Testing for Insulin Resistance
Request a fasting plasma glucose (FPG) and hemoglobin A1C (HbA1c) as your initial screening tests—these are the most practical and guideline-recommended approaches for detecting insulin resistance in clinical practice. 1
Primary Screening Tests
The American Diabetes Association endorses three equally appropriate screening options for detecting prediabetes and insulin resistance 1:
- Fasting Plasma Glucose (FPG): Values of 100-125 mg/dL indicate impaired fasting glucose, reflecting insulin resistance 1
- Hemoglobin A1C: Values of 5.7-6.4% suggest prediabetes with underlying insulin resistance 1
- Oral Glucose Tolerance Test (OGTT): 2-hour glucose values of 140-199 mg/dL indicate impaired glucose tolerance 1
The FPG and A1C are more convenient, reproducible, and easier to perform than the OGTT, making them preferred for routine screening. 1
Most Sensitive Approach for Early Detection
If you want the most sensitive test to catch insulin resistance earliest, request an oral glucose tolerance test (OGTT) with 2-hour glucose measurement—this detects abnormalities before fasting glucose becomes elevated. 2, 3 However, the OGTT is poorly reproducible and difficult to perform in practice. 1, 2
For OGTT testing, ensure adequate carbohydrate intake (at least 150 g/day) for 3 days prior to testing. 1
Additional Confirmatory Tests
Fasting insulin levels provide direct confirmation of insulin resistance 2, 4, 5:
- Normal: <15 mU/L 2, 4
- Borderline high: 15-20 mU/L 2, 5
- Clearly elevated (confirms insulin resistance): >20 mU/L 2, 5
The Endocrine Society states that fasting insulin >15 mU/L directly confirms insulin resistance. 2 While fasting insulin is highly specific for insulin resistance, it may be less sensitive than combined measures. 6, 7
Testing Requirements and Critical Pitfalls
Always test in the fasting state (minimum 8 hours without caloric intake) to avoid postprandial variations that invalidate results. 2, 4, 5
Normal glucose levels do not exclude insulin resistance—hyperinsulinemia can exist with euglycemia, which is why measuring fasting insulin alongside glucose improves diagnostic accuracy. 4, 8
Who Should Be Tested
Request testing if you have 1:
- BMI ≥25 kg/m² (or ≥23 kg/m² if Asian ancestry) plus any additional risk factor 1, 4
- First-degree relative with diabetes 1, 4
- High-risk race/ethnicity (African American, Latino, Native American, Asian American) 1, 4
- Physical signs: acanthosis nigricans, central obesity, skin tags 4, 5
- Associated conditions: hypertension (≥130/80 mmHg), dyslipidemia (HDL <35 mg/dL or triglycerides >250 mg/dL), polycystic ovary syndrome, history of gestational diabetes 1, 4
- Physical inactivity 1, 4
For all other individuals without risk factors, screening should begin at age 35 years. 1
Practical Testing Algorithm
Start with FPG and HbA1c as your initial screening panel—these are convenient, reproducible, and guideline-endorsed. 1
If you want maximum sensitivity or if initial screening is borderline, add fasting insulin to directly confirm insulin resistance. 2, 4, 5
If results remain equivocal or you need the most sensitive assessment, request an OGTT with 2-hour glucose measurement. 2, 3 The OGTT is more sensitive than fasting glucose alone but is cumbersome in practice. 1, 2
Interpreting Your Results
Prediabetes/insulin resistance is present if any of the following are met 1:
- FPG 100-125 mg/dL
- HbA1c 5.7-6.4%
- 2-hour OGTT glucose 140-199 mg/dL
- Fasting insulin >15 mU/L
Individuals with HbA1c >6.0% are at very high risk (20 times higher than those with A1C 5.0%) and require aggressive intervention. 1
Population-Specific Considerations
Asian Americans have increased diabetes risk at lower BMI thresholds—use ≥23 kg/m² rather than ≥25 kg/m² as the screening cutoff. 1, 4
African Americans may have equivalent diabetes risk at BMI 26 kg/m² compared to BMI 30 kg/m² in non-Hispanic whites. 4