Would measuring a fasting insulin level be helpful in a patient with borderline high hemoglobin A1C (HbA1C), difficulty building muscle, and ease of weight loss, to assess insulin sensitivity and secretion?

Measuring Fasting Insulin: Limited Clinical Utility in Your Patient

Measuring fasting insulin in this patient would provide minimal clinical benefit, as current guidelines explicitly state that routine insulin testing is not recommended for most individuals with diabetes or at risk for diabetes—these assays are primarily useful for research purposes. 1, 2

Why Fasting Insulin Testing Is Not Recommended

The 2023 American Diabetes Association laboratory guidelines are clear: in most people with diabetes or at risk for diabetes or cardiovascular disease, routine testing for insulin or proinsulin is not recommended. 1 This recommendation applies directly to your patient with borderline high HbA1c.

Key Limitations of Fasting Insulin Measurement

• No impact on clinical management: There is no evidence that measuring plasma insulin helps adapt treatment for diabetic patients, except for determining the need for insulin therapy. 3

• Research tool, not clinical tool: Fasting insulin measurements serve primarily research purposes and lack established clinical decision thresholds for routine practice. 1, 2

• Poor reproducibility: Even among insulin sensitivity indices, fasting insulin-based measurements show significant variability between repeated measurements, limiting their clinical reliability. 4

What You Should Do Instead

Assess Insulin Resistance Through Standard Clinical Measures

Your patient's borderline high HbA1c already provides the critical diagnostic information you need. 1 The appropriate next steps are:

• Fasting plasma glucose: Values between 100-125 mg/dL indicate impaired fasting glucose and insulin resistance. 5

• Oral glucose tolerance test (OGTT): 2-hour glucose values of 140-199 mg/dL indicate impaired glucose tolerance, confirming insulin resistance. 5

• HbA1c interpretation: Values between 5.7-6.4% suggest prediabetes and potential insulin resistance. 5

Clinical Evaluation for Insulin Resistance

Look for physical signs that directly indicate insulin resistance without laboratory testing: 5

• Acanthosis nigricans (darkened, velvety skin in body folds)
• Central obesity (measure waist circumference)
• Hypertension
• Dyslipidemia (check lipid profile—elevated triglycerides are particularly useful)
• Polycystic ovary syndrome (if female)

The Most Practical Approach

If you want to assess insulin resistance beyond standard glucose testing, measure fasting triglycerides alongside fasting glucose. 6 A weighted combination of fasting insulin and triglycerides provides better screening for insulin resistance than fasting insulin alone, but since routine insulin testing isn't recommended, elevated fasting triglycerides combined with impaired fasting glucose or HbA1c 5.7-6.4% effectively identifies insulin resistance. 5, 6

When Fasting Insulin IS Indicated

The rare clinical scenarios where fasting insulin measurement has value: 1, 2

• Suspected insulinoma: Investigating non-diabetic hypoglycemia with inappropriately elevated insulin during documented low glucose. 2

• Distinguishing diabetes types: C-peptide (not insulin) is preferred when fasting plasma glucose is ≤220 mg/dL to differentiate type 1 from type 2 diabetes in ambiguous presentations. 1

• Surreptitious insulin administration: Ruling out factitious hypoglycemia. 2

Common Pitfalls to Avoid

• Don't use insulin testing to "confirm" insulin resistance when clinical criteria and standard glucose testing already provide this information. 1, 2

• Don't interpret fasting insulin in isolation: Even in research settings, insulin levels correlate poorly with insulin resistance in patients with impaired glucose tolerance or diabetes (correlation only -0.47 to -0.48). 7

• Don't order insulin testing during acute illness: Stress hyperglycemia temporarily mimics insulin resistance and invalidates interpretation. 5

Bottom Line for Your Patient

Your patient's difficulty building muscle and ease of weight loss, combined with borderline high HbA1c, warrant metabolic evaluation—but fasting insulin measurement won't change your management. 1, 2 Focus on standard diagnostic tests (fasting glucose, OGTT, or repeat HbA1c), assess for clinical signs of insulin resistance, check lipid profile and blood pressure, and implement lifestyle interventions or pharmacotherapy based on established glucose-based diagnostic criteria. 5