Fasting Blood Glucose and HbA1c
Blood sugar tests are among the most common laboratory tests, but they don't always tell the whole story.
Glucose is the sugar circulating in our blood. Insulin, a hormone produced by the pancreas, helps regulate blood glucose and allows many cells to take up and use it.
When cells become less responsive to insulin—a condition called insulin resistance—the pancreas may compensate by producing more insulin. Blood glucose can remain normal for years, even while insulin levels are elevated.
This is why some researchers and clinicians believe that measuring insulin, in addition to glucose, can provide useful information about metabolic health.
In this article, we'll look at fasting glucose, HbA1c, fasting insulin, and HOMA-IR—what they measure, how to interpret them, and what their limitations are.
Fasting Blood Glucose
This test measures the amount of glucose in your blood after you have not eaten for at least eight hours.
It is commonly used to screen for diabetes and prediabetes.
How to Interpret Fasting Glucose
Below 100 mg/dL — Normal
100–125 mg/dL — Prediabetes
126 mg/dL or higher — Diabetes range
A result in the diabetes range generally requires confirmation with repeat testing or another diagnostic test.
An important limitation: Fasting glucose can remain normal even when the body is producing unusually large amounts of insulin to keep it there.
This means that a normal fasting glucose result does not necessarily rule out insulin resistance.
HbA1c (Also Called A1C): Average Blood Sugar Over Time
The HbA1c test estimates your average blood glucose over approximately the past two to three months. Unlike fasting glucose, it does not require fasting.
It measures the percentage of hemoglobin in red blood cells that has glucose attached to it.
How to Interpret HbA1c
Below 5.7% — Normal
5.7%–6.4% — Prediabetes
6.5% or higher — Diabetes range
A result in the diabetes range generally requires confirmation unless there is clear evidence of diabetes.
An important limitation: HbA1c measures average glucose, not insulin levels. It may remain normal even when insulin resistance is developing.
HbA1c can also be misleading in people with anemia, certain blood disorders, recent blood loss, or conditions that affect the lifespan of red blood cells.
For older adults, HbA1c is particularly useful for monitoring diabetes, but treatment targets should be individualized according to overall health and the risks of low blood sugar.
Fasting Insulin: What Glucose Tests May Miss
Fasting insulin measures the amount of insulin circulating in your blood after an overnight fast, usually 8–12 hours.
Insulin helps move glucose from the bloodstream into cells. When cells become resistant to insulin, the pancreas often compensates by producing more of it.
As a result, fasting glucose and A1C may remain normal even while insulin levels are elevated.
What Is a Normal Fasting Insulin Level?
Unlike glucose and A1C, fasting insulin does not have universally accepted clinical cutoffs for diagnosing insulin resistance.
Some metabolic health researchers favor fasting insulin levels below approximately 5 µIU/mL, while many laboratories use considerably broader reference ranges. However, this lower target has not been established as a universal clinical standard.
Why Might This Test Be Useful?
Fasting insulin can provide additional information about insulin resistance, particularly when considered alongside fasting glucose, triglycerides, HDL cholesterol, and waist circumference.
However, insulin levels vary between individuals and laboratory methods. Fasting insulin is not currently recommended as a routine screening test for everyone.
The important point: Normal glucose does not always mean normal insulin metabolism.
HOMA-IR: Estimating Insulin Resistance
HOMA-IR stands for Homeostatic Model Assessment of Insulin Resistance.
It is a calculation that combines fasting glucose and fasting insulin to estimate how resistant the body may be to insulin.
When glucose is measured in mg/dL, the formula is:
HOMA-IR = Fasting Insulin × Fasting Glucose ÷ 405
For example, someone with a fasting glucose of 95 mg/dL and fasting insulin of 10 µIU/mL would have a HOMA-IR of approximately 2.35.
What Does the Result Mean?
Generally, a higher HOMA-IR suggests greater insulin resistance.
Some researchers use values around 2.0–2.5 as possible indicators of insulin resistance. However, there is no universally accepted cutoff. Results vary with age, population, laboratory methods, and metabolic health.
Why Isn't HOMA-IR Routinely Ordered?
HOMA-IR is widely used in research, but it is not a standard diagnostic test for insulin resistance in everyday medical practice.
One limitation is that insulin measurements are not standardized across laboratories. HOMA-IR also becomes less reliable in certain situations, including when insulin production is substantially impaired.
The important point: HOMA-IR can provide another perspective on metabolic health, but it should not be interpreted as a definitive diagnosis by itself.
Which Tests Should You Consider?
For most adults, fasting glucose and A1C are the standard tests used to screen for prediabetes and diabetes.
But these tests measure blood sugar, not how much insulin the body needs to maintain it.
Fasting insulin may provide additional information about metabolic health, particularly when glucose levels are normal but other signs suggest insulin resistance.
HOMA-IR combines fasting glucose and insulin into a single estimate of insulin resistance. It is primarily a research tool rather than an established diagnostic test.
Other useful clues include triglycerides, HDL cholesterol, blood pressure, and waist circumference.
For people in their 70s and 80s, interpreting these results should also take into account medications, other health conditions, and changes over time.
The bottom line: Normal blood sugar is reassuring, but it does not necessarily mean that insulin metabolism is normal. Additional testing may be worth discussing with your healthcare professional, although it is not routinely necessary for everyone.
Learn More
Information about A1C testing, diabetes diagnosis, and blood sugar management.
Emphasizes low-carbohydrate nutrition, insulin resistance, and metabolic approaches to preventing and managing type 2 diabetes.
Explains insulin resistance, prediabetes, risk factors, and established approaches to prevention.