When is drawing an insulin level recommended?

When to Draw an Insulin Level

Drawing an insulin level is primarily indicated when evaluating suspected endogenous hyperinsulinism (insulinoma or other causes) in patients presenting with documented hypoglycemia, not for routine diabetes management or assessment of insulin resistance in clinical practice.

Primary Indication: Evaluation of Hypoglycemia

The main clinical scenario requiring insulin level measurement is during documented hypoglycemia to diagnose endogenous hyperinsulinism 1:

• Draw insulin level when blood glucose is ≤55 mg/dL (3.0 mmol/L) with concurrent symptoms to evaluate for inappropriate insulin secretion 1
• Simultaneously measure C-peptide and proinsulin levels along with insulin, as insulin levels alone can be misleading, particularly with modern insulin-specific assays 1
• Critical diagnostic finding: Detectable insulin (>3 mIU/L), elevated C-peptide (>0.6 ng/ml), and elevated proinsulin (>35 pmol/L) during documented hypoglycemia suggest endogenous hyperinsulinism 1

Important Caveat About Assay Selection

Modern insulin-specific assays without proinsulin cross-reactivity may show insulin levels below traditional diagnostic thresholds (even <3-6 mIU/L) in patients with true insulinomas, making C-peptide measurement mandatory for diagnosis 1. Older radioimmunoassays with 40% proinsulin cross-reactivity may give falsely elevated insulin readings 1.

NOT Recommended for Routine Clinical Use

Insulin levels should NOT be routinely drawn for:

Diabetes Management in Hospitalized Patients

• Hospital glucose management relies on point-of-care glucose monitoring before meals (or every 4-6 hours if NPO), not insulin levels 2
• Insulin therapy should be initiated when glucose persistently exceeds 180 mg/dL, with target range 140-180 mg/dL for most hospitalized patients 2
• Treatment decisions are based on glucose values and clinical response, not serum insulin measurements 2

Assessment of Insulin Resistance

While fasting insulin has been used in research settings to estimate insulin resistance, it is not recommended as a clinical diagnostic tool 3:

• Fasting insulin correlates moderately with insulin resistance (r = -0.58 to -0.74) only in subjects with normal glucose tolerance 3
• Correlation becomes substantially weaker in patients with impaired glucose tolerance or diabetes (r = -0.47 to -0.48) 3, 4
• The hyperinsulinemic-euglycemic clamp technique remains the gold standard for measuring insulin resistance in research, but has no role in routine clinical practice 4
• Clinical assessment using waistline, fasting glucose, and HbA1c is more practical than insulin levels for evaluating metabolic status 4

Hypoglycemia Risk Assessment in Diabetes

• Risk stratification for hypoglycemia in insulin-treated patients relies on clinical factors, not insulin levels: history of severe hypoglycemia, impaired hypoglycemia awareness, long disease duration, strict glycemic targets, and high glycemic variability 5
• Real-time continuous glucose monitoring is strongly recommended for high-risk patients with type 1 diabetes on multiple daily injections to prevent hypoglycemia 6
• Patient questionnaires and glucose monitoring data—not insulin levels—guide hypoglycemia prevention strategies 5

Practical Algorithm

Draw insulin level only when:

1. Patient has documented hypoglycemia (glucose ≤55 mg/dL) with symptoms
2. Endogenous hyperinsulinism is suspected (recurrent fasting hypoglycemia, no exogenous insulin administration)
3. Always obtain simultaneously: glucose, insulin, C-peptide, and proinsulin during the hypoglycemic episode 1

Do NOT draw insulin level for:

• Routine diabetes management or monitoring
• Assessing insulin resistance in clinical practice
• Determining insulin dosing in hospitalized patients
• Risk stratification for hypoglycemia in known diabetes