High Fasting Insulin Level: Definitions, Clinical Use, and Limitations

Definition and Thresholds

The American Heart Association defines fasting insulin levels as normal < 15 μU/mL, borderline high 15–20 μU/mL, and high ≥ 20 μU/mL; these cut‑offs are used as a practical alternative to the euglycemic insulin clamp, although they are less precise than the clamp method. 1

Fasting insulin should be interpreted only when the simultaneous fasting plasma glucose is ≤ 220 mg/dL (12.5 mmol/L), ensuring the assay reflects true insulin secretion rather than hyperglycemia‑driven changes. 2
The test is most appropriate in individuals who have risk factors for insulin resistance (e.g., obesity, family history of type 2 diabetes, hypertension, dyslipidemia, polycystic ovary syndrome) and/or physical signs such as acanthosis nigricans. 1

Routine fasting insulin testing is not recommended for most patients with diabetes or those at risk for diabetes; the assay is primarily valuable for research purposes rather than routine clinical decision‑making. 2

Instead of fasting insulin, the American Heart Association recommends evaluating insulin resistance syndrome through a combination of:

Although fasting insulin concentrations decrease with age and are slightly higher in men than women, the ≥ 20 μU/mL threshold for “high” insulin remains applicable across all age groups for identifying clinically significant hyperinsulinemia. 1