What is Considered a High Fasting Insulin Level?
A fasting insulin level ≥20 μU/mL (or mU/L) is considered high, with borderline high defined as 15-20 μU/mL, and normal as <15 μU/mL. 1
Established Clinical Thresholds
The American Heart Association provides clear cutoffs for fasting plasma insulin levels in the assessment of insulin resistance 1:
- Normal: <15 mU/L
- Borderline high: 15-20 mU/L
- High: ≥20 mU/L
These thresholds are used clinically as a reasonable alternative to the euglycemic insulin clamp (the gold standard research method) for evaluating insulin resistance, though they are less accurate than the clamp technique 1.
Population-Based Reference Values
Research studies provide additional context for normal ranges across different populations:
Iranian adults: The 95th percentile upper reference limit is approximately 11-12 μU/mL in healthy, non-obese adults aged 24-83 years 2
French population: In a large study of over 4,000 individuals, the 95th percentile reference values were:
- Women (30-49 years): 11.2 μU/mL
- Men (30-64 years) and women (50-64 years): 12.6 μU/mL 3
These population studies suggest that values above approximately 12-13 μU/mL represent the upper 5% of the healthy population, supporting the clinical threshold of 15 μU/mL as borderline elevated 2, 3.
Clinical Context for Assessment
Fasting insulin measurement is most useful when 1:
- Fasting plasma glucose is simultaneously ≤220 mg/dL (12.5 mmol/L) for accurate interpretation 4
- The patient has risk factors including obesity, family history of type 2 diabetes, hypertension, dyslipidemia, or polycystic ovary syndrome 1
- Physical signs of insulin resistance are present, such as acanthosis nigricans 1
Important Caveats
Routine insulin testing is not recommended for most patients with diabetes or at risk for diabetes, as these assays are primarily useful for research purposes 4. The clinical diagnosis of insulin resistance syndrome is better made through assessment of:
- Body mass index and waist circumference 1
- Blood pressure measurements 1
- Fasting plasma glucose 1
- Lipid profile (triglycerides and HDL cholesterol) 1
Age and sex influence insulin levels: Fasting insulin concentrations show a decreasing trend with age in both genders, and levels are slightly higher in men compared to women across all age groups 2, 3. However, the clinical threshold of ≥20 mU/L for "high" remains applicable across age groups for identifying clinically significant hyperinsulinemia 1.