Short answer: a fasting insulin test measures how much insulin is in the blood after an overnight fast. Together with fasting glucose it gives HOMA-IR, an estimate of insulin resistance: HOMA-IR = glucose (mmol/L) × insulin (µIU/mL) ÷ 22.5. Insulin tests are not standardised between laboratories and HOMA-IR has no universal cut-off, so it is a research and follow-up index, not a test that diagnoses diabetes. Diabetes is diagnosed with glucose and HbA1c.
At a glance
| Measures | Insulin in serum or plasma after fasting |
|---|---|
| Units | µIU/mL (= mU/L) or pmol/L; 1 µIU/mL ≈ 6 pmol/L (some labs use 6.945) |
| LOINC | 20448-7 (insulin, serum or plasma) |
| Derived index | HOMA-IR = glucose (mmol/L) × insulin (µIU/mL) ÷ 22.5 |
| Not for | Diagnosing diabetes (glucose and HbA1c are used) |
| Keep | Insulin and glucose from the same sample, with date and fasting time |
What the test measures
Insulin lets cells take up glucose. When cells respond less well (insulin resistance), the pancreas makes more insulin to keep glucose in range, so fasting insulin can be high while glucose is still normal. That is why some doctors look at insulin, and why the HOMA-IR index combines both.
How HOMA-IR is calculated
With glucose in mmol/L: HOMA-IR = glucose × insulin ÷ 22.5. With glucose in mg/dL: HOMA-IR = glucose × insulin ÷ 405. Example: glucose 5.0 mmol/L (90 mg/dL) and insulin 10 µIU/mL give 2.2. Insulin and glucose must come from the same fasting sample.
Why there is no single cut-off
Different insulin assays give different numbers for the same blood, and HOMA-IR depends on age, weight, ethnicity and the population studied. Published thresholds range widely. Read the result with your doctor and compare it mainly with your own earlier results from the same laboratory.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| Insulin, serum or plasma | 20448-7 | µIU/mL or mU/L | pmol/L (×6.0 or ×6.945) |
| Glucose, fasting (same sample) | 1558-6 | mg/dL | mmol/L (÷18.016) |
Keeping the results in your record
Keep insulin and glucose together, with the date and how long you fasted. Anpheros Daily keeps both with their units and charts each over time; the free lab results tracker does the same for one test without an account.
What this page does not do
It explains the test and the formula. It does not diagnose insulin resistance, prediabetes or diabetes, and it does not recommend treatment.
Sources
- MedlinePlus: Insulin in Blood
- Matthews DR, et al. Homeostasis model assessment. Diabetologia 1985;28:412–419.
Frequently asked questions
What is a normal HOMA-IR?
There is no single normal value: thresholds depend on the insulin method and the population. Your doctor reads it with your glucose, HbA1c, weight and history.
Do I need to fast for an insulin test?
Yes, usually 8 to 12 hours, because insulin rises after eating. Follow your laboratory's instructions.
Can a fasting insulin test diagnose diabetes?
No. Diabetes is diagnosed with fasting glucose, HbA1c or a glucose tolerance test. Insulin can add information but is not a diagnostic criterion.
How do I convert insulin from µIU/mL to pmol/L?
Multiply by about 6 (some laboratories use 6.945). The report usually says which unit it uses.
Try it in Anpheros Daily
Photograph your lab report and Anpheros Daily lines up each value with its reference range, so you can compare reports from different laboratories.
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