as wellness, but the product is still left out of the policy because its micro-needles pierce the skin. The 2024 warning is still on the FDA’s site, unchanged. For anyone whose treatment depends on the number, the advice stands: an authorized device. More on where the line runs: can a smart ring track your health in real time?

What does measure glucose

A continuous glucose monitor has a small sensor that goes into the skin and reads glucose every few minutes, sending the numbers to a phone. In March 2024 the FDA cleared the first one that can be bought without a prescription, for adults who do not use insulin and do not have problematic low blood sugar. Each sensor is worn for up to 15 days and reports every 15 minutes, and the FDA says not to make medical decisions from it without talking to a doctor. A watch that shows numbers from such a monitor is a different thing from a watch that claims to make them up from light.

What still needs a blood sample

Cholesterol, iron, kidney and liver values, and most of what a doctor orders as a blood test, still come from a sample of blood taken with a needle or a finger prick and read by a laboratory or an authorized test. A ring can tell you that something in your nights or your heart rate has changed. It cannot tell you why.

What we are working on

The first Lychnos Ring has no health sensors: its battery goes to conversation. For a later model we are working on wellness trends from heart rate, skin temperature and sleep, presented as wellness insights, not medical measurements. We will not claim to read blood sugar from a ring. If you already wear an approved glucose monitor, Lychnos is planned to read its numbers out to you. A micro-needle that tests a drop of blood is a research idea for us, not a product: it would be a medical device, with clinical studies first: why our first ring skips health. It is in development.

Sources, read 4 November 2026: US Food and Drug Administration, do not use smartwatches or smart rings to measure blood glucose levels, safety communication, 21 February 2024, FDA clears first over-the-counter continuous glucose monitor, 5 March 2024, and general wellness: policy for low risk devices, guidance, 6 January 2026. General information, not medical advice.