Measure to make a decision
The useful question about any measurement is what you would do differently depending on the result. If the answer is nothing, the number is entertainment, and it can be expensive entertainment when it triggers a chain of follow-up tests.
This is the filter to apply to the whole direct-to-consumer testing market. Some of it answers real questions. Much of it produces a dashboard nobody acts on.
Numbers worth watching
A short list covers most people and comes largely from ordinary clinical care.
- Blood pressure, measured at home. Out-of-office blood pressure predicts cardiovascular risk over and above readings taken in a clinic, and the American Heart Association and American Medical Association now recommend home monitoring as part of diagnosis and management — using a validated oscillometric upper-arm device and a standardised protocol, not whatever the pharmacy had on the shelf.
- Standard blood panels your doctor already orders — lipids, glucose or HbA1c, kidney and liver function. Interpretation belongs with a clinician who knows your history.
- Functional measures you can repeat yourself: grip strength, how long you can stand on one leg, how fast you cover a fixed walking route, how many repetitions of a movement you can control.
Grip strength has the largest dataset behind it of anything on that last line. In the PURE study, following 139,691 adults across 17 countries for a median of four years, each 5 kg lower grip strength was associated with a 16% higher rate of death from any cause. It is a cheap, fast test with real prognostic information in it.
Two caveats travel with that figure. It is observational, so weaker grip marks people who are already less well rather than causing the outcome. And in the same study grip strength showed no significant association with incident diabetes, hospital admission for pneumonia or lung disease, falls, or fractures — it predicted death, not the arrival of specific diseases.
Functional measures rarely appear in longevity marketing because nobody can sell you a subscription to a stopwatch. They are still among the most informative things you can track.
What a single reading means
Very little on its own. Biological measurements vary across the day, with hydration, with recent food and exercise, with illness, with stress, and with the laboratory that ran them. A value slightly outside a reference range is common and frequently unremarkable.
Compare a reading to your own previous readings, taken under similar conditions. Direction over time is the signal. One point is a rumour.
Biological age scores
Products that convert a panel of markers into a single age figure are making a modelling choice, not a measurement.
There is real research behind the underlying epigenetic clocks, and it is that research that shows the problem with the consumer number on top. When investigators ran the same biological sample twice through six well-known clocks, the two results disagreed by a median of 0.5 to 2.4 years depending on the clock, and by as much as 8.6 years at the extreme — from technical noise alone, with no biology changing in between. Tracked over time, the same instability produced trajectories that wandered by decades.
That is a measurement problem in the research instrument, before any consumer product wraps a report around it. It is why Endurvity does not publish a biological age number.
A sustainable approach
Track a small number of things, on a fixed schedule, with a written note of what you would change at what threshold. Review quarterly rather than daily.
When a number belongs with a clinician
Some readings are not yours to interpret, and a few need attention the same day.
- Blood pressure that is repeatedly high on home readings. Take the readings to your doctor rather than adjusting anything yourself; home monitoring is designed to feed into clinical management, not replace it.
- A very high reading with symptoms — severe headache, chest pain, breathlessness, visual change, weakness on one side. That is urgent care, not a repeat measurement.
- Any result flagged as outside range on a blood panel, particularly kidney or liver markers. Repeat testing and interpretation belong with the clinician who ordered it.
- A functional measure that drops sharply — a walking route that suddenly takes much longer, grip that has fallen noticeably, new unsteadiness. A rapid change is worth investigating.
- Anything from a direct-to-consumer panel that alarms you. Take it to a doctor rather than to a search engine, and take the raw values with you.
What the evidence does not show
- That home blood pressure monitoring improves cardiovascular outcomes. It lowers blood pressure and improves control in trials, especially alongside other support. Trials measuring heart attacks and strokes as the endpoint are still needed, and the AHA and AMA say so directly.
- That improving your grip strength lowers your risk of dying. PURE is a cohort study. It shows grip strength is a marker. It does not show that training grip changes the outcome it marks.
- That a biological age score means anything for you personally. The underlying clocks are not reliable enough at the individual level to support a personal decision, and the consumer layer on top adds assumptions nobody publishes.
- That more testing produces better health. Nothing here supports broad panels in people without symptoms. False positives are common, and each one costs money, time, and worry.
- That any consumer wearable measures a clinical parameter. Estimates of blood pressure, blood glucose, or sleep stages from a wrist device are model outputs, and none of them substitutes for the validated instrument.