When should fasting insulin be ordered in patients with risk factors for insulin resistance or diabetes, such as obesity (body mass index (BMI) greater than 30), family history of diabetes, history of gestational diabetes, or signs of metabolic syndrome?

When to Order Fasting Insulin

Fasting insulin is not routinely recommended as a screening or diagnostic test for diabetes or prediabetes by major diabetes guidelines, which instead prioritize fasting plasma glucose, HbA1c, or oral glucose tolerance testing. 1

Standard Screening Tests for Diabetes Risk

The American Diabetes Association does not include fasting insulin measurement in their standard screening recommendations for diabetes or prediabetes. Instead, the following tests are recommended 1:

• Fasting plasma glucose (FPG)
• HbA1c
• 2-hour plasma glucose during 75-g oral glucose tolerance test (OGTT)

These three tests are considered equally appropriate for screening and diagnosis of prediabetes and type 2 diabetes 1.

When Screening Should Occur (Using Standard Tests, Not Fasting Insulin)

For adults, testing should be considered in 1:

• Adults of any age with BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans) who have one or more additional risk factors:

  • First-degree relative with diabetes 1
  • High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander) 1
  • History of cardiovascular disease 1
  • Hypertension (≥140/90 mmHg or on therapy) 1
  • HDL cholesterol <35 mg/dL or triglycerides >250 mg/dL 1
  • Women with polycystic ovary syndrome 1
  • Physical inactivity 1
  • Other conditions associated with insulin resistance (severe obesity, acanthosis nigricans, metabolic dysfunction-associated steatotic liver disease) 1

• All adults beginning at age 35 years, regardless of risk factors 1

• Yearly testing for those with prediabetes (A1C ≥5.7%, impaired glucose tolerance, or impaired fasting glucose) 1

• Every 1-3 years for women with history of gestational diabetes 1

For children and adolescents, screening should be considered after puberty onset or age 10 years (whichever is earlier) in those with overweight (≥85th percentile) or obesity (≥95th percentile) plus one or more risk factors 1:

• Maternal history of diabetes or gestational diabetes during gestation 1
• Family history of type 2 diabetes in first- or second-degree relative 1
• Signs of insulin resistance (acanthosis nigricans, hypertension, dyslipidemia, PCOS) 1

Limited Role of Fasting Insulin in Clinical Practice

While fasting insulin can theoretically help assess insulin resistance, it is not included in standard diabetes screening guidelines because 1:

• Fasting plasma glucose, HbA1c, and OGTT are better validated for predicting diabetes risk and cardiovascular outcomes
• These standard tests have established diagnostic thresholds and are more cost-effective
• Insulin assays lack standardization across laboratories

Research Context for Insulin Resistance Assessment

In research settings, fasting insulin combined with fasting triglycerides has been shown to predict insulin sensitivity better than fasting insulin alone 2. However, this approach remains primarily a research tool rather than a guideline-recommended clinical screening method 2.

For women with history of gestational diabetes, insulin resistance is a key underlying factor for future metabolic syndrome and type 2 diabetes risk 3, 4. However, even in this high-risk population, guidelines recommend standard glucose-based testing (FPG, HbA1c, or OGTT) rather than fasting insulin measurement 1.

Common Pitfalls to Avoid

• Do not order fasting insulin as a first-line screening test when standard tests (FPG, HbA1c, OGTT) are guideline-recommended and more appropriate 1
• Do not delay screening until symptoms develop—most patients with prediabetes and early type 2 diabetes are asymptomatic 1
• Do not screen less frequently than every 3 years in normal-risk adults over age 35, or more frequently if BMI is increasing or risk factors are worsening 1