What Does an Insulin Blood Test Mean?
An insulin blood test measures the amount of insulin hormone circulating in your blood, but routine measurement of insulin levels is NOT recommended for most patients with diabetes or at risk for diabetes, as it has minimal clinical utility for diagnosis or management. 1
Clinical Utility and Recommendations
When Insulin Testing is NOT Recommended
In most people with diabetes or at risk for diabetes or cardiovascular disease, routine testing for insulin or proinsulin is not recommended. These assays are useful primarily for research purposes. 1
- Insulin measurement has no clinical value in the diagnosis or management of diabetic patients, with rare exceptions 2
- The test does not help adapt treatment for diabetic patients, except in determining the need for insulin therapy 3
- Diabetes is diagnosed solely on the basis of chronic hyperglycemia, not insulin levels 2
When Insulin Testing IS Clinically Useful
C-peptide measurements (a marker of insulin production) may help distinguish type 1 from type 2 diabetes in ambiguous cases, such as individuals who have a type 2 phenotype but present in ketoacidosis. 1
Specific clinical scenarios where insulin/C-peptide testing has value include:
- Investigating hypoglycemia: Diagnosis of islet cell tumors requires demonstrating inappropriately increased plasma insulin concentrations despite low glucose, with an increased proinsulin-to-insulin ratio strongly suggesting an insulinoma 1
- Ruling out surreptitious insulin administration: C-peptide measurement is essential in investigating nondiabetic hypoglycemia to exclude factitious hypoglycemia from self-administered insulin 1
- Insulin pump therapy coverage: If required by payers, measure fasting C-peptide when simultaneous fasting plasma glucose is ≤220 mg/dL (12.5 mmol/L) 1
Understanding Insulin Resistance Detection
While direct insulin measurement has limited utility, fasting insulin levels >15 mU/L directly confirm insulin resistance, with borderline high values at 15-20 mU/L and clearly elevated values >20 mU/L. 4 Normal fasting insulin is <15 mU/L. 4
Algorithmic Approach for Insulin Resistance Assessment
The oral glucose tolerance test (OGTT) is more sensitive and modestly more specific than fasting plasma glucose for detecting early insulin resistance, though it is poorly reproducible and difficult to perform in practice. 4
For early detection, tests rank from most to least sensitive: 4
- OGTT with 2-hour glucose
- Combined fasting insulin + glucose (QUICKI or HOMA indices)
- Combined fasting insulin + triglycerides
- Fasting plasma glucose
- HbA1c
- Fasting insulin alone
Critical Testing Considerations
Always test in the fasting state (minimum 8 hours without caloric intake) to avoid postprandial variations. 4 This is a common pitfall that can invalidate results.
Population-Specific Thresholds
- Asian Americans: Use lower BMI threshold (≥23 kg/m²) rather than the standard ≥25 kg/m² for identifying insulin resistance risk 4
- African Americans: May have equivalent diabetes risk at BMI 26 kg/m² compared to BMI 30 kg/m² in non-Hispanic whites 4
What to Use Instead for Diabetes Monitoring
HbA1c should be measured routinely every 3 months until acceptable targets are reached, then at least every 6 months to assess long-term glycemic control. 5 This is the gold standard for monitoring, not insulin levels.
Fasting plasma glucose (FPG) should be measured in venous plasma, with samples collected after at least 8 hours of fasting. 5 For diagnosis, values ≥126 mg/dL (7.0 mmol/L) on two occasions confirm diabetes. 6