Who Should Have Fasting Insulin Testing?
Fasting insulin testing is not routinely recommended for clinical diagnosis or screening of diabetes, prediabetes, or cardiovascular disease risk assessment. Current evidence-based guidelines from the American Diabetes Association and other major organizations do not include fasting insulin as a standard diagnostic or screening test 1.
The Standard Approach: What Guidelines Actually Recommend
The established screening tests for diabetes and prediabetes are:
- Fasting plasma glucose (FPG)
- 2-hour plasma glucose during 75-g oral glucose tolerance test (OGTT)
- Hemoglobin A1C
These three tests are considered equally appropriate and are the only tests recommended by major guidelines 1, 2, 3.
Why Fasting Insulin Is Not Standard Practice
The Evidence Gap
While research demonstrates that insulin resistance is associated with cardiovascular disease risk 4, 5, 6, and fasting insulin levels correlate with insulin resistance 7, 8, no major clinical guidelines recommend routine fasting insulin testing for screening or diagnosis. The American Diabetes Association Standards of Care, which represent the most authoritative guidance in this field, make no mention of fasting insulin testing in their comprehensive screening and diagnostic algorithms 1.
Practical Limitations
- Fasting insulin has significant variability and lacks standardized reference ranges across laboratories 8
- More sophisticated calculations like QUICKI (which uses fasting insulin) may be more sensitive than fasting insulin alone, but even these are not incorporated into clinical guidelines 8
- The test identifies insulin resistance, but standard glucose-based tests already identify the populations at risk for diabetes and cardiovascular disease 1
Who Should Be Screened (Using Standard Tests, Not Fasting Insulin)
Begin screening at age 35 years for all adults using FPG, A1C, or OGTT 1, 2.
Screen earlier (any age) if the patient has:
- BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans) PLUS one or more of the following 1:
- First-degree relative with diabetes
- High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
- History of cardiovascular disease
- Hypertension (≥130/80 mmHg or on therapy)
- HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL
- Polycystic ovary syndrome
- Physical inactivity
- Other conditions associated with insulin resistance (severe obesity, acanthosis nigricans)
Repeat testing intervals:
The Research Context (Not Clinical Practice)
Fasting insulin testing may have value in research settings to assess insulin resistance 4, 7, 8, 6, but this has not translated into clinical guideline recommendations. Studies show that insulin resistance measured by various methods predicts cardiovascular events 4, 5, but A1C has already been demonstrated to be a stronger predictor of both diabetes and cardiovascular events than fasting glucose 1, making additional insulin testing redundant for clinical decision-making.
Common Pitfalls to Avoid
- Don't order fasting insulin as a "more sensitive" diabetes screening test - it is not validated or recommended for this purpose
- Don't use fasting insulin to diagnose insulin resistance in routine practice - the diagnosis of prediabetes (using standard tests) already identifies the insulin-resistant population that benefits from intervention 3, 5
- Don't confuse research utility with clinical utility - while fasting insulin correlates with insulin resistance in studies 7, 8, this hasn't been incorporated into evidence-based clinical algorithms
The Bottom Line for Clinical Practice
Use fasting plasma glucose, A1C, or OGTT for screening and diagnosis 1, 2, 3. These tests are standardized, validated, and directly linked to treatment algorithms and outcomes. Fasting insulin testing remains a research tool without established clinical utility in routine diabetes or cardiovascular risk assessment.