Fasting Insulin Interpreter

Metformin reduces insulin resistance and will lower your fasting insulin level. If you are on metformin, your unmedicated fasting insulin would likely be higher than shown.

Insulin therapy invalidates fasting-insulin interpretation. The test measures endogenous insulin output; exogenous insulin distorts the reading and the downstream HOMA-IR calculation.

Corticosteroids (prednisone, dexamethasone) and some antipsychotics (olanzapine, clozapine) are associated with increased insulin resistance and may elevate fasting insulin independent of underlying baseline.

If you are taking any of these medications, your results reflect the medicated state. Discuss with your healthcare provider for proper context.

Formula

Fasting insulin is measured directly from a fasting blood draw, reported in micro-International Units per milliliter (μIU/mL). No formula — the value is read off the lab report.

Classification

Optimal: below 5 μIU/mL. Acceptable: 5-9.99. Borderline (early IR signal): 10-14.99. Elevated (IR likely present): 15 and above. These targets are stricter than the standard lab reference range (2-25 μIU/mL) because 'normal' on a lab report is not 'optimal' for metabolic health — fasting insulin in the 5-15 range typically reflects rising insulin output to compensate for early insulin resistance.

Known Limitations

Requires a true fasting state (8-12 hours, water only). Day-to-day variability of 20-30% is normal — a single measurement is directional, not definitive. Invalid during insulin therapy. Acute stress or illness on the day of the draw can elevate readings. For ethnicity-specific considerations and a fuller picture, pair with HbA1c, HOMA-IR, and TG:HDL ratio from a lipid panel.

Sources

Reaven GM. Banting Lecture 1988. Role of insulin resistance in human disease. Diabetes, 1988. Kraft JR. Diabetes Epidemic & You. 2008. Various IR-screening protocols (Ascaso, Gayoso-Diz, Kraft).