In this article
Insulin and A1C are both blood tests connected to how the body handles glucose (blood sugar), but they measure different things. An insulin in blood test measures the amount of insulin in a blood sample (MedlinePlus). An A1C test reflects average blood glucose over the past few months (NIDDK).
This guide explains what each test measures, in general terms.
What insulin does
Insulin is a hormone made by the pancreas that helps control the amount of glucose in the blood. MedlinePlus describes how insulin and blood glucose normally rise and fall together (MedlinePlus):
- After eating, blood glucose goes up.
- The rise tells the pancreas to release more insulin.
- Insulin helps glucose move from the blood into cells, where it's used for energy.
- As blood glucose goes down, the pancreas releases less insulin.
What the insulin in blood test measures
An insulin in blood test measures the amount of insulin in a sample of blood. Other names for it include fasting insulin, insulin serum, and total and free insulin (MedlinePlus).
MedlinePlus says the test may be used together with other tests. Finding the cause of low blood glucose (hypoglycemia) is the main reason for insulin testing. Other uses include helping evaluate how much insulin the pancreas is making (MedlinePlus).
MedlinePlus also notes that an insulin in blood test is not usually used to diagnose diabetes. People with diabetes may have insulin levels that vary, so diabetes is usually diagnosed with tests that measure blood glucose (MedlinePlus).
What the A1C test measures
The A1C test is a blood test that reflects average blood glucose levels over the past 3 months (NIDDK). It's also called hemoglobin A1C, HbA1c, glycated hemoglobin or glycohemoglobin (NIDDK).
Glucose in the blood attaches to hemoglobin, a protein in red blood cells. The A1C test measures the amount of hemoglobin with glucose attached, and the result is reported as a percentage (NIDDK). Because red blood cells live about three months, the test reflects an average over that period (MedlinePlus).
According to NIDDK, the A1C test can be used to diagnose type 2 diabetes and prediabetes, and it is the primary test used for diabetes management (NIDDK).
How the two tests compare
| Insulin in blood test | A1C test | |
|---|---|---|
| What it measures | The amount of insulin in a blood sample (MedlinePlus) | The amount of hemoglobin with glucose attached (NIDDK) |
| What it reflects | Insulin in the blood at the time of the sample | Average blood glucose over about 3 months (NIDDK) |
| Main uses described | Finding the cause of low blood glucose; evaluating how much insulin the pancreas makes (MedlinePlus) | Diagnosing type 2 diabetes and prediabetes; diabetes management (NIDDK) |
| Used to diagnose diabetes? | Not usually (MedlinePlus) | Yes, alone or with other tests (NIDDK) |
| Other names | Fasting insulin, insulin serum | HbA1c, glycated hemoglobin |
When to see a clinician
Questions about lab tests, or about any change in health, are best discussed with a licensed clinician who knows a person's history. In a medical emergency, call 911 or go to the nearest emergency room.
Frequently asked questions
Is a fasting insulin test the same as an insulin in blood test? MedlinePlus lists "fasting insulin" as another name for the insulin in blood test (MedlinePlus).
What does A1C measure? The amount of hemoglobin in the blood that has glucose attached, reported as a percentage. It reflects average blood glucose over the past 3 months (NIDDK).
Why does A1C reflect about three months? Glucose stays attached to hemoglobin for as long as a red blood cell is alive, and red blood cells live about three months (MedlinePlus).
What's the difference between insulin and glucose? Glucose is a sugar that is the body's main source of energy. Insulin is a hormone that helps glucose move from the blood into cells (MedlinePlus).
- MedlinePlus. Insulin in blood test.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The A1C test & diabetes.
- MedlinePlus. Hemoglobin A1C (HbA1c) test.
The information in this article is for general educational purposes only. It is not medical advice and is not a substitute for diagnosis or treatment from a qualified clinician who knows your history. Don't start, stop or change any treatment based on this article. If you think you're having a medical emergency, call 911 or go to the nearest emergency room. Reading this article does not create a clinician-patient relationship with Tenor Health or any provider on our platform.
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