Why test insulin?

Insulin testing assesses the body’s insulin production and its response to glucose. It helps identify conditions related to insulin resistance (the body’s reduced response to insulin), hyperinsulinemia (high insulin levels), and can indicate risk or presence of conditions like:

• Prediabetes and diabetes
• Polycystic ovary syndrome (PCOS)
• Metabolic syndrome
• Hypoglycemia evaluation
• Insulin-producing tumors (insulinomas)

Why a fasting insulin?

Fasting insulin measures insulin levels after 8–12 hours without food or caloric beverages. This provides a stable baseline measurement and clearer insights into:

• Insulin resistance (high fasting insulin)
• Early signs of metabolic dysfunction (before glucose levels become elevated)
• Predictive risk of type 2 diabetes and cardiovascular diseases
• Identifying hyperinsulinemia (often seen before fasting glucose abnormalities occur)

How do you test insulin?

Insulin levels are tested with a blood test:

Preparation: Typically fasting overnight (8–12 hours).

Procedure: Blood drawn from a vein (often involves an extra fee)

Interpretation:

• Normal fasting insulin usually <10 µIU/mL (varies by lab).
• Elevated insulin levels indicate insulin resistance or hyperinsulinemia.
• Low insulin levels may indicate type 1 diabetes or pancreatic dysfunction.

What does fasting insulin show?

Fasting insulin primarily indicates:

• Insulin resistance (high fasting insulin levels)
• Pancreatic function and insulin secretion status
• Metabolic health and risk factors for type 2 diabetes, PCOS, and cardiovascular disease
• High fasting insulin levels, even with normal glucose levels, suggest metabolic dysfunction occurring before more overt glucose abnormalities (such as elevated fasting glucose) develop.

What is a HOMA-IR?

HOMA-IR (Homeostasis Model Assessment of Insulin Resistance) is a mathematical calculation that uses fasting insulin and fasting glucose to estimate insulin resistance:

HOMA-IR= 

Fasting insulin (µIU/mL)×Fasting glucose (mg/dL)

​

Interpretation:

• Normal: Typically <2.0 (varies slightly by guideline/lab).
• Elevated (>2.0–3.0) indicates insulin resistance.

Usefulness:

• Identifies insulin resistance early
• Helpful in risk assessment for diabetes, PCOS, and cardiovascular disease
• Monitors improvements with lifestyle or medication interventions

Why test insulin over C-peptide?

Insulin and C-peptide both reflect pancreatic insulin secretion but have different strengths and clinical utilities:

Insulin 

- Direct measure of active hormone levels

- More sensitive to rapid fluctuations

- Reflects immediate metabolic response to glucose 

- Diagnosing insulin resistance

- Evaluating metabolic syndrome, prediabetes, PCOS, insulinomas

- Rapid response assessments

C-peptide  

- Stable marker reflecting endogenous insulin production (not influenced by exogenous insulin therapy)

- Longer half-life (less variability) 

- Assessing residual pancreatic function in diabetes

- Differentiating between type 1 and type 2 diabetes

- Useful when patients are on insulin therapy

Summary

Use insulin testing for acute or early detection of insulin resistance, hyperinsulinemia, PCOS, metabolic syndrome, or insulinomas.

Use C-peptide testing to assess longer-term endogenous pancreatic function, especially useful in diabetics receiving insulin injections (exogenous insulin obscures insulin measurements but not C-peptide).

Quick Takeaways

• Fasting insulin identifies insulin resistance early.
• HOMA-IR quantifies insulin resistance clearly.
• Insulin testing helps diagnose metabolic dysfunction early; C-peptide assesses pancreatic function independently from exogenous insulin.