A1c, fasting glucose or a CGM: what each test tells you

A1c: the long-term average

Hemoglobin A1c measures how much glucose has attached to hemoglobin in your red blood cells. Because red cells live for a few months, the result reflects your average glucose over roughly the previous two to three months. You do not need to fast, which makes it convenient.

The CDC uses these cut points: below 5.7 percent is normal, 5.7 to 6.4 percent is prediabetes, and 6.5 percent or higher indicates diabetes. A1c is excellent for tracking trends over time, but it is an average. Two people with the same A1c can have very different days: one steady, one swinging high and low.

When A1c can mislead

MedlinePlus notes that A1c may not be accurate if you have anemia or another condition affecting red blood cells, and that kidney failure, liver disease, pregnancy and some medicines can also affect results. In these situations your clinician may lean more on fasting glucose, a glucose tolerance test or CGM data.

Recent blood loss, iron deficiency or a recent transfusion can also shift the result, which is one reason a full blood count is useful alongside A1c.

Fasting glucose: a morning snapshot

Fasting glucose is measured after at least eight hours without food. According to the CDC, 99 mg/dL or lower is normal, 100 to 125 mg/dL is prediabetes and 126 mg/dL or higher suggests diabetes. It is cheap, widely available and part of most routine panels.

Its weakness is that it captures one moment. A short night's sleep, stress, illness or a late heavy meal can nudge it up. Many people with early insulin resistance have normal fasting glucose but larger rises after meals, which a fasting test cannot see. Pairing it with fasting insulin, to estimate insulin resistance, adds context.

CGM: the full movie

A continuous glucose monitor uses a small sensor under the skin to estimate glucose every few minutes. Over one or two weeks, you see your overnight baseline, how high and how long you rise after different meals, and how exercise, alcohol, stress and sleep shift the pattern.

That detail is the strength of a CGM, but it is not a diagnostic test, and readings can lag rapid changes. The FDA advises against making medical decisions from over-the-counter CGM readings without talking to a healthcare provider. Evidence that routine CGM use improves long-term outcomes in people without diabetes is limited.

Using them together

In practice, these tests complement each other. Lab values set the diagnosis and track risk over years. A CGM explains the day-to-day behavior behind those numbers and shows which changes actually move your curve.

• A1c rising but fasting glucose normal: a CGM can reveal large after-meal rises
• Fasting glucose high but A1c normal: a CGM can show whether early-morning rises are the main issue
• A1c may be unreliable because of anemia or kidney disease: CGM data add another view
• Starting a GLP-1, metformin or lifestyle plan: a CGM shows early changes before the next A1c
• Normal labs but strong family history: a CGM can show how your body handles your usual meals

Which test should you start with?

For most adults over 45, the starting point is lab work. The CDC uses A1c, fasting glucose and the glucose tolerance test to diagnose prediabetes and diabetes, and these results are what guidelines and medication decisions are built on. If your results are normal and you feel well, a CGM is optional.

A CGM becomes more useful when your labs raise a question. That might be a rising A1c despite a normal fasting result, an A1c that may be unreliable because of anemia, a strong family history, or a new medication or lifestyle plan you want feedback on. In those cases, a few weeks of continuous data can explain what a single number cannot.

Whichever test you use, the next step is the same: a clear plan for what to change and when to recheck.

Getting both through HealifyMD

Your clinician can order the Comprehensive Lab Panel ($199 one-time), which includes A1c and fasting glucose among other markers, and recommend a CGM when the data would change your plan. The HealifyMD CGM is $189 one-time (one 14-day sensor, clinician visit included). There is no membership fee, and prices are confirmed at the eligibility review.

Your clinician reviews your CGM data alongside your labs, medicines and habits and gives you a written summary with practical next steps.