Diabetes & metabolic
Fasting insulin blood test
A fasting insulin blood test measures the hormone that moves sugar from your blood into your cells, taken after an overnight fast. Alongside fasting glucose it reveals insulin resistance, often years before blood sugar rises, which makes it a useful early marker of metabolic health.
- What it measures
- Insulin (fasting), the hormone that controls blood sugar
- Sample
- Venous blood sample
- Fasting
- Required (an overnight fast, needed to interpret the result)
- Also known as
- insulin, fasting serum insulin
- Included in 57 Health tests
- 1 panel, from £100
Fasting insulin reference range
Guide only| Category | Guide range |
|---|---|
| Fasting adults (guide) | < 25 mIU/L |
The result is only meaningful when fasting, and it's interpreted alongside fasting glucose to estimate insulin resistance (the HOMA-IR calculation). An “optimal” fasting insulin is often well below the upper limit. Ranges and units vary by laboratory.
These are general UK adult guide ranges from public health sources, shown for orientation only. Reference ranges vary by laboratory, assay, age and sex, so your result is always interpreted against the range quoted by the analysing laboratory on your report.
What does a high fasting insulin level mean?
A high fasting insulin suggests insulin resistance: your body is producing extra insulin to keep blood sugar normal. This can appear years before glucose or HbA1c rise and is linked with weight gain, polycystic ovary syndrome and a higher risk of type 2 diabetes.
It's interpreted alongside fasting glucose (as HOMA-IR), HbA1c and your wider metabolic picture rather than on its own.
What does a low fasting insulin level mean?
A low fasting insulin is usually normal, particularly if blood sugar is also normal.
A low insulin with a high blood sugar is a different pattern that points to reduced insulin production, and is interpreted alongside glucose and other tests.
When to consider a fasting insulin test
- Assessing insulin resistance or prediabetes risk
- Weight gain, especially around the middle
- Polycystic ovary syndrome (PCOS)
- A family history of type 2 diabetes
- A fuller metabolic health check
Frequently asked questions
This page is general information about a blood marker and is not medical advice or a diagnosis. A fasting insulin test screens and gives context; results are interpreted by a clinician alongside your symptoms, history and other markers. Always follow the guidance on your own report.