Jun 4, 2025
Testing for Diabetes: The Two Key Blood Tests You Should Know
Early detection and monitoring of diabetes starts with two important blood tests: Fasting Blood Glucose and HbA1c.
Biomarkers
Written by
Mito Team
If you’re curious about your blood sugar levels or if you might be at risk for diabetes, two tests can help you out: the Fasting Blood Glucose (FBG) test and the Hemoglobin A1c (HbA1c) test.
And the need for testing is bigger than you might think. Nearly 1 in 4 adults with diabetes don’t even know they have it. That’s 8.7 million undiagnosed people living with high blood sugar, silently damaging their health.
These tests are the most commonly used to screen for prediabetes, type 2 diabetes, and sometimes gestational diabetes. While both are useful, they don’t measure the same thing. In fact, they often give different answers. That’s why understanding the difference between A1c and glucose can help you take control of your health.
What Is Fasting Blood Glucose (FBG)?
Fasting Blood Glucose, or FBG, measures how much sugar (glucose) is in your blood after you haven’t eaten for 8 to 12 hours. Doctors usually ask you to come in for a morning blood draw after an overnight fast.
This test gives a snapshot of your current blood sugar level, a glimpse into how your body handles sugar without the influence of a recent meal.
What the Results Mean
Normal: Less than 100 mg/dL
Prediabetes: 100 to 125 mg/dL
Diabetes: 126 mg/dL or higher (confirmed with a second test)
If your glucose serum is high, especially in the morning before eating, it could be an early sign of insulin resistance or diabetes. But this doesn’t always tell the full story.
What Is Hemoglobin A1c (HbA1c)?
On the other hand, Hemoglobin A1c tells you your average blood sugar level over the past 2 to 3 months. It works by measuring how much sugar is stuck to your red blood cells.
You might hear people ask, “Do you need to fast for A1c?” The answer is no. That’s one of its biggest advantages. You can take this test anytime—no fasting required.
Hemoglobin A1c Range
Normal HbA1c range: Below 5.7%
Prediabetes: 5.7% to 6.4%
Diabetes: 6.5% or higher (confirmed with a second test)
Doctors use this test to diagnose diabetes and to monitor how well your blood sugar is controlled over time. If you already have diabetes, your provider may suggest testing your HbA1c every 3 to 6 months.
HbA1c Levels vs. Fasting Glucose: What's the Difference?
This can get confusing, so here’s the simple version.
Fasting blood glucose looks at your blood sugar right now (after fasting).
Hemoglobin A1c looks at your average blood sugar over a few months.
People often ask: Which is more accurate—HbA1c or fasting glucose? The truth is, both are helpful, but for different reasons. FBG can be affected by short-term things like stress, illness, or lack of sleep. HbA1c levels aren’t as sensitive to daily changes but may be off if you have certain health conditions that affect red blood cells.
This explains a common question: Why is my fasting blood sugar high but my HbA1c is normal? That happens if your blood sugar spikes in the morning but is mostly normal throughout the day. Since A1c averages things out, it may miss those brief highs. That’s why doctors sometimes recommend using both tests to get a clearer picture.
How HbA1c and Glucose Levels Work Together
If you’re screening for diabetes or trying to keep it under control, using both FBG and A1c can be a smart move. Each test fills in gaps that the other might miss.
FBG test is best for spotting immediate problems
HbA1c test is best for seeing long-term trends
Some people may have a normal A1c range but an abnormal fasting glucose, or the other way around. When both are borderline or elevated, your risk of developing diabetes is higher. That’s especially true if you also have risk factors like weight gain, family history, or high blood pressure.
What If Your HbA1c Is High?
An elevated hemoglobin A1c means your blood sugar has been running high for several months. Even if you feel fine, that can quietly increase your risk for heart disease, nerve damage, kidney problems, and more.
The good news? Small changes in diet, exercise, sleep, and stress management can often bring your numbers back into a healthy range, especially if you catch it early.
So Which Test Should You Get?
That depends on your personal health, convenience, and what your doctor recommends.
Choose FBG if you're looking for a quick screen, are pregnant, or have conditions that affect HbA1c test result accuracy.
Choose HbA1c if you want a more complete view of long-term blood sugar, and don’t want to fast.
Use both if you want the most accurate picture of your HbA1c and glucose levels combined.
Final Thoughts
If you’re trying to catch diabetes early or stay on top of it, don’t skip your blood work. These two simple tests can reveal a lot, even before symptoms appear.
At Mito Health, we make it easy to get both HbA1c and fasting glucose tests. Our comprehensive blood panel covers these biomarkers and more. Join now to monitor your prediabetes, adjust your diet, or optimize your energy.