[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Endocrine system](https://durgical.com/en/general-pathology/areas/endocrine/) › Diabetes mellitus: diagnosis and classification

# Diabetes mellitus: diagnosis and classification

- Topic

Diabetes means blood sugar stays too high, because the body lacks insulin or does not respond to it, or both.

Updated Nov 12, 2026

## Key points

- Diagnosis: fasting glucose, two-hour glucose after a load, or HbA1c above the diagnostic thresholds, or a random glucose above threshold with symptoms; results below these but above normal define prediabetes.
- Type 1: autoimmune beta-cell destruction, absolute insulin deficiency.
- Type 2: insulin resistance plus inadequate beta-cell compensation.
- Other specific types: monogenic (MODY, neonatal), exocrine pancreas disease (pancreatitis, cystic fibrosis, haemochromatosis, pancreatectomy), endocrinopathies (Cushing, acromegaly, pheochromocytoma, glucagonoma), drugs (glucocorticoids), infections.
- Gestational diabetes: first recognised in pregnancy.
- Insulin is released in two phases after glucose enters the beta cell through GLUT2 and closes ATP-sensitive potassium channels.

## What it is

Diabetes mellitus is a group of metabolic disorders with chronic hyperglycaemia from defective insulin secretion, insulin action, or both. It is diagnosed when fasting glucose, a two-hour glucose after an oral glucose load, HbA1c, or a random glucose with classic symptoms is above the diagnostic thresholds (the condition pages give the numbers). Type 2 accounts for the large majority of cases and is rising fast in Latin America with obesity.

## Step by step

- Glucose enters the beta cell through GLUT2 and is phosphorylated by glucokinase, the glucose sensor.
- Rising ATP closes KATP channels (KCNJ11 and ABCC8 subunits) and depolarises the cell.
- Calcium enters and triggers release of stored insulin (first phase), then newly made insulin (second phase); incretins (GLP-1, GIP) amplify it.
- Insulin binds its tyrosine-kinase receptor; through IRS and PI3K-AKT it moves GLUT4 to the membrane of muscle and fat and stores glycogen, fat and protein.
- If insulin is missing or its signal is blunted, glucose stays high, lipolysis and gluconeogenesis rise, and diabetes follows.

## Type 1 and type 2 diabetes compared

| Feature | Type 1 | Type 2 |
| --- | --- | --- |
| Mechanism | Autoimmune beta-cell loss | Insulin resistance with beta-cell failure |
| Onset | Usually childhood or young adult; can be any age | Usually adult, now also adolescents |
| Body weight | Normal or thin | Usually obese |
| Insulin levels | Very low, C-peptide low | Normal or high early, falls later |
| [Islet autoantibodies](https://durgical.com/en/conditions/type-1-diabetes-mellitus/) | Present | Absent |
| Genetics | HLA-DR3/DR4-DQ8, strong | Polygenic, stronger family concordance |
| [Acute crisis](https://durgical.com/en/conditions/diabetic-ketoacidosis/) | Ketoacidosis | Hyperosmolar hyperglycaemic state |
| Islet morphology | Insulitis, loss of beta cells | Islet amyloid, modest beta-cell loss |

## Clinical notes

- Classic symptoms: polyuria, polydipsia, polyphagia, weight loss.
- HbA1c reflects average glucose over the preceding two to three months.
- In Colombia and Latin America, diabetes is a leading cause of kidney failure, blindness and amputation.

## Memory aid

Type 1 has no key (insulin); type 2 has a rusty lock (resistance) and, later, a worn-out key-maker (beta cell).

## Test yourself

Think of the answer, then tap the question to see it.

## Which tests can diagnose diabetes?

Fasting plasma glucose, two-hour glucose after an oral glucose load, HbA1c, or a random glucose with classic symptoms, each above its diagnostic threshold.

## What closes the KATP channel in a beta cell after a meal?

A rise in intracellular ATP from glucose metabolism, after glucose enters through GLUT2 and is phosphorylated by glucokinase.

## Which two endocrine diseases commonly cause secondary diabetes?

Cushing syndrome and acromegaly (also pheochromocytoma and glucagonoma).

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## Elsewhere in DurgiPedia

- [Islets and insulin: secretion and actions](https://durgical.com/en/physiology/islets-and-insulin-secretion-and-actions/)
- [Glucagon and the counter-regulatory hormones](https://durgical.com/en/physiology/glucagon-and-the-counter-regulatory-hormones/)
- [Insulin signalling](https://durgical.com/en/biochemistry/topics/insulin-signalling/)
- [Glycated haemoglobin (HbA1c)](https://durgical.com/en/biochemistry/topics/glycated-haemoglobin/)
- [Islets of Langerhans: α, β, δ cells](https://durgical.com/en/histology/islets-of-langerhans-cells/)

## Connections

## Related 6

- [Type 1 diabetes](https://durgical.com/en/general-pathology/type-1-diabetes/)
- [Type 2 diabetes](https://durgical.com/en/general-pathology/type-2-diabetes/)
- [Monogenic and gestational diabetes](https://durgical.com/en/general-pathology/monogenic-and-gestational-diabetes/)
- [Diabetes: acute complications (DKA, HHS)](https://durgical.com/en/general-pathology/diabetes-acute-complications/)
- [Chronic pancreatitis: fibrosis, ductal stones and lost acini](https://durgical.com/en/general-pathology/chronic-pancreatitis-fibrosis-ductal-stones-and-lost-acini/)
- [Hemochromatosis: hepcidin, HFE and where iron settles](https://durgical.com/en/general-pathology/hemochromatosis-hepcidin-hfe-and-where-iron-settles/)

## Related conditions 9

- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)
- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [Gestational diabetes mellitus](https://durgical.com/en/conditions/gestational-diabetes-mellitus/)
- [Chronic pancreatitis](https://durgical.com/en/conditions/chronic-pancreatitis/)
- [Cushing syndrome](https://durgical.com/en/conditions/cushing-syndrome/)
- [Acromegaly and gigantism](https://durgical.com/en/conditions/acromegaly-and-gigantism/)
- [Cystic fibrosis](https://durgical.com/en/conditions/cystic-fibrosis/)
- [Hereditary hemochromatosis](https://durgical.com/en/conditions/hereditary-hemochromatosis/)
- [Phaeochromocytoma](https://durgical.com/en/conditions/phaeochromocytoma/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Diabetes](https://www.who.int/news-room/fact-sheets/detail/diabetes/) (World Health Organization, 2025); [Diabetes Type 2](https://medlineplus.gov/diabetestype2.html) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/diabetes-mellitus-diagnosis-and-classification/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/diabetes-mellitus-diagnosis-and-classification/