Understanding Blood Sugar Labs: Fasting Glucose, HbA1c, and Fasting Insulin

Julene Montgomery
Sep 24
4 min read

If you’ve ever gotten lab results back and stared at the numbers thinking, What in the world do these even mean?, you’re not alone. Blood sugar labs can feel like alphabet soup—fasting glucose, HbA1c, fasting insulin—it’s enough to make anyone’s head spin. And if you've just been diagnosed with prediabetes, it's important to understand these tests because they become tools and indicators to reverse the condition.

But here’s the good news: once you understand what these tests actually measure, they stop being confusing numbers on a page and start becoming powerful tools for taking control of your health.

So let’s break down the big three blood sugar labs—fasting glucose, HbA1c, and fasting insulin—what they mean, how they’re different, and why knowing all three gives you the clearest picture of your metabolic health.

Fasting Glucose: The Snapshot

Think of fasting glucose as a quick snapshot.

This test measures how much sugar (glucose) is floating around in your bloodstream after you haven’t eaten for at least 8 hours. It’s usually the first thing doctors check when they’re screening for diabetes or prediabetes.

Normal: under 100 mg/dL
Prediabetes: 100–125 mg/dL
Diabetes: 126 mg/dL or higher

Here’s the thing, though: fasting glucose is just one moment in time. It’s like looking at a single photo instead of the whole photo album. Your blood sugar can fluctuate based on stress, sleep, what you ate the night before, or even whether you had a fight with your spouse that morning.

So yes, it’s useful. But it doesn’t always tell the whole story.

HbA1c: The Three-Month Report Card

If fasting glucose is a snapshot, HbA1c (hemoglobin A1c) is the highlight reel.

This test measures the percentage of your red blood cells that have sugar “stuck” to them. Since red blood cells live about 3 months, HbA1c gives you an average of your blood sugar over that timeframe.

Normal: below 5.7%
Prediabetes: 5.7–6.4%
Diabetes: 6.5% or higher

It’s super helpful because it smooths out the daily ups and downs and shows your overall blood sugar trends. But like any test, it has its quirks:

If you have anemia, recent blood loss, or certain blood conditions, HbA1c might not be accurate.
It’s an average, so it can hide spikes. For example, two people could both have an HbA1c of 5.7%. One might run steady at 100 mg/dL all day, while the other swings wildly from 60 to 180. Big difference!

So while HbA1c is a great long-term marker, it doesn’t always show the hidden rollercoaster rides your blood sugar may be taking.

Fasting Insulin: The Early Warning System

Now here’s where things get interesting: fasting insulin.

This test doesn’t just measure sugar—it measures the hormone (insulin) that helps control your sugar.

Think of insulin as the “key” that unlocks your cells and lets glucose in. If your body needs to make more and more keys just to get the same job done, that’s a sign of insulin resistance—the root cause of prediabetes and type 2 diabetes.

Here’s the catch: insulin resistance shows up long before your blood sugar numbers climb high enough to trigger a red flag. That means you could have a “normal” fasting glucose and HbA1c, but your fasting insulin is already elevated, quietly signaling trouble.

Optimal fasting insulin: usually 2–6 µIU/mL
Borderline/early resistance: 7–9 µIU/mL
Insulin resistance likely: 10+ µIU/mL

Unfortunately, fasting insulin isn’t always included in routine blood work, so you may need to specifically ask your doctor for it. But if you want a fuller picture of your blood sugar health, it’s worth the request.

Why You Want All Three Blood Sugar Labs

Imagine trying to put together a puzzle with only half the pieces. That’s what it’s like relying on just fasting glucose or HbA1c.

Here’s how the three tests work together:

Fasting glucose = current snapshot.
HbA1c = three-month average.
Fasting insulin = early warning system for insulin resistance.

Together, they give you the whole picture: how your blood sugar looks right now, how it’s been trending, and whether your body is working overtime behind the scenes to keep things in range.

Real-Life Example

Let’s say two people both have a fasting glucose of 95 mg/dL.

Person A: HbA1c is 5.4%, fasting insulin is 4. Their body is handling sugar well, and they’re metabolically flexible.
Person B: HbA1c is 5.7%, fasting insulin is 12. Their blood sugar looks “fine” at first glance, but insulin is already sky-high. That’s the canary in the coal mine—early insulin resistance that could progress to prediabetes if nothing changes.

See the difference? Fasting insulin is like the backstage pass—it shows you what’s happening behind the curtain.

Fun Analogies to Remember

Fasting glucose = the selfie you take right now.
HbA1c = your highlight reel from the last 3 months.
Fasting insulin = the behind-the-scenes footage that explains how you got there.

What to Do With This Info

Ask your doctor for fasting glucose, HbA1c, and fasting insulin at your next checkup.
Track your trends over time. One number isn’t the full story, but patterns are powerful.
Don’t wait for “prediabetes” to take action. Elevated fasting insulin is a nudge from your body saying, “Hey, let’s make some changes before things get serious.”
Support your blood sugar naturally:
- Eat balanced meals with protein, fiber, and healthy fats.
- Move your body daily (even a 10-minute walk after meals helps).
- Prioritize sleep.
- Manage stress (your hormones will thank you).

Bottom Line

Fasting glucose and HbA1c are helpful, but they don’t tell the whole story. Adding fasting insulin to the mix gives you a 360-degree view of your blood sugar health—and the chance to catch problems years earlier.

Because when it comes to blood sugar, knowledge really is power. The more you understand your labs, the more proactive you can be in protecting your energy, your metabolism, and your future health.

Want to dive deeper into other important blood sugar tests? Check out my YouTube video .

Did you know that a steam sauna can improve blood sugar?

Yes, that’s right! And there are many other easy ways to improve blood sugar. Addressing prediabetes doesn’t have to be hard. Ready to improve your blood sugar without changing your diet? Sign up for my FREE mini course, 10 Ways to Improve Blood Sugar Without Changing Your Diet. It’s a free email course, and it’s a fast and easy way to learn 10 impactful tips to begin balancing your blood sugar TODAY! Learn more