Tracking Your Health

Wearable trackers give you a great many numbers, and they all look equally precise, but some are far more reliable than others. In this video, I'll explain which figures are worth acting on, which deserve only a glance and how to use a tracker around treatment. It helps to think of tracker numbers in three groups, starting with those measured directly, such as heart rate and steps. Others, like sleep stages, calories and fitness scores, are estimated by the device. Then there's readiness or recovery scores that combine several estimates, using formulas that the manufacturers don't publish. Sleep is a good example of an estimate, because tracking is less accurate than its precise-looking charts suggest. Trackers are good at telling when you're asleep but poor at spotting when you're lying awake. They also identify the right stage of sleep only around half to two thirds of the time. By contrast, your resting heart rate is the most reliable and most under-used number on your device. If it stays above your usual level for several days, that can be a real signal of infection; illness; poor sleep or alcohol. Readiness scores haven't been independently tested, so resting heart rate is the better guide. A related number, heart rate variability, measures the small changes in the gap between one heartbeat and the next. It's only useful as a trend over time for one person, because it shifts with your menstrual cycle, contraception and the menopause. A single night tells you very little, and your figure can't be compared with anyone else's. Steps, which are measured directly, are the most useful number for changing what you do, and simply wearing a tracker tends to increase them. During treatment, a steady fall in your steps from week to week is worth mentioning to your team. Fitness scores are less reliable, as the estimate on your wrist can be quite far out for any one person. Even so, fitness itself is one of the strongest predictors of long-term health; including after cancer. So treat the figure as a rough trend, and a prompt to stay active. The same caution applies to skin temperature and blood oxygen readings, which haven't been independently tested on consumer devices. A tracker's oxygen reading is no substitute for a medical pulse oximeter, the small clip placed on your finger. Rhythm alerts differ, because some trackers look for atrial fibrillation, a heart rhythm problem that raises stroke risk if undetected. For these alerts, roughly one in three is actually triggered by atrial fibrillation; so if you do get an alert, treat it as a need to arrange a heart tracing called an ECG, rather than a diagnosis. Some situations need help straight away; whatever your tracker says. During chemotherapy, a temperature, shivering or feeling very unwell means ringing your 24-hour helpline. If an irregular or fast heartbeat comes with chest pain; breathlessness; faintness; sweating; weakness on one side, or difficulty speaking - call 999. Outside those emergencies, expect your numbers to be lower during treatment than before your diagnosis. A sustained rise in your resting heart rate is worth telling your team about, particularly after chemotherapy or surgery. A poor sleep score on its own isn't a reason to seek a review, but if you feel unwell, contact us anyway. Watching your numbers closely can also create a new kind of worry, especially about getting perfect sleep. If a number is making you anxious about a night that felt fine, it's reasonable to stop looking at it. Protecting your personal data deserves some thought as well. For each tracker, read the privacy notice; and assume that what you record in a commercial app is commercial data. Find out how to export or delete it before you need to. Used well, a tracker is best at showing trends in your own numbers over weeks, and months. And try not to compare yourself with your numbers from before your diagnosis, or with anyone else's. If a reading or an alert is worrying you, please get in touch with me or my team.