If your doctor checks your blood sugar, they typically order one of two tests: fasting glucose or HbA1c. Some order both. Most order just one. The question is whether one is enough — and the answer, if you care about catching metabolic problems early, is no.

What each test measures

Fasting glucose is a snapshot. It tells you what your blood sugar level is right now, after at least 8 hours without eating. Normal is under 100 mg/dL. Between 100 and 125 is prediabetes. 126 or higher is diabetes.

HbA1c (glycated hemoglobin) is a 3-month average. It measures the percentage of your hemoglobin molecules that have glucose permanently attached to them. Because red blood cells live for about 3 months, HbA1c reflects your average blood sugar over that entire period. Normal is under 5.7%. Between 5.7% and 6.4% is prediabetes. 6.5% or higher is diabetes. A diabetes diagnosis normally needs confirmation by a second abnormal result.

Both tests are useful. Each alone has blind spots.

Where fasting glucose fails

Fasting glucose is measured at a single point in time. Your blood sugar at 7 AM on a Tuesday depends on what you ate the night before, how well you slept, your stress level, whether you exercised yesterday, and whether your liver decided to dump a bunch of stored glucose overnight (the "dawn phenomenon").

A person with early insulin resistance can have a fasting glucose of 92 one day and 108 the next. The first result is "normal." The second is "prediabetes." Same person, same metabolism — just biological noise.

Fasting glucose also misses postprandial hyperglycemia — the blood sugar spikes that happen after meals. Some people have normal fasting glucose but wildly abnormal glucose responses to food. Fasting glucose can't see this.

Where HbA1c fails

HbA1c smooths out the noise that fasting glucose is vulnerable to — which is its strength. But it has blind spots of its own.

HbA1c can be misleading in people with conditions that affect red blood cell lifespan: iron deficiency anemia (artificially high HbA1c), hemolysis or blood loss (usually artificially low), hemoglobin variants (common in African American, Mediterranean, and Southeast Asian populations), recent blood transfusions, or chronic kidney disease.

It can also miss early-stage problems. Because HbA1c is an average, short post-meal glucose spikes may barely move it, so it can still look normal in the early stages.

Why both — plus insulin — make sense

A thorough metabolic work-up therefore includes fasting glucose, HbA1c, and fasting insulin. The third marker is the one most standard check-ups miss — and it may give the earliest signal.

Fasting insulin, combined with fasting glucose to calculate HOMA-IR, reflects insulin resistance. In the Whitehall II study, insulin sensitivity was already falling steeply in the five years before diabetes was diagnosed, while fasting glucose rose sharply only in the last three. The caveat: insulin assays are not well standardized, there is no universally agreed HOMA-IR cut-off, and diabetes guidelines do not recommend it for routine screening.

Together, the three markers cover different time horizons and different failure modes:

  • Fasting insulin/HOMA-IR: years before diagnosis. Can point to the earliest stage of metabolic dysfunction.
  • HbA1c: months -3 to now. Detects sustained glycemic elevation. Catches what fasting glucose misses on a bad day.
  • Fasting glucose: right now. Provides the standard diagnostic criterion and tracks acute changes.

If your last check-up included only one of these, it's reasonable to ask your doctor whether adding the others makes sense for you.

The bottom line

Fasting glucose and HbA1c each have blind spots; together they catch more than either alone. Fasting insulin can add an early hint of insulin resistance, although it is not a guideline-recommended screening test. Which combination makes sense for you is worth discussing with your doctor.

References

  1. American Diabetes Association Professional Practice Committee. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes—2025. Diabetes Care. 2025;48(Suppl 1):S27-S49.
  2. Tabák 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;373(9682):2215-2221.
  3. Herman WH, et al. Differences in A1C by race and ethnicity among patients with impaired glucose tolerance in the Diabetes Prevention Program. Diabetes Care. 2007;30(10):2453-2457.
Prof. Dr. med. Markus Guba

Head of Liver Surgery and Transplantation at LMU University Hospital Munich. Edits this independent journal in a private capacity; views are his own. About →

This article is general information and not individual medical advice. Decisions about tests and treatment belong in a conversation with your own physician.