Insulin Level Lab: Preparation and Interpretation
Insulin level testing is not a standard diagnostic test for diabetes and has limited clinical utility in routine practice, as diabetes diagnosis relies on glucose-based measurements (HbA1c, fasting glucose, or OGTT), not insulin levels. 1
When Insulin Testing May Be Considered
While not part of standard diabetes diagnosis, insulin levels may be measured in specific clinical scenarios:
- Fasting insulin can serve as a marker of insulin resistance in research settings, though its clinical utility is limited 2
- Insulin measurement may help evaluate suspected insulinoma or factitious hypoglycemia (though C-peptide is typically preferred) 3
- In subjects with normal glucose tolerance, fasting insulin correlates moderately with insulin resistance (correlation -0.58 to -0.74), but this relationship weakens substantially in those with impaired glucose tolerance or diabetes 2
Patient Preparation
- Fasting requirement: The patient must fast for at least 8 hours before blood collection 1
- No caloric intake is permitted during the fasting period 1
- The test should be performed in the morning after an overnight fast for standardization 4
Interpretation of Results
Reference Values (Using Enzyme Immunoassay)
- Men aged 30-59 years: Median 5.8 microU/mL, 95th percentile 14.0 microU/mL 4
- Women aged 30-49 years: Median 5.1 microU/mL, 95th percentile 11.2 microU/mL 4
- Men ≥60 years and women ≥50 years: Median 5.6 microU/mL, 95th percentile 12.6 microU/mL 4
Clinical Context
- Insulin levels increase 50% when BMI is 25-30 and double when BMI exceeds 30 5
- In persons with impaired glucose tolerance or newly diagnosed type 2 diabetes, fasting insulin typically increases 2-3 fold depending on BMI and gender 5
- Critical limitation: Only fasting insulin should be used as a marker of insulin resistance in population studies, particularly in subjects with abnormal glucose tolerance, as postload insulin correlations with insulin resistance are substantially weaker 2
Important Caveats
- Insulin levels are NOT used for diabetes diagnosis—diagnosis requires HbA1c ≥6.5%, fasting glucose ≥126 mg/dL, 2-hour OGTT glucose ≥200 mg/dL, or random glucose ≥200 mg/dL with symptoms 1
- Hyperinsulinemia reflects insulin resistance but does not establish a diagnosis of diabetes 3, 5
- The assay method matters: enzyme immunoassays that don't cross-react with proinsulin provide more specific results than conventional methods 4
- In subjects with established diabetes, the correlation between insulin levels and insulin resistance becomes unreliable 2