Blood Test

Fasting Insulin Test

Name: Fasting Insulin Test: Normal Range | HealthReview
Creator: Suman Konda
Published: 2026-07-05

A fasting insulin test measures how much insulin your pancreas is producing to keep blood sugar normal. High fasting insulin is one of the earliest signs of insulin resistance, often detectable years before diabetes develops.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: 01 August 2026 · How we check our content

What is a fasting insulin test?

Fasting insulin measures the level of insulin in your blood after fasting for 8-12 hours. Insulin is a hormone made by the beta cells of the pancreas. Its job is to move glucose from the blood into cells for energy. In insulin resistance, cells no longer respond well to insulin, so the pancreas compensates by making more and more insulin to keep blood sugar normal. High fasting insulin with normal blood sugar is the hallmark of early insulin resistance. The test is typically ordered alongside fasting glucose to calculate the HOMA-IR index.

Fasting insulin normal range

Category	Fasting Insulin (microIU/mL or mU/L)	Interpretation
Optimal	< 5	Excellent insulin sensitivity
Normal	5 – 10	Normal range
Borderline high	10 – 15	Early insulin resistance possible
High	> 15	Likely insulin resistance
Very high	> 25	Significant insulin resistance or insulinoma

Fasting Insulin Test: reference intervals by group (mU/L), drawn from the table above. Intervals differ between laboratories: compare your result against the range printed on your own report.

What is HOMA-IR?

HOMA-IR (Homeostasis Model Assessment of Insulin Resistance) is calculated from fasting insulin and fasting glucose:

HOMA-IR = (Fasting Insulin x Fasting Glucose) / 405 (glucose in mg/dL)

HOMA-IR Value	Interpretation
< 1.0	Optimal insulin sensitivity
1.0 – 1.9	Normal
2.0 – 2.9	Early insulin resistance
≥ 3.0	Significant insulin resistance

Why HOMA-IR Has No Agreed Cut-Off

HOMA-IR is calculated from your fasting insulin and fasting glucose, and it is genuinely useful — but it is worth knowing what kind of tool it is before treating a number as a diagnosis. It was developed for comparing groups in research, and it has never acquired a single threshold that separates insulin resistance from normality across populations.

Reviews of its use are explicit that the appropriate cut-off differs by ethnicity, age, sex, and by which conditions someone already has, and that this variability is exactly what has prevented an agreed value from being established.² Studies that derive a cut-off for one specific population produce figures that apply to that population and not necessarily beyond it — and even then, performance is modest. One such study set a threshold of 2.7 for its group and reported sensitivity and specificity around 70%, describing the resulting prediction as of moderate strength.³

Two numbers from two laboratories may not be comparableInsulin assays are not standardised between manufacturers the way glucose is. The same sample can yield different insulin values, and therefore different HOMA-IR figures, depending on which laboratory ran it. Comparing your result to a threshold you read online, or to a friend’s result from another lab, is not a like-for-like comparison.

None of this makes the test useless. It makes it a tracking tool rather than a verdict. Measured at the same laboratory over time, a falling HOMA-IR alongside weight loss or increased activity is meaningful evidence that something is working. What it cannot do is convert a single reading into a diagnosis, and there is no threshold at which a specific treatment automatically becomes appropriate.

Practical consequences follow. Fasting insulin must genuinely be fasting, since anything eaten beforehand raises it. And an insulin-resistance panel bought privately, interpreted against a generic cut-off and acted on without the rest of your clinical picture, is the least reliable way to use this test.

What causes high fasting insulin?

Overweight and obesity, especially excess belly fat
Sedentary lifestyle
High-carbohydrate diet (especially refined carbs and sugar)
PCOS (polycystic ovary syndrome): insulin resistance is a core feature
Sleep deprivation and chronic stress (raises cortisol which antagonises insulin)
Metabolic syndrome
Family history of type 2 diabetes
Certain medications (corticosteroids, antipsychotics, thiazide diuretics)

Why does this matter?

Insulin resistance is the foundation of type 2 diabetes, metabolic syndrome, PCOS, non-alcoholic fatty liver disease (NAFLD) and cardiovascular disease. Detecting it early through fasting insulin, when fasting glucose and HbA1c are still normal, allows for intervention through lifestyle change before damage occurs.

Questions to ask your doctor

What is my HOMA-IR score?
Does this mean I am at risk for diabetes?
Should I have an OGTT (oral glucose tolerance test)?
What diet and lifestyle changes reduce insulin resistance?
If I have PCOS, is insulin resistance driving it?

Frequently asked questions

Should a borderline Fasting Insulin result be repeated?

Usually. A value just outside the interval is common in people who are entirely well, since about one in twenty healthy results falls outside any 95% reference range by construction. A borderline Fasting Insulin is normally rechecked rather than acted on.

Do Fasting Insulin results from two different laboratories compare directly?

Not reliably. Methods differ, so a change between labs can reflect the assay rather than a change in you. Where Fasting Insulin is being tracked over time, staying with one laboratory makes the comparison meaningful.

Can medicines change a Fasting Insulin result?

Frequently, and this includes things people do not think of as medicines: supplements, herbal products and over-the-counter painkillers. Bringing a current list to the appointment often explains a result faster than repeating the test.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

Biochemistry, Hormones. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK541112
Tang Q, Li X, Song P, Xu L. Optimal cut-off values for the homeostasis model assessment of insulin resistance (HOMA-IR) and pre-diabetes screening: developments in research and prospects for the future. Drug Discov Ther. 2015;9(6):380–5. PMID 26781921 · doi:10.5582/ddt.2015.01207
Harbuwono DS, Nenfiati, Tahapary DL, Setiati S, Yunihastuti E, Tarigan TJE. Defining the optimum HOMA-IR cut-off value for insulin resistance among ARV-treated HIV patients in Indonesia. Diabetes Metab Syndr. 2023;17(2):102719. PMID 36739722 · doi:10.1016/j.dsx.2023.102719