A glucose tracking app does not measure anything on its own. It keeps, arranges and draws numbers that were measured somewhere else. Here is where those numbers come from, what an app can do with them, and what it cannot. This is general information from the sources listed at the end, not advice for your situation. Which tools suit you, and how often to check, are questions for your own care team.
Three places a glucose number comes from
A meter. Most blood sugar meters save your results for you and your health care team to review, and some meters use an app on your phone to track your levels [1].
A continuous glucose monitor (CGM). A CGM estimates your glucose level every few minutes and keeps track of it over time. Its sensor estimates the glucose in the fluid between your cells, which is very similar to the glucose level in your blood [2].
A lab A1C. The A1C test is a blood test that reflects your average blood glucose over the past 3 months [3].
What an app can do with them
Many CGMs work with apps that have features such as ways to track the food and drink you consume, your physical activity and the medicines you take, and the ability to download data so you can easily see trends in your glucose levels [2]. Most CGMs send information without wires to an app on a smartphone [2]. With a CGM, you can review how your glucose changes over a few hours or days and spot trends [2].
For a meter, an app can hold the record in one place. If you prefer not to use a phone, a written record works too [1]. See the post on keeping a glucose log for what a log usually contains.
Where Apple Health fits
On an iPhone, the Health app is one place several sources can send data. You can set up apps and accessories to send data to Health, and Health takes similar data from different sources and organizes it based on where it comes from. When several sources contribute the same data type, the source at the top of the list takes priority, and you can change the order [4]. So the numbers an app that reads from Health can show depend on which sources are sending data there.
What an app cannot do
It cannot make a number more accurate than the device that measured it. For some CGM models you may need a finger-stick test with a standard blood glucose monitor to calibrate the system and make sure the readings are correct [2].
It also cannot turn a CGM trace into a lab result. A CGM-based estimate of A1C is called the glucose management indicator (GMI). It is calculated from mean glucose, and its authors chose the name so that it does not imply a precise match with a laboratory A1C [5]. A lab A1C reflects the past 3 months [3]; the two can tell related but different stories. See the HbA1c post.
And an app does not decide your targets or how often you check. Your doctor will tell you when and how often to check your blood sugar levels [1].
Sources
- Centers for Disease Control and Prevention, Monitoring Your Blood Sugar.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), Continuous Glucose Monitoring.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), The A1C Test & Diabetes.
- Apple Support, Manage Health data on your iPhone, iPad, or Apple Watch.
- Bergenstal RM, et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care, 2018;41(11):2275–2280.
We made Steadion, an iPhone tracker for people with diabetes or prediabetes, and it is free to download. It reads blood glucose from Apple Health (a CGM included, if you use one), lets you log a fingerstick reading by hand or photograph your meter, and shows trend charts. Steadion Pro is an optional subscription that adds extras such as a doctor report, longer glucose history and data export. Learn more at steadion.fit.