By the Primary Care Evidence Guide Team
Short answer: A number on a glucose-monitor app is an estimate. A diagnosis comes from a clinician using specific blood tests. This guide helps you bring the right context and questions to that conversation.
What a glucose sensor can and can't tell you
A continuous glucose monitor (CGM) is a wearable sensor. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), it estimates your glucose level every few minutes by reading the fluid between your cells, which NIDDK says is very similar to blood glucose.
The key word is estimate. NIDDK also says researchers are still working to make CGMs more accurate and easier to use.
A diagnosis is a different process. NIDDK explains that doctors diagnose diabetes and prediabetes using blood tests. It names the fasting plasma glucose test, the A1C test, the oral glucose tolerance test and, when symptoms are present, the random plasma glucose test. NIDDK also says not to diagnose yourself, and that over-the-counter equipment such as a blood glucose meter cannot diagnose diabetes. The diagnostic ranges on that page are for those lab tests. CGM readings are not listed among them.
Why one pattern on a graph isn't an answer
- It's an estimate. NIDDK says you may sometimes need to compare CGM readings with a finger-stick on a standard meter, for example if you doubt the readings or the device gives a warning alert. Some models need finger-stick calibration. The FDA review summary for one over-the-counter system says it is factory calibrated only.
- Devices differ in what they show. The same FDA summary says that system's app displays readings only between 70 and 250 mg/dL, updates every 15 minutes, and has no glucose alarms. Another device may work differently, so read your device's own label.
- Some medicines can affect certain sensors. For that system, the FDA summary says acetaminophen above the standard maximum dose, or hydroxyurea, can make readings falsely high. Ask a pharmacist or clinician whether anything you take affects your device.
- Diagnostic tests have limits too. NIDDK notes that A1C might not be accurate with certain types of anemia, certain hemoglobin variants, or in the second or third trimester of pregnancy. If A1C and glucose results don't match, a doctor should consider that A1C may not be reliable for you.
- Confirmation is normal. NIDDK says doctors will usually use a second test to confirm a diabetes diagnosis.
- Glucose tests don't name the type. NIDDK says the common tests can confirm diabetes but can't identify which type you have. That takes other testing.
- Some questions are still open. NIDDK says researchers are studying how CGMs might be used for different types of diabetes and for other medical conditions. The NIDDK pages reviewed here do not explain how to interpret readings in people without diabetes, so that is a question for your clinician.
A reading-context-and-confirmation checklist
Before you see a clinician, write down the context around any pattern that concerns you. NIDDK notes that many CGM apps let you track food and drinks, activity and medicines, which can make this easier.
Reading context
- Which device you used, and whether it allows calibration
- Where the sensor was worn, and how long it had been in place
- Time of day, and whether you had recently eaten or fasted
- What you ate and drank, and your activity level
- Every medicine and supplement you take, listed exactly as you take it
- Any symptoms at the time of the reading
- Whether you cross-checked with a finger-stick, and what it showed
- Any alerts the device gave, or whether your device has alerts at all
Confirmation
- Which blood test, if any, your clinician wants to use to check the pattern
- Whether a second test is needed to confirm
- Whether A1C is a reliable test for you, given anemia, a blood-cell variant or pregnancy
- Whether you need to fast beforehand. NIDDK says the fasting plasma glucose test and the oral glucose tolerance test call for at least 8 hours without food or drink other than sips of water.
- If a diagnosis is made, whether more testing is needed to identify the type
Questions to take to your clinician
- What does this pattern show, and what can't it show?
- Which lab test should check it, and when?
- Could anything about my body, my other conditions or my medicines make a test or my device less reliable?
- Do I have risk factors that make testing worthwhile? (NIDDK's tests and diagnosis page lists factors for which routine testing is recommended, such as age 35 or older.)
- How often should I be retested?
- Should I keep wearing the device, and what should I do if it gives an alert?
When to get professional advice
- If you have symptoms. NIDDK says anyone with symptoms of diabetes should be tested. For type 1 diabetes, it gives feeling very thirsty and needing to urinate more often as examples.
- If you are pregnant or planning to be. NIDDK says some tests use different cutoffs in pregnancy, so ask your doctor what your results mean.
- If you already have diabetes or use diabetes medicine. Follow your treatment plan and your care team's instructions. NIDDK says to act quickly when a CGM alarm signals a low or high glucose level. Ask your care team before changing anything in your plan.
- If you bought an over-the-counter CGM. Read its intended-use label. The FDA review summary for one such system describes it as intended for people 18 and older who are not on insulin. It also says users are not intended to take medical action based on the device output without consulting a qualified healthcare professional. Bring your device data to your visit.
What this article can't do
This guide doesn't diagnose, doesn't tell you what any number means for you, and doesn't suggest starting, stopping or adjusting any medicine or diet. It is not a recommendation for or against any device.
Sources and editorial independence
- NIDDK, Diabetes Tests & Diagnosis (last reviewed July 2022; source checked October 6, 2026)
- NIDDK, Continuous Glucose Monitoring (last reviewed June 2023; source checked October 6, 2026)
- FDA, 510(k) decision summary K234070, an over-the-counter integrated CGM, used here as an example (source checked October 6, 2026)
The NIDDK tests page carries a notice that NIDDK's website content is not being updated regularly, so ask your clinician about current guidance.
Primary Care Evidence Guide is an independent educational publication. It is not a medical practice and offers no care. This web address was previously used by a primary care practice with which we have no connection. See our Independence and Domain-History Notice. This article contains no affiliate links and is for general education only, not medical advice.