Those usual tests are fasting glucose and hemoglobin A1c. A1c is the share of your hemoglobin, the protein in red blood cells, that has glucose attached, and it works as an average of blood sugar over about three months. Both have cutoffs agreed by guideline: a fasting glucose of 100 or an A1c of 5.7 marks the start of the prediabetes range, and a fasting glucose of 126 is the diabetes line. They are the tests used to diagnose diabetes, and a normal answer is good news that I take at face value.
Fasting insulin is not on that list, and no guideline defines an ideal value. What we have are long studies. In one, 515 healthy adults with normal blood sugar were followed for 24 years, and the fifth with the highest fasting insulin at the start were the most likely to develop prediabetes or diabetes. Most laboratories print values up to about 20 or 25 as normal. In the long studies, the lowest-risk groups sat in the single digits.
I like to see fasting insulin in the single digits. That is my target, not a universal cutoff, and no trial has tested a target number.
The test has real limits:
- It is noisy. It moves by about a quarter from day to day in the same person.
- Laboratories do not agree with each other, so compare results from the same one.
- A normal fasting value does not rule out high insulin after meals.
You may also see HOMA-IR. It is a calculation that multiplies fasting insulin by fasting glucose. It is not a separate test, and no guideline defines a normal value for it.
One marker is a clue. I read insulin beside glucose, A1c, triglycerides and waist size, and one high reading earns a repeat, not a label.