What does the Insulin, Fasting, Serum test measure?

Why is Insulin, Fasting, Serum measured?

Fasting insulin is the most sensitive and clinically actionable single marker for insulin resistance detectable through routine clinical laboratory testing — identifying compensated insulin resistance before fasting glucose, HbA1c, or oral glucose tolerance test results become overtly abnormal. In the metabolic risk assessment context, fasting insulin above 5–7 µIU/mL in a fasted, lean individual is increasingly recognised as evidence of early insulin resistance; values above 10–15 µIU/mL indicate moderate-to-significant insulin resistance; and values above 20–25 µIU/mL indicate severe insulin resistance even when fasting glucose is normal. The HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) — calculated as (fasting glucose mg/dL × fasting insulin µIU/mL) ÷ 405, or (fasting glucose mmol/L × fasting insulin µIU/mL) ÷ 22.5 — translates fasting insulin and glucose into a dimensionless index of hepatic insulin resistance that is the most widely used research and clinical insulin resistance metric, with a HOMA-IR above 1.5–2.0 indicating meaningful insulin resistance in most population datasets. Beyond diabetes prevention, elevated fasting insulin is the mechanistic driver of PCOS-associated androgen excess (insulin stimulates ovarian theca cell androgen production and suppresses SHBG), cardiovascular risk intensification through endothelial dysfunction and hypertension, visceral adiposity and metabolic syndrome, and the dyslipidaemia of insulin resistance (elevated triglycerides, low HDL-C, small dense LDL particles).

Reference ranges

Standard reference

2.6–24.9 µIU/mL (18.1–173.0 pmol/L) # REVIEWER FLAG: Confirm LabCorp reference range for fasting serum insulin — values vary substantially between immunoassay platforms due to non-standardised insulin assay calibration. The range of 2.6–24.9 µIU/mL is one commonly reported LabCorp range; confirm the current LabCorp platform and whether age-stratified or BMI-adjusted reference ranges are applied. The upper limit of 24.9 µIU/mL is widely recognised in functional medicine as being far too permissive — patients with fasting insulin in the 15–25 µIU/mL range frequently have significant insulin resistance. Confirm the specific LabCorp assay platform (e.g., Roche Cobas ECLIA) and its calibration traceability to the WHO International Reference Preparation 66/304.

Functional medicine optimal

< 5 µIU/mL (functional medicine optimal for minimal insulin resistance and lowest cardiometabolic risk; HOMA-IR < 1.0 calculated from concurrent fasting glucose). Many functional medicine practitioners use < 7 µIU/mL as the practical functional target and > 10 µIU/mL as the action threshold regardless of fasting glucose status. # REVIEWER FLAG: The < 5 µIU/mL functional optimal is not a mainstream clinical guideline intervention threshold — mainstream standards use the laboratory reference range as the normal benchmark. Reviewer to confirm appropriate framing that clearly distinguishes the functional optimal from the analytical reference range and communicates the evidence basis for these insulin resistance thresholds.

Units

µIU/mL, pmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.