What labs are used to assess insulin sensitivity?

Laboratory Tests for Assessing Insulin Sensitivity

The most appropriate laboratory tests for assessing insulin sensitivity include fasting insulin levels, fasting glucose, HOMA-IR calculation, and oral glucose tolerance test (OGTT) when indicated, rather than routine insulin or proinsulin testing which is not recommended for most patients. 1

Primary Assessment Methods

First-Line Tests

• Fasting Plasma Glucose (FPG)

  • Preferred screening test due to convenience, cost-effectiveness, and patient acceptability 2
  • Values of 100-125 mg/dL indicate impaired fasting glucose 1

• Hemoglobin A1C

  • Values of 5.7-6.4% indicate prediabetes range 1
  • Should be performed using FDA-approved or NGSP-certified methods to ensure accuracy 1

• Fasting Insulin Levels with HOMA-IR Calculation

  • Normal fasting insulin: <15 mU/L
  • Borderline high: 15-20 mU/L
  • High: >20 mU/L
  • HOMA-IR ≥3.8 indicates insulin resistance 1
  • HOMA-IR calculation: (fasting insulin × fasting glucose)/22.5 3

Second-Line Tests

• Oral Glucose Tolerance Test (OGTT)
  • Indicated when FPG is normal but insulin resistance is still suspected 2
  • 2-hour post-load values of 140-199 mg/dL indicate impaired glucose tolerance 1
  • More sensitive than FPG alone for detecting early insulin resistance 4

Clinical Application

When to Test

• BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans) 1
• Family history of type 2 diabetes 1
• High-risk ethnicity (Native Americans, African-Americans, Hispanic Americans, Asians/South Pacific Islanders) 2
• Signs of insulin resistance:
  • Acanthosis nigricans
  • Hypertension
  • Dyslipidemia (high triglycerides, low HDL)
  • Polycystic ovary syndrome 2, 1

Testing Frequency

• If initial tests are normal, repeat at minimum 3-year intervals 1
• More frequent testing if BMI is increasing 1

Important Considerations

Test Limitations

1. Fasting requirements must be strictly followed to avoid falsely elevated glucose levels 1
2. Sample handling is critical as improper handling can cause glycolysis and falsely low glucose values 1
3. Diurnal variation affects some parameters; blood pressure and glucose measurements should be performed at the same time of day (preferably morning in fasting state) 2

Research vs. Clinical Practice

The American Diabetes Association explicitly states that routine testing for insulin or proinsulin is not recommended for most people with diabetes or risk for diabetes or cardiovascular disease, as these assays are useful primarily for research purposes 2. This is a key distinction between research settings and clinical practice.

Special Populations

• Children and adolescents: Testing should be considered after puberty onset or after age 10 (whichever occurs earlier) if BMI ≥85th percentile with additional risk factors 2, 1
• Women with PCOS: Have profound insulin resistance independent of obesity and abnormalities in β-cell function, warranting screening 2
• Patients with acanthosis nigricans: This skin condition strongly correlates with insulin resistance and warrants testing, especially in teenagers with additional risk factors 1

Advanced Testing (Research Settings)

• Euglycemic insulin clamp: Gold standard technique but primarily used in research settings 5
• Minimal model analysis: Research method requiring specialized expertise 4
• Insulin suppression test: Used in research to assess steady-state insulin action 5

By following these laboratory testing guidelines, clinicians can appropriately assess insulin sensitivity and identify patients at risk for developing diabetes, allowing for timely intervention to prevent disease progression and complications.