About the Fasting Insulin Test
Purpose of the test
The fasting insulin test checks whether your body is making more insulin than it should to keep blood sugar in a normal range. That pattern is called insulin resistance (a condition where your cells don’t respond well to insulin). It can be present for years before your fasting glucose or A1c goes up. Insulin serves as a gatekeeper by allowing glucose to move from your blood to cells that use it for energy. Insulin resistance is a condition where cells in your muscles, fat, and liver do not respond appropriately thus forcing the pancreas to step up production in making more insulin. Eventually, the pancreas cannot keep up and blood glucose levels start to rise, setting the stage for prediabetes and type 2 diabetes.
As the National Institute of Diabetes and Digestive and Kidney Diseases explains, insulin resistance is closely linked to prediabetes, type 2 diabetes, metabolic syndrome, polycystic ovary syndrome (PCOS), acromegaly (excess growth hormone), Cushing’s syndrome (excess cortisol levels), viral diseases, and cardiovascular disease. Left unaddressed, it can contribute to type 2 diabetes, unhealthy levels of cholesterol and triglycerides, high blood pressure, nonalcoholic fatty liver disease, obesity, and metabolic syndrome (increases the risk of heart disease, stroke).
It doesn’t replace a fasting glucose or A1c test that would suggest diabetes, or replace other tests that would identify other complicating disorders.
What does the fasting insulin test measure?
The test measures the insulin level in your blood after fasting to determine how well your body manages glucose and any potential related metabolic issues.
You might hear about two other tests that sound similar but work differently:
- LP-IR score (Lipoprotein Insulin Resistance score): An NMR-based composite biomarker that estimates insulin resistance from lipoprotein particle size and number.
- HOMA-IR (Homeostatic Model Assessment of Insulin Resistance): A calculated index using fasting insulin and fasting glucose together.
When should I get a fasting insulin test?
Consider testing if any of these apply:
- Unexplained weight gain, especially around your waist
- Fatigue, brain fog, or energy crashes after eating
- Dark, velvety patches on skin folds (acanthosis nigricans)
- Low blood sugar symptoms between meals (shakiness, sweating, chills, clammy skin, palpitations, confusion, anxiety, or sudden hunger)
- A diagnosis of PCOS or a history of gestational diabetes
- A family history of type 2 diabetes or metabolic syndrome
- A sedentary lifestyle, shift work, or a diet high in refined carbs
- A prediabetes diagnosis, with a desire to track your response to lifestyle changes
For routine screening, no major guidelines currently recommend universal fasting insulin screening. The American Diabetes Association’s Standards of Care in Diabetes — 2025 uses fasting glucose and A1c for routine diabetes screening. Fasting insulin testing is most often ordered when:
- Routine glucose or A1c results are borderline and your provider wants more context
- You have PCOS and your provider is checking insulin’s role
- You’re tracking your response to dietary or lifestyle changes over time
The test isn’t appropriate if you’ve already been diagnosed with diabetes or you’re on glucose-lowering medications. Those conditions change how results are read.
How It Works
How to get tested
Your provider can order this test through a clinic, hospital lab, or outpatient facility. Testing.com connects you with CLIA-certified labs, including LabCorp and Quest Diagnostics. Results are delivered through a secure online account. Most are typically available within one to three business days after the lab receives your sample, though timing can vary by lab.
Before the test
Fast for eight to 12 hours before your appointment. Water is fine. Skip food, coffee, juice, and everything else. Eating triggers insulin release, so a non-fasting sample reflects a post-meal spike rather than your true baseline. That’s the whole reason fasting matters here.
Tell your provider about any medications you take. Corticosteroids, oral contraceptives, and certain other drugs can raise insulin levels. High-dose biotin supplements (above 5 mg daily) can interfere with immunoassay results, per the FDA’s safety communication on biotin in-lab tests. Don’t stop any medication without talking to your provider first.
Avoid intense exercise for several hours before your blood draw. Hard workouts can temporarily decrease insulin levels.
During the test
- A technician draws blood from a vein in your arm. The draw takes about two to three minutes.
- A small bandage covers the spot afterward.
- Keep the bandage on for at least 15 minutes to ensure bleeding stops. Call your provider if you notice lasting pain, swelling, or signs of infection at the site.
That’s it. No drink to consume, no multiple draws, no waiting around. It’s a single blood draw.
After the test
Results are typically returned within one to three business days after the lab receives your sample, though timing can vary by lab. If a provider’s office ordered the test, timing depends on their system.
What Do My Results Mean?
Your result is reported in microinternational units per milliliter (µIU/mL). The number shows how much insulin your pancreas released while you were fasting.
| Result | Fasting Insulin Level |
| Functional medicine target (non-standard) | Below 10 µIU/mL |
| Within lab reference range | Check your lab report |
| Above lab reference range | Above your lab’s upper bound |
Reference ranges vary between labs and assay methods. The “optimal” threshold below 10 µIU/mL comes from functional medicine literature and isn’t a standard medical reference range. Always read your result alongside the reference range printed on your lab report.
Your provider may calculate your HOMA-IR using both your fasting insulin and fasting glucose. A HOMA-IR above 2.0 to 2.5 is a commonly used cutoff for insulin resistance, though it can vary based on gender, age, and ethnicity. Cleveland Clinic’s overview of insulin resistance provides additional context on how these values are interpreted clinically.
If your results are normal
Your fasting insulin falls within the lab’s reference range, and your pancreas isn’t overworking to keep blood sugar stable. That’s a good sign.
A normal result doesn’t rule out all metabolic concerns. If you still have fatigue, weight gain, or blood sugar swings, follow-up testing for glucose, A1c, or a full metabolic panel may give more context. Talk with your provider about what to check next.
If your results are high
Your body is producing more insulin than expected to maintain normal blood sugar. That pattern may point to insulin resistance. It doesn’t confirm diabetes on its own, but it’s a signal worth acting on.
Your provider will likely look at your fasting glucose and A1c alongside this result. High fasting insulin is actionable: cutting refined carbs and added sugars, moving more, and sleeping better all improve insulin sensitivity. Retesting after three to six months of lifestyle changes is common. In some cases, your provider may discuss medications like metformin. The Mayo Clinic’s guidance on insulin resistance and type 2 diabetes outlines how lifestyle changes and medication work together to address elevated insulin.
Low fasting insulin is less common in this testing context. If your result is unexpectedly low and you have symptoms of high blood sugar, your provider will likely order more testing.