Interpreting HbA1c, Fasting Glucose, Fasting Insulin and HOMA IR Together
Level 3, Clinical application
In brief
HbA1c, fasting glucose, fasting insulin and HOMA IR each capture a different aspect of glucose regulation, and they are most informative read together. High insulin with normal glucose suggests compensated insulin resistance; high glucose with low or normal insulin suggests failing insulin secretion or other forms of diabetes; discordant HbA1c and glucose suggest red cell or measurement issues. Insulin assays are poorly standardised, so trends in the same laboratory matter more than universal cut offs.
Glucose regulation depends on a balance between insulin secretion and insulin sensitivity. No single test captures both. HbA1c reflects average glucose over months; fasting glucose reflects overnight regulation, largely by the liver; fasting insulin reflects how much insulin is needed to maintain that glucose; and HOMA IR combines glucose and insulin into an estimate of insulin resistance. Read together, these markers tell a more complete story than any one alone. This article explains how.
What each marker reflects
| Marker | Reflects | Strengths | Limitations |
|---|---|---|---|
| HbA1c | Average glucose over about 2 to 3 months | No fasting needed; standardised; used for diagnosis | Distorted by anaemia, haemoglobin variants, kidney disease, pregnancy and recent blood loss or transfusion |
| Fasting glucose | Overnight glucose regulation, mainly hepatic | Widely available; diagnostic thresholds established | Day to day variation; affected by stress, illness, poor sleep and the dawn phenomenon |
| Fasting insulin | The insulin needed to maintain fasting glucose | Can reveal compensated insulin resistance before glucose rises | Assays poorly standardised; no universal reference range; affected by timing and recent food |
| HOMA IR | Estimate of insulin resistance from fasting glucose and insulin | Simple; useful for trends and research | Inherits insulin assay variability; not valid in people taking insulin; no universal clinical cut off |
| C peptide | Endogenous insulin secretion | Distinguishes insulin deficiency from resistance; valid in people taking insulin | Affected by kidney function |
HOMA IR is commonly calculated as fasting glucose in mmol/L multiplied by fasting insulin in microunits per millilitre, divided by 22.5. Because insulin results differ between laboratories, HOMA IR values should be compared within the same laboratory rather than against a universal threshold.
Reading the patterns
| Pattern | Likely interpretation | What to consider |
|---|---|---|
| Normal glucose and HbA1c, high fasting insulin, high HOMA IR | Compensated insulin resistance | Look for clinical features: central adiposity, raised triglycerides, low HDL, fatty liver, PCOS. Intervene early. |
| Raised fasting glucose, normal HbA1c | Isolated impaired fasting glucose, or a transient rise | Repeat; consider sleep, stress, illness and the dawn phenomenon; an oral glucose tolerance test may clarify |
| Normal fasting glucose, raised HbA1c | Post meal glucose may be raised; or HbA1c may be falsely high | Check for iron deficiency and haemoglobin variants; consider glucose tolerance testing or continuous glucose monitoring |
| Raised glucose, low or inappropriately normal insulin | Failing insulin secretion | Consider advanced type 2 diabetes, autoimmune diabetes or pancreatic diabetes; C peptide and islet antibodies |
| Raised glucose and raised insulin | Insulin resistance with partially preserved secretion | Typical early type 2 diabetes |
| Low fasting glucose with raised insulin | Unusual; may indicate inappropriate insulin secretion or insulin use | Needs medical assessment |
| HbA1c much lower than expected from glucose readings | Shortened red cell survival | Haemolysis, recent blood loss, treatment of anaemia, some haemoglobin variants |
The dawn phenomenon
Early morning rises in cortisol, growth hormone and other hormones increase hepatic glucose output. In people with insulin resistance, this can produce a higher fasting glucose than expected from their overall control. It is one reason fasting glucose alone can be misleading, and why patterns across several markers are more reliable.
Beyond fasting measures
Oral glucose tolerance test
A glucose tolerance test shows how the body handles a glucose load and can identify impaired glucose tolerance that fasting tests miss. It is recommended in some situations, such as for women with polycystic ovary syndrome in international guidance, and after gestational diabetes. Measuring insulin during the test can show the pattern of insulin response, but interpretation of insulin curves is not standardised for clinical use.
Fructosamine
Fructosamine reflects average glucose over 2 to 3 weeks and can be useful when HbA1c is unreliable.
Continuous glucose monitoring
Continuous monitoring shows glucose patterns across days and nights, including post meal responses. It is established in diabetes management. In people without diabetes, it can be educational, but normal ranges and the clinical significance of fluctuations are still being defined.
Practical guidance
- Interpret in context: body composition, triglycerides, HDL, liver tests, blood pressure, family history and PCOS all inform the picture.
- Standardise conditions: fasting insulin and glucose should be measured after an overnight fast, in the morning, when the patient is well.
- Use the same laboratory for serial insulin measurements.
- Repeat abnormal results before drawing conclusions, as diagnosis of diabetes requires confirmation.
- Check HbA1c reliability in people with anaemia, kidney disease, haemoglobin variants or pregnancy.
- Look for other diabetes types when glucose is high but insulin is not, or when the presentation is atypical.
- Track trajectories: changes over time are often more informative than single values.
3 worked cases
A lean woman with high insulin
A 29 year old woman with irregular cycles and acne has a BMI of 22, a fasting glucose of 4.9 mmol/L, an HbA1c of 36 mmol/mol and a fasting insulin well above her laboratory’s typical range, giving a raised HOMA IR. Triglycerides are mildly raised. The pattern suggests compensated insulin resistance, consistent with polycystic ovary syndrome. Her glucose markers alone would have been reassuring.
A man with discordant results
A 56 year old man has an HbA1c of 49 mmol/mol, suggesting diabetes, but repeated fasting glucose results are normal. His blood count shows iron deficiency anaemia. Iron deficiency can falsely raise HbA1c. After investigation of the cause and iron replacement, his HbA1c is rechecked and a glucose tolerance test is performed, which is normal.
A woman with high glucose and low insulin
A 41 year old woman with a BMI of 23 has a fasting glucose of 9.2 mmol/L and a low fasting insulin. She has lost weight recently. C peptide is low and islet antibodies are positive, indicating autoimmune diabetes. The pattern of high glucose without high insulin was the clue that this was not typical type 2 diabetes.
4 markers, one picture
Glucose markers and insulin markers answer different questions. Read together, and in context, they show not only whether glucose is raised but why: whether insulin resistance is being compensated, whether insulin secretion is failing, or whether a test result itself is misleading.
Questions about glucose and insulin
What does high insulin with normal glucose mean?
How is HOMA IR calculated?
Why might HbA1c and glucose disagree?
References
- Matthews DR, Hosker JP, Rudenski AS, et al. Homeostasis model assessment: insulin resistance and beta cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. 1985;28(7).
- Wallace TM, Levy JC, Matthews DR. Use and abuse of HOMA modeling. Diabetes Care. 2004;27(6).
- Miller WG, Thienpont LM, Van Uytfanghe K, et al. Toward standardization of insulin immunoassays. Clinical Chemistry. 2009;55(5).
- English E, Idris I, Smith G, et al. The effect of anaemia and abnormalities of erythrocyte indices on HbA1c analysis: a systematic review. Diabetologia. 2015;58(7).
- Teede HJ, Tay CT, Laven JJE, et al. Recommendations from the 2023 international evidence based guideline for the assessment and management of polycystic ovary syndrome. Journal of Clinical Endocrinology and Metabolism. 2023;108(10).
- Jones AG, Hattersley AT. The clinical utility of C peptide measurement in the care of patients with diabetes. Diabetic Medicine. 2013;30(7).
Continue your learning
Reading glucose and insulin markers together
Assessing insulin resistance accurately, beyond a single fasting value, is the central skill of the Blood Sugar Dysfunction module in IIFM’s 6 month Functional Medicine and Clinical Nutrition program.
This article is for the education of healthcare professionals. It is not a substitute for individual medical advice, diagnosis or treatment, and does not change any practitioner’s scope of practice. Regulation and guidelines differ between countries.