Fasting insulin and HOMA-IR: assessing insulin resistance

Insulin resistance can develop and progress for years — before fasting glucose or HbA1c ever move outside the normal range. That is why researchers looked for a way to quantify this “hidden” phase: fasting insulin measurement and the calculated HOMA-IR index emerged from that effort.

Understanding the limitations of both tools before you see your result matters. Insulin levels in the blood are unstable — the same sample measured in different laboratories with different reagents will produce different numbers. There is no measurement standard comparable to ISO-standardised HbA1c. HOMA-IR is an epidemiological index proposed in 1985 for research purposes, not a clinical diagnostic test.

This article explains what these tests actually measure, how to interpret results correctly, and when they are genuinely useful — and when ordering them is not warranted.

What insulin resistance is

In normal physiology, insulin acts as a key — unlocking muscle, fat, and liver cells so that glucose from the blood can enter. With insulin resistance, cells become less sensitive to that signal. The pancreas responds by producing more insulin to achieve the same effect. Blood glucose can remain within the normal range for years while the pancreas is still compensating for the elevated demand.

This is the hidden phase. Metabolic changes are already under way — elevated insulin promotes visceral fat deposition, contributes to vascular inflammation, and alters the lipid profile. But standard glucose screening is not yet signalling a problem.

When the compensatory mechanism is exhausted — when beta cells can no longer meet the increased demand — glucose begins to rise. That is the moment standard prediabetes tests detect.

This is why fasting insulin and HOMA-IR are viewed as an attempt to identify that phase before glucose moves out of range. An attempt — with substantial caveats.

Fasting insulin: reference range and limitations

The reference range for fasting insulin listed by most laboratories is approximately 2–25 µIU/mL. This figure, however, depends heavily on the assay method and reagents used by the specific laboratory.

Unlike HbA1c, insulin results are not interchangeable between laboratories. Two tests taken at different places a week apart can differ by 20–30% with no actual change in metabolism. Even within the same laboratory, repeat measurements in the same individual show considerable variability.

The practical implication: monitoring insulin trends is only meaningful when measurements are taken at the same laboratory, using the same method, under identical preparation conditions.

One additional nuance is the form of insulin being measured. Laboratories measure total immunoreactive insulin, which includes proinsulin and other precursors. The proportion of these fractions varies between individuals, which further limits comparability of results. For clinical purposes this is generally accepted, but it matters when interpreting HOMA-IR in a research context.

If your doctor has ordered serial insulin monitoring, always use the same laboratory.

HOMA-IR: formula and interpretation

HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) is calculated using the following formulas:

If glucose is in mmol/L:

HOMA-IR = (fasting insulin [µIU/mL] × fasting glucose [mmol/L]) / 22.5

If glucose is in mg/dL:

HOMA-IR = (fasting insulin [µIU/mL] × fasting glucose [mg/dL]) / 405

Orientational thresholds commonly cited in the literature:

These values are orientational, not absolute. Thresholds vary by population, insulin assay method, body mass index, and other factors. The American Diabetes Association (ADA) does not include HOMA-IR in the diagnostic criteria for prediabetes or diabetes. The index can be a useful contextual measure — but it does not replace standardised tests.

Relationship to HbA1c and fasting glucose

The standard for prediabetes and diabetes screening is HbA1c and fasting glucose. These are the markers the USPSTF (2021) recommends for screening adults aged 35–70 with overweight or obesity. The ADA Standards of Care 2024 also define prediabetes exclusively by glucose, HbA1c, and the oral glucose tolerance test — without insulin.

Fasting insulin and HOMA-IR add information about mechanism: whether glucose is rising because of declining insulin secretion or because of tissue resistance. This is useful for understanding pathophysiology, but not for making a diagnosis.

The practical value: if both glucose and HbA1c are normal but fasting insulin is consistently elevated, that is a reason to discuss lifestyle adjustments with your doctor before standard markers deteriorate.

It is equally important to understand that an elevated HOMA-IR with normal glucose is not a diagnosis and does not indicate a need for medication. It is a signal for monitoring and, where appropriate, lifestyle change: physical activity and dietary adjustment remain the most effective tools for improving tissue insulin sensitivity.

PCOS and insulin resistance

Polycystic ovary syndrome (PCOS) is associated with insulin resistance in the majority of affected women, even those with normal body weight. In this context, fasting insulin measurement and HOMA-IR calculation are part of a comprehensive work-up — alongside hormonal assessment (LH, FSH, testosterone, AMH) and pelvic ultrasound.

In PCOS, an endocrinologist or reproductive endocrinologist may order insulin deliberately to assess the degree of resistance and guide the treatment approach. This is one of the well-supported clinical scenarios for the test.

The relationship between PCOS and insulin resistance is bidirectional: elevated insulin stimulates excess androgen production by the ovaries, and hyperandrogenaemia in turn deepens resistance. This feedback loop is important when deciding on a management strategy. Insulin testing in PCOS is therefore not just “another marker” — it is part of evaluating the core pathogenic mechanism.

When NOT to order this test

The USPSTF classifies screening for insulin resistance using insulin as Grade I — meaning evidence is insufficient to recommend for or against it in the general population. The ADA Standards of Care 2024 do not include insulin testing in routine prediabetes screening.

Routine fasting insulin testing is not indicated for a healthy woman over 40 without specific clinical indications. It may be appropriate in these situations:

• work-up for suspected PCOS (ordered by an endocrinologist or reproductive endocrinologist)
• evaluation of metabolic syndrome — alongside waist circumference, triglycerides, HDL, and blood pressure
• research or clinical study contexts
• presence of acanthosis nigricans or other clinical signs of hyperinsulinaemia

The standard for prediabetes screening in women over 40 is HbA1c and fasting glucose. These are standardised, reproducible, and have well-established clinical thresholds.

Why does this matter? Because fasting insulin is a paid-for test and not always available in basic laboratory panels. Ordering it “just to check” or “out of curiosity” is not an optimal use of resources. If, however, you have a specific clinical picture — abdominal adiposity, an abnormal lipid profile, elevated blood pressure, signs of PCOS, or a family history of type 2 diabetes — it is worth discussing with your doctor whether an extended metabolic work-up, which may include insulin, is warranted.

How HealthLab helps track trends

Insulin resistance is a process over time. A single insulin or HOMA-IR measurement gives only a snapshot, while a trend across several data points is far more informative. The same applies to glucose and HbA1c: what matters most is not the isolated number but the direction it is moving from one test to the next.

HealthLab automatically recognises insulin, HOMA-IR, glucose, and HbA1c from PDF lab reports issued by any laboratory and displays their trend on a single timeline chart. You see the direction without manual data entry or scattered paper printouts. All results are stored in one place — convenient to show your doctor at an appointment or to track how your body responds to dietary changes.

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