Blood glucose, glucose tolerance test and HbA1c !
- Measurement
- Exam priority: medium
Three tests show how well the body handles sugar: a fasting level, a curve after a sugary drink and HbA1c, the average of the last months.
Updated 2026-11-01
Also called: OGTT, Fasting plasma glucose, Glycated hemoglobin
See it in 3D: Pancreas · Liver
What it is
Fasting plasma glucose shows how well insulin restrains the liver overnight; the oral glucose tolerance test (OGTT) shows how well insulin clears a standard glucose load; HbA1c shows the average glucose over the red cells' lifetime. Each sees a different face of the same physiology, so they do not always agree. The thresholds below are the internationally used ones for diagnosing diabetes and prediabetes in non-pregnant adults.
How it works, step by step
- After an overnight fast, glucose comes almost only from the liver; its level depends on how well basal insulin suppresses hepatic output.
- Oral glucose is absorbed and raises plasma glucose within 30–60 minutes.
- Beta cells release stored insulin (first phase) and then newly made insulin (second phase); gut incretins amplify the response.
- Insulin suppresses the liver and drives glucose into muscle and fat: in a healthy person glucose returns under 140 mg/dL by 2 hours.
- In insulin resistance or beta-cell failure, the curve stays high; impaired tolerance often appears before fasting glucose rises.
- Glucose binds hemoglobin non-enzymatically and irreversibly in proportion to its average level, so HbA1c records the past months.
Equations
- eAG
- estimated average glucose
- HbA1c
- glycated hemoglobin, %
An HbA1c of 7% corresponds to about 154 mg/dL (8.6 mmol/L).
- 18
- factor from the molar mass of glucose (180 g/mol)
Diagnostic thresholds (venous plasma, non-pregnant adults)
| Parameter | Value | Note |
|---|---|---|
| Fasting glucose, normal | < 100 mg/dL | < 5.6 mmol/L |
| Impaired fasting glucose | 100–125 mg/dL | 5.6–6.9 mmol/L |
| Fasting glucose, diabetes | ≥ 126 mg/dL | ≥ 7.0 mmol/L |
| OGTT 2 h, normal | < 140 mg/dL | < 7.8 mmol/L |
| OGTT 2 h, impaired tolerance | 140–199 mg/dL | 7.8–11.0 mmol/L |
| OGTT 2 h, diabetes | ≥ 200 mg/dL | ≥ 11.1 mmol/L |
| HbA1c: normal / prediabetes / diabetes | < 5.7% / 5.7–6.4% / ≥ 6.5% | < 39 / 39–47 / ≥ 48 mmol/mol |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- A sample left unprocessed keeps consuming glucose (red-cell glycolysis), so the reported value falls with delay; fluoride tubes or prompt separation avoid it.
- Capillary glucometers estimate glucose for monitoring; diagnosis uses laboratory venous plasma.
- Hemoglobin variants, kidney failure and recent transfusion make HbA1c unreliable; then glucose-based tests are used.
- Acute illness or glucocorticoids raise glucose temporarily (stress hyperglycemia): the diagnosis is confirmed when the person is stable.
Test yourself
Think of the answer, then tap the question to see it.
Why can impaired glucose tolerance appear before the fasting glucose rises?
Early on, basal insulin still restrains the liver overnight, but the beta cells cannot raise insulin enough, or tissues resist it, when a large load arrives.
A patient with hemolytic anemia has HbA1c 5.2% but fasting glucose 140 mg/dL on two days. How do you explain the mismatch?
Red cells survive less, so they have less time to be glycated; HbA1c underestimates glycemia. The two fasting values already meet the diabetes threshold.
Why is the OGTT usually more sensitive than fasting glucose in pregnancy screening?
Pregnancy hormones (placental lactogen, progesterone, cortisol) cause rising insulin resistance mainly after meals; the load reveals it while the fasting value can stay normal.
Why does an unseparated blood sample report a lower glucose a few hours later?
Red and white cells keep metabolizing glucose by glycolysis in the tube unless glycolysis is inhibited or the plasma is separated.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Islets and insulin: secretion and actions
- Physiology of diabetes and ketoacidosis
- Appetite hormones: leptin, ghrelin and incretins
- The red cell life cycle and the spleen
- Maternal physiological changes in pregnancy
- The endocrine stress response
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 17.9 The Endocrine Pancreas (OpenStax (Rice University), 2022); Glucose tolerance test - non-pregnant: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025); Hemoglobin A1C (HbA1c) Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025); Type 2 Diabetes (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 79). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.