Fasting insulin
Blood test · fasting
Also known as: HOMA-IR, Serum insulin
There is no agreed cutoff for fasting insulin or HOMA-IR. Labs print a reference range, but it describes the people tested, not a healthy target.
Fasting insulin is the insulin in your blood after an overnight fast, and HOMA-IR combines it with fasting glucose into one estimate. When the body responds poorly to insulin, the pancreas makes more to keep blood sugar normal. So a high fasting insulin is a rough sign of insulin resistance.
What it tells you
- The level reflects two things: how resistant the body's tissues are, and how much insulin the pancreas can still make.
- As the pancreas starts to fail on the way to type 2 diabetes, insulin can fall while blood sugar rises. A lower insulin is not automatically better without the glucose beside it.
- HOMA-IR is worked out from fasting insulin and fasting glucose drawn at the same time.
- In its original study, HOMA-IR tracked the gold-standard test of insulin sensitivity closely. But its estimate for one person carried about 31% variation.
- Its designers meant it for studies of large groups, not for judging one person.
- No guideline uses insulin or HOMA-IR to diagnose diabetes. US laboratory and diabetes guidelines advise against routine insulin testing.
What can throw it off
- The lab method matters most. When 12 insulin tests were compared, results varied by a median of about 24% between methods.
- So compare results from the same lab and method. A change between labs may reflect the test, not you.
- Eating before the draw raises it.
- A long fast or a large calorie deficit lowers it. In one study, a two-day fast cut average insulin by more than a third.
- Red blood cells breaking open in the tube can make it falsely low.
- High-dose biotin supplements can interfere with at least one widely used insulin test. Stopping biotin for a few days before the draw is sensible.
- Medicines that change insulin sensitivity or output move it. Incretin drugs lower it in trials, and growth-hormone-releasing drugs can raise it.
- In people treated with insulin, the result no longer reflects the body's own insulin.
Evidence
Detecting insulin resistance
No accuracy study exists, because no cutoff has been validated. In correlation studies, HOMA-IR tracked the gold-standard clamp test fairly closely. It agreed only moderately when sorting people into sensitivity groups.
Predicting type 2 diabetes
In about 2,700 Framingham adults followed for 7 to 11 years, HOMA-IR predicted diabetes fairly well. But simple facts like family history, weight, blood pressure, cholesterol and fasting glucose predicted it better. Adding insulin or HOMA-IR to those facts barely changed the result.
Predicting heart disease
In a review of 65 studies with over 500,000 people, fasting insulin alone did not significantly predict heart disease. HOMA-IR did, modestly. People in the top group had about 1.6 times the risk of those in the bottom group.
Limits
- Insulin tests are not standardized. Results are not comparable across labs or with cutoffs seen online.
- There is no agreed cutoff. Published HOMA-IR thresholds range from about 1.5 to 3.8, depending on the population and method.
- One reading is imprecise. HOMA-IR works best for comparing groups, not for judging one person.
- It adds little to standard risk checks for diabetes or heart disease.
- A falling insulin can mean better insulin sensitivity or a tiring pancreas. It needs to be read with glucose and HbA1c.
References
This page is for education only. It is not medical advice. Decisions about any treatment belong with a physician who knows your history.