Insulin (Fasting) Test
The fasting insulin test measures circulating insulin to assess beta-cell secretion, insulin resistance, and hyperinsulinaemia. Paired with fasting glucose it enables indices such as HOMA-IR, and it supports the workup of hypoglycaemia alongside C-peptide.
1
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
Yes — 8–12 hours; fasting is required for insulin-resistance assessment and HOMA-IR, with a concurrent fasting glucose recommended
Fasting
What is the Insulin test?
The Insulin Test measures circulating insulin, usually in the fasting state, to assess beta-cell secretion and the degree of insulin resistance or hyperinsulinaemia. It provides a window into metabolic dysfunction that can precede abnormalities in glucose, making it useful in early metabolic assessment and in the investigation of hypoglycaemia. Within metabolic assessment, fasting insulin complements glucose and HbA1c rather than replacing them. Paired with fasting glucose it enables insulin-resistance indices such as HOMA-IR, and it helps characterise polycystic ovary syndrome, metabolic syndrome, and the early insulin-resistant phase of type 2 diabetes. In hypoglycaemia, insulin is interpreted alongside C-peptide and glucose to distinguish endogenous hyperinsulinism from exogenous insulin administration. Because the assay reflects only the peripheral fraction after hepatic extraction and is sensitive to exogenous insulin and antibodies, C-peptide is preferred where endogenous secretion must be assessed in insulin-treated patients.
What does the Insulin test measure?
Insulin is an anabolic peptide hormone secreted by pancreatic beta cells in response to rising glucose and other secretagogues. It is released into the portal circulation, where the liver extracts a substantial first-pass fraction, and it has a short plasma half-life of only a few minutes. Insulin promotes cellular glucose uptake, glycogen and lipid synthesis, and suppression of hepatic glucose output. It is co-secreted with C-peptide in equimolar amounts, but unlike C-peptide, exogenous insulin and significant hepatic clearance complicate the interpretation of measured insulin.
Fasting insulin is used to assess insulin resistance and hyperinsulinaemia, to evaluate hypoglycaemia, and to characterise metabolic dysfunction earlier than glucose alone, since compensatory hyperinsulinaemia can precede overt glucose abnormalities. Combined with fasting glucose it allows calculation of indices such as HOMA-IR. Its interpretation is limited by assay variability, the influence of exogenous insulin and insulin antibodies, and the fact that it reflects only the peripheral, post-hepatic fraction.
Why is the Insulin test ordered?
- Assessment of insulin resistance in metabolic syndrome, obesity, or non-alcoholic fatty liver disease, often paired with fasting glucose for HOMA-IR
- Evaluation of polycystic ovary syndrome where hyperinsulinaemia and insulin resistance are clinically relevant
- Investigation of fasting or postprandial hypoglycaemia, interpreted with C-peptide and glucose to identify endogenous hyperinsulinism
- Characterisation of early metabolic dysfunction where compensatory hyperinsulinaemia may precede glucose abnormalities
- Workup of suspected insulinoma, where inappropriately elevated insulin during hypoglycaemia is a key finding
- Distinguishing endogenous hyperinsulinism from exogenous (factitious or therapeutic) insulin, alongside C-peptide
- Monitoring of metabolic response to lifestyle or pharmacological intervention targeting insulin sensitivity
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
Yes — 8–12 hours; fasting is required for insulin-resistance assessment and HOMA-IR, with a concurrent fasting glucose recommended
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Draw fasting in the morning with a paired fasting glucose where insulin resistance is being assessed. Process promptly and avoid haemolysis, as red-cell insulin-degrading enzyme lowers the result. Document insulin therapy, secretagogues, and relevant medications.
Specimen requirements
Minimum 0.5 mL serum, non-haemolysed. Separate serum from cells promptly per Quest protocol. Specimen stable refrigerated/frozen per laboratory guidance; follow Quest transport guidelines.
What can affect Insulin results?
Insulin rises sharply after meals, so a non-fasting sample does not reflect basal secretion. A fasting specimen is required for insulin-resistance assessment and HOMA-IR.
Administered insulin is measured by many assays and cannot be distinguished from endogenous insulin, while some analogues are under- or over-recovered. C-peptide is preferred to assess endogenous secretion in insulin-treated patients.
Endogenous insulin antibodies can interfere with immunoassays, producing spuriously high or low results depending on assay design, and may cause autoimmune hypoglycaemia.
The liver clears a large first-pass fraction of insulin and the kidney contributes to its degradation, so hepatic or renal impairment alters circulating insulin independent of secretion.
Sulfonylureas and other secretagogues raise insulin, while metformin and lifestyle change lower it; glucocorticoids increase resistance and compensatory insulin. Medication history should be documented.
Red cells contain insulin-degrading enzyme, so haemolysis and delayed separation can degrade insulin and lower the measured result. Prompt processing is required.
What do Insulin results mean?
Elevated
Fasting hyperinsulinaemia indicates insulin resistance, as in metabolic syndrome, obesity, or PCOS, where the pancreas compensates for reduced sensitivity. Inappropriately elevated insulin during documented hypoglycaemia, with raised C-peptide, suggests endogenous hyperinsulinism such as insulinoma; raised insulin with suppressed C-peptide points to exogenous insulin.
Normal
A normal fasting insulin is interpreted only in conjunction with glucose. Normal insulin with normal glucose suggests preserved metabolic function, whereas normal insulin with elevated glucose may indicate inadequate beta-cell compensation.
Reduced
Low fasting insulin may reflect good insulin sensitivity or, in the context of hyperglycaemia, impaired beta-cell secretion as seen in advanced type 2 diabetes or type 1 diabetes. Interpretation requires the concurrent glucose.
Note: Insulin assays are poorly standardised and not interchangeable, so results should be read against the performing laboratory's method. Insulin reflects only the post-hepatic peripheral fraction, is sensitive to exogenous insulin and antibodies, and is degraded by haemolysis. Where endogenous secretion must be assessed — especially in insulin-treated patients or hypoglycaemia workup — C-peptide is the preferred complementary measure, always interpreted with concurrent glucose.
Frequently asked questions
It measures circulating insulin in the fasting state to assess beta-cell secretion and the degree of insulin resistance or hyperinsulinaemia. Paired with fasting glucose it enables insulin-resistance indices such as HOMA-IR.
How to access the Insulin test
The Insulin reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: June 2, 2026
Last reviewed: June 2, 2026
- Matthews DR, et al. Homeostasis model assessment: insulin resistance and beta-cell function from fasting glucose and insulin. Diabetologia. (1985)
- Cryer PE, et al. Evaluation and Management of Adult Hypoglycemic Disorders: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. (2009)
- Manley SE, et al. Comparison of insulin assays and the implications for clinical practice. Ann Clin Biochem. (2007)
- Tohidi M, et al. Fasting insulin and risk of incident type 2 diabetes: a prospective cohort study. BMC Endocr Disord. (2018)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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