Insulin, Fasting, Serum
The Fasting Insulin Serum test quantifies insulin concentration following a 9–12 hour fast — the earliest and most sensitive routine laboratory marker of insulin resistance, detecting compensated beta cell hyperinsulinaemia before fasting glucose or HbA1c become abnormal. Combined with fasting glucose, it enables HOMA-IR calculation — the validated index of hepatic insulin resistance.
1
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
Yes — mandatory 9–12 hour fast. This is the single most critical collection requirement for fasting insulin. Only water is permitted during the fast. The patient must not consume any food, caloric beverages, coffee, tea (caffeinated), chewing gum, or non-water supplements. Document fasting duration precisely on the requisition form — if fasting cannot be confirmed, the specimen should not be collected and the appointment rescheduled. Oral medications with a sip of water are permissible.
Fasting
What is the Fasting Insulin test?
The Fasting Insulin test quantifies serum insulin concentration following a 9–12 hour fast, providing the earliest and most sensitive biochemical indicator of insulin resistance available through routine clinical laboratory testing. In the context of metabolic risk assessment, fasting insulin is uniquely positioned to detect compensated insulin resistance — the phase when the pancreatic beta cells are secreting excess insulin to overcome peripheral insulin receptor insensitivity and maintain normal fasting glucose — a stage when standard glycaemic markers (fasting glucose and HbA1c) remain within normal limits, appearing falsely reassuring. By the time fasting glucose rises above 100 mg/dL or HbA1c reaches 5.7%, insulin resistance has typically been present for years and significant cardiometabolic, hormonal, and inflammatory consequences may have already accumulated. Fasting insulin is most clinically informative when interpreted alongside concurrent fasting glucose for HOMA-IR calculation — which provides a standardised, validated index of hepatic insulin resistance that facilitates comparison across clinical encounters and with published population data. It is ordered as a first-tier metabolic risk assessment component in any patient with features of the insulin resistance syndrome — central adiposity, hypertriglyceridaemia, low HDL-C, elevated blood pressure, PCOS, acanthosis nigricans, skin tags, polycystic ovary morphology, or a strong family history of type 2 diabetes or premature cardiovascular disease — where a normal fasting glucose and HbA1c might otherwise provide false metabolic reassurance. It is also the primary monitoring marker for lifestyle, dietary, and pharmacological interventions targeting insulin resistance — responding to low-carbohydrate dietary patterns, time-restricted eating, exercise, metformin, and GLP-1 receptor agonist therapy before glucose-based markers change.
What does the Fasting Insulin test measure?
Insulin is a 51-amino acid peptide hormone synthesised as preproinsulin by pancreatic beta cells of the islets of Langerhans, processed through cleavage of the signal peptide and C-peptide to form the mature two-chain disulphide-linked insulin molecule. It is co-secreted with equimolar amounts of C-peptide — a clinically important distinction, as C-peptide persists longer in circulation (half-life approximately 35 minutes vs insulin 5–10 minutes) and is not removed by hepatic first-pass metabolism, making C-peptide a more reliable index of endogenous insulin secretion when hepatic extraction is a variable. Insulin secretion has two phases: a rapid first-phase release of pre-formed granules within 2–5 minutes of glucose stimulation, and a slower second-phase synthesis-dependent release over 60–90 minutes. Fasting insulin reflects basal insulin secretion — the minimal level required to suppress hepatic glucose output (gluconeogenesis and glycogenolysis) and prevent lipolysis between meals, in the absence of postprandial stimulation. In insulin resistance states, the pancreatic beta cells compensate for reduced insulin receptor sensitivity by secreting proportionally more insulin to maintain euglycaemia — fasting hyperinsulinaemia develops before fasting glucose rises, making fasting insulin the earliest biochemical marker of insulin resistance in individuals with preserved beta cell compensatory capacity.
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).
Why is the Fasting Insulin test ordered?
- Insulin resistance detection before diabetes — patients with risk factors for type 2 diabetes (central adiposity, family history, sedentary lifestyle, dyslipidaemia, hypertension) where fasting glucose and HbA1c are normal but insulin resistance may already be present and driving cardiometabolic risk accumulation
- PCOS diagnosis and management — fasting insulin is a mandatory component of the PCOS metabolic workup; insulin-driven androgen excess (insulin stimulates ovarian thecal androgen synthesis and suppresses SHBG) is the primary metabolic mechanism in the insulin-resistant PCOS phenotype, and elevated fasting insulin directly guides the decision to use metformin, inositol, or lifestyle modification alongside hormonal treatments
- Cardiovascular risk stratification beyond standard lipid panels — fasting insulin and HOMA-IR as independent cardiometabolic risk markers identifying the insulin-resistant phenotype — high triglycerides, low HDL-C, small dense LDL-C, elevated hs-CRP — that carries ASCVD risk underestimated by standard LDL-C and Framingham/PCE risk scoring
- Metabolic syndrome characterisation — fasting insulin alongside fasting glucose, triglycerides, HDL-C, blood pressure, and waist circumference for complete metabolic syndrome phenotyping per IDF and AHA/NHLBI criteria, providing the insulin-driven mechanism underlying the cardiometabolic risk cluster
- Non-alcoholic fatty liver disease (NAFLD) and NASH evaluation — insulin resistance is the central pathophysiological driver of hepatic steatosis progression; elevated fasting insulin in a patient with elevated liver enzymes contextualises the hepatic lipid accumulation mechanism and guides lifestyle and pharmacological treatment priority
- Monitoring response to insulin-sensitising interventions — serial fasting insulin before and after initiating low-carbohydrate dietary patterns, caloric restriction, exercise protocols, intermittent fasting, metformin, inositol supplementation, berberine, or GLP-1 receptor agonist therapy to document objective insulin resistance reduction
- Hypoglycaemia investigation — fasting insulin alongside fasting glucose and C-peptide in the evaluation of fasting hypoglycaemia (Whipple's triad — documented glucose below 55 mg/dL with symptoms, relieved by glucose administration); an inappropriately elevated fasting insulin (>3 µIU/mL at the time of documented hypoglycaemia) is diagnostic of endogenous hyperinsulinism — insulinoma or non-insulinoma pancreatogenous hypoglycaemia syndrome (NIPHS)
- Integrative and functional medicine comprehensive metabolic assessment — fasting insulin as the insulin resistance anchor of a comprehensive cardiometabolic panel alongside fasting glucose (for HOMA-IR), HbA1c, lipid panel, hs-CRP, and uric acid, providing the metabolic mechanistic picture needed to stratify cardiovascular and metabolic risk and personalise interventions
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
Yes — mandatory 9–12 hour fast. This is the single most critical collection requirement for fasting insulin. Only water is permitted during the fast. The patient must not consume any food, caloric beverages, coffee, tea (caffeinated), chewing gum, or non-water supplements. Document fasting duration precisely on the requisition form — if fasting cannot be confirmed, the specimen should not be collected and the appointment rescheduled. Oral medications with a sip of water are permissible.
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Standard venipuncture into a serum separator tube (SST / gold top). Prompt specimen processing is important — erythrocyte insulin-degrading enzyme (IDE) degrades insulin in haemolysed specimens and in whole blood left uncentrifuged at room temperature for extended periods. Centrifuge the SST within 30–60 minutes of collection and separate serum promptly. Inspect for haemolysis — visibly haemolysed specimens should be rejected and recollected. Document on the requisition form: (1) fasting duration confirmed in hours — mandatory; (2) all current antidiabetic medications — exogenous insulin (type, dose, timing of most recent injection), sulphonylureas, meglitinides, GLP-1 receptor agonists (fasting insulin cannot be used for insulin resistance assessment when these are present); (3) metformin, SGLT2 inhibitors, thiazolidinediones — these reduce insulin resistance and will appropriately lower fasting insulin; (4) concurrent fasting glucose ordered simultaneously — essential for HOMA-IR calculation; (5) acute illness or significant physiological stress within 4 weeks; (6) time of last meal and content if fast duration is uncertain. # REVIEWER FLAG: Confirm LabCorp SST tube type, minimum serum volume, maximum acceptable time from collection to centrifugation for insulin stability, cold-chain shipping requirements, and haemolysis rejection threshold for fasting insulin before publication.
Specimen requirements
Minimum 0.5 mL serum. Centrifuge and separate serum within 30–60 minutes of collection — insulin stability is more time-sensitive than most routine serum analytes due to IDE degradation risk in the whole blood phase. Stable at 2–8°C for up to 5 days after separation; stable frozen at −20°C for extended storage. # REVIEWER FLAG: Confirm minimum volume and stability data — particularly maximum acceptable whole blood time before centrifugation — with LabCorp assay documentation.
What can affect Fasting Insulin results?
Insulin is the primary postprandial hormone — it rises sharply within 2–5 minutes of carbohydrate and protein consumption, peaks within 30–90 minutes, and remains elevated for 2–4 hours after meals depending on the glycaemic and insulinaemic index of the food consumed. A non-fasting specimen collected within 4–6 hours of a carbohydrate-rich meal will produce a substantially elevated insulin that reflects the postprandial secretory response rather than the basal fasting secretory rate, producing a false-positive insulin resistance result. A strict 9–12 hour fast — ideally overnight — with only water permitted is mandatory for fasting insulin to be interpretable. Fasting duration must be precisely confirmed and documented at the time of collection.
Unlike many clinical chemistry analytes, serum insulin immunoassay results are not standardised across laboratory platforms — different immunoassay platforms produce systematically different insulin values from the same specimen due to differences in antibody specificity, calibration tracers, and cross-reactivity with proinsulin and insulin degradation products. LabCorp's specific assay platform calibration must be known for HOMA-IR thresholds and published cutpoints to be applied correctly. Functional medicine optimal thresholds (e.g., < 5 µIU/mL) derived from one platform study may not directly translate to another platform's numerical scale. Serial fasting insulin monitoring requires using the same platform throughout — results from different laboratories should not be directly compared without confirming assay platform equivalence.
Sulphonylureas (glibenclamide, glipizide, gliclazide) and meglitinides (repaglinide) stimulate pancreatic insulin secretion directly by closing ATP-sensitive potassium channels on beta cells, producing elevated fasting insulin that reflects pharmacological stimulation rather than endogenous insulin resistance. GLP-1 receptor agonists enhance glucose-dependent insulin secretion and may elevate fasting insulin. These medications must be documented on the requisition form; fasting insulin values cannot be used for insulin resistance assessment in patients on insulin secretagogues — C-peptide provides the endogenous secretion index when exogenous insulin or secretagogue confounding is present.
Patients using exogenous insulin (any formulation — rapid-acting, intermediate, or long-acting) will have serum insulin that reflects a combination of endogenous pancreatic secretion and injected insulin. Standard insulin immunoassays detect both endogenous and exogenous human insulin and most insulin analogues (though some modern analogues are detected to different degrees by different antibodies). Fasting insulin results in patients on exogenous insulin therapy cannot be used for insulin resistance assessment — C-peptide measurement is the appropriate endogenous beta cell secretion marker in this population, as C-peptide is not present in exogenous insulin preparations.
Erythrocytes contain insulin-degrading enzyme (IDE, also called insulysin), which rapidly degrades insulin in whole blood in vitro when cells are haemolysed. Haemolysed specimens produce artifactually low fasting insulin results through IDE-mediated insulin degradation — the opposite direction of interference from most other serum analytes. Prompt specimen centrifugation and serum separation within 30–60 minutes of collection is critical for insulin stability. Haemolysed specimens should be rejected. # REVIEWER FLAG: Confirm LabCorp specimen rejection criteria for haemolysis for fasting insulin, and whether they specifically flag the IDE degradation risk in their specimen handling guidance.
Acute systemic illness, infection, and physiological stress activate counter-regulatory hormones — cortisol, glucagon, epinephrine, and growth hormone — that antagonise insulin action and promote insulin resistance as an adaptive response to the acute stress state. Fasting insulin measured during or within 2–4 weeks of an acute illness reflects acute stress-induced insulin resistance rather than the patient's habitual metabolic baseline. Testing should be deferred until full clinical recovery from acute illness.
What do Fasting Insulin results mean?
Elevated
Fasting insulin above 10 µIU/mL in a well-fasted patient without an exogenous insulin or secretagogue explanation confirms clinically meaningful insulin resistance — the pancreatic beta cells are secreting excess insulin to compensate for peripheral insulin receptor insensitivity. Calculate HOMA-IR: (fasting glucose mg/dL × fasting insulin µIU/mL) ÷ 405 — HOMA-IR above 2.0 is consistent with moderate insulin resistance; above 3.5 indicates significant insulin resistance in most population datasets, though thresholds vary by study population and assay platform. In the presence of elevated fasting insulin, assess the full metabolic phenotype: triglycerides and HDL-C (atherogenic dyslipidaemia pattern), blood pressure, waist circumference, ALT (hepatic steatosis), uric acid (hyperuricaemia of insulin resistance), and sex hormones (SHBG suppression, androgen excess in females). In a woman with oligomenorrhoea, hirsutism, or ovarian cysts, elevated fasting insulin confirms the insulin-resistant PCOS phenotype requiring an insulin-sensitising intervention as a primary therapeutic priority.
Normal
Fasting insulin within the LabCorp reference range (2.6–24.9 µIU/mL) in a confirmed fasting specimen does not exclude insulin resistance in the functional medicine context — the upper reference limit of 24.9 µIU/mL encompasses values that in most population-based insulin resistance research would be classified as significant hyperinsulinaemia. In functional medicine practice, a fasting insulin above 7–10 µIU/mL — even within the laboratory reference range — warrants clinical attention as a cardiometabolic risk signal requiring dietary and lifestyle assessment. A HOMA-IR above 1.5–2.0 calculated from concurrent fasting glucose contextualises a 'normal' fasting insulin value that may still reflect meaningful insulin resistance relative to the patient's glucose.
Reduced
Fasting insulin below 2.6 µIU/mL in a patient not on exogenous insulin warrants consideration of reduced beta cell secretory capacity — most commonly from type 1 diabetes or latent autoimmune diabetes in adults (LADA), advanced type 2 diabetes with beta cell exhaustion, or pancreatogenic diabetes. In a patient with type 1 diabetes, very low fasting insulin alongside elevated fasting glucose and the clinical presentation confirms absent endogenous insulin production. Inappropriately low insulin relative to simultaneously low glucose (below 55 mg/dL) does NOT confirm insulinoma — insulinoma presents with inappropriately ELEVATED insulin during hypoglycaemia. At fasting glucose below 55 mg/dL, fasting insulin above 3 µIU/mL is inappropriately elevated and consistent with endogenous hyperinsulinism.
Note: The most clinically critical caveat is that fasting insulin reference ranges are not standardised across immunoassay platforms — the LabCorp range of 2.6–24.9 µIU/mL reflects this platform's calibration and should not be directly compared with published cutpoints from studies using different assay platforms without knowing the cross-platform conversion factor. Specifically, functional medicine thresholds of < 5 µIU/mL for optimal insulin sensitivity were developed from studies using particular immunoassay platforms that may not be directly numerically equivalent to the LabCorp platform result. The second critical caveat is the mandatory fasting requirement — a non-fasting specimen is the most common source of spuriously elevated fasting insulin results in clinical practice, potentially misclassifying insulin-sensitive patients as insulin-resistant. The third caveat is that fasting insulin is a marker of hepatic insulin resistance (HOMA-IR reflects hepatic glucose output suppression) and does not directly assess peripheral (skeletal muscle) insulin resistance — the euglycaemic-hyperinsulinaemic clamp remains the gold standard for peripheral insulin sensitivity, and the oral glucose tolerance test with insulin measurements provides more complete assessment of both hepatic and peripheral responses than fasting insulin alone.
Frequently asked questions
Fasting insulin measures the basal pancreatic insulin secretion rate in the post-absorptive state — reflecting how much insulin the pancreas must produce to maintain normal fasting glucose through hepatic glucose output suppression. When peripheral insulin resistance develops, the pancreas compensates by secreting proportionally more insulin to maintain euglycaemia — fasting insulin rises while fasting glucose remains normal. This compensated phase, where fasting hyperinsulinaemia precedes fasting hyperglycaemia by years, is the critical early intervention window that fasting glucose misses entirely and that fasting insulin identifies directly.
How to access the Fasting Insulin test
The Fasting 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: May 24, 2026
Last reviewed: May 24, 2026
- Matthews DR, et al. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. (1985)
- Stern SE, et al. Identification of individuals with insulin resistance using routine clinical measurements. Diabetes. (2005)
- Teede HJ, et al. Recommendations from the international evidence-based guideline for the assessment and management of polycystic ovary syndrome. Hum Reprod. (2018)
- Tabak AG, et al. Trajectories of glycaemia, insulin sensitivity, and insulin secretion before diagnosis of type 2 diabetes: an analysis from the Whitehall II study. Lancet. (2009)
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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