Prediabetes

In shortPrediabetes is caught by four instruments: fasting glucose, glycated hemoglobin (HbA1c), the oral glucose tolerance test (OGTT) and the HOMA-IR index of insulin resistance. They do not duplicate each other: glucose is the moment, HbA1c is three months, OGTT is the response to a load, HOMA-IR is the background of resistance. Fructosamine covers the last two to three weeks, when HbA1c does not work.

Step one: fasting glucose — the snapshot

Fasting glucose is the most accessible test and simultaneously the narrowest: it is one point in time. The morning value depends on last night's dinner, sleep, stress and even the dawn rise of hormones. A borderline result requires confirmation: by the rules of diabetes diagnostics, any deviating value is repeated on another day.

Type 2 diabetes guidelines set diagnostic thresholds for fasting glucose, HbA1c and the OGTT; the intermediate zone between normal and diabetes is prediabetes: impaired fasting glucose or impaired glucose tolerance.

Step two: HbA1c — the three-month average

Glycated hemoglobin shows what average glucose was over the red cell's lifespan — about three months. The test is convenient because it needs no fasting and does not depend on yesterday: it smooths fluctuations and shows the background. That is why HbA1c became a full diagnostic criterion alongside glucose.

HbA1c has blind spots: anemias, recent blood loss, hemolysis and some hemoglobin variants distort it in both directions. In those situations fructosamine helps — the glycation of plasma proteins with a horizon of two to three weeks, independent of red cells.

Step three: the OGTT — the load test

The glucose tolerance test shows what fasting cannot: how the body handles a carbohydrate load. In the morning, fasting glucose is measured, then a standard glucose solution is drunk and the measurement repeats after two hours. Impaired tolerance is when fasting is still decent but post-load sugar runs higher than it should.

Preparation decides the test's quality: three days of usual eating with enough carbohydrates, habitual activity, fasting the night before. A strict diet before the test 'improves' the result deceptively: the body falls out of practice with carbohydrates and answers the load worse than it actually lives.

Step four: HOMA-IR — the background of insulin resistance

The HOMA-IR index is calculated from a fasting pair — glucose and insulin: it shows how much insulin the body must spend to hold sugar normal. A high index with normal glucose means compensation comes at a high price — this is insulin resistance, the soil of prediabetes.

HOMA-IR is a calculated guide, not a diagnostic criterion: it depends on the insulin assay method and has no single threshold across laboratories. It is read together with waist size, blood pressure and lipids: insulin resistance rarely arrives alone.

What distorts carbohydrate metabolism tests

Food and sweet drinks within 8 hours before the draw
A strict low-carb diet before the OGTT — a false worsening
Acute infection and stress — a temporary glucose rise
Anemia and recent blood loss — HbA1c distortion
A workout the day before — a shift in fasting glucose

Markers covered in this article

Blood glucose Hemoglobin A1C (HbA1c)Glucose tolerance test (OGTT)HOMA-IR insulin resistance Insulin Fructosamine

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Frequently asked questions

Fasting glucose is normal — can prediabetes still exist?

Yes: impaired glucose tolerance is visible only in the OGTT after a load, and early insulin resistance in the HOMA-IR index. Normal fasting glucose does not close the question when risk factors are present.

How is HbA1c better than ordinary glucose?

It does not depend on fasting and shows average sugar over three months rather than one point. Limitations are anemias and hemoglobin variants; then fructosamine is used.

What is fructosamine for?

Fructosamine reflects average glucose over two to three weeks and does not depend on red cells. It is needed when HbA1c is distorted (anemias, blood loss) and when fast dynamics matter — for example, assessing changes over recent weeks.

HOMA-IR is high but sugar is normal — what does it mean?

That is insulin resistance: the pancreas holds sugar normal by overproducing insulin. The state is reversible, and catching it before any glucose deviations is the most favourable moment for lifestyle correction with a clinician.

How often is prediabetes monitored?

Guidelines imply regular monitoring of glucose and HbA1c at intervals the clinician sets by baseline risk. Self-monitoring without a plan is less useful than rare but meaningful laboratory checks.

Is the OGTT mandatory if HbA1c is normal?

No: with normal glucose and HbA1c the test is assigned by risk factors — obesity, family history, past gestational diabetes. The decision follows the whole picture, not a single number.

Sources

Type 2 diabetes in adults — clinical guideline context — Minzdrav of Russia (cr.minzdrav.gov.ru). Accessed: August 29, 2026.
Glucose tolerance test - non-pregnant: MedlinePlus Medical Encyclopedia — MedlinePlus, US National Library of Medicine. Accessed: August 31, 2026.
Fructosamine — test information — North Bristol NHS Trust, Severn Pathology. Accessed: August 31, 2026.

Prediabetes: the checking sequence — glucose, HbA1c, OGTT and HOMA-IR