6.1 Diabetes Mellitus: Diagnostic Criteria & Chronic Management
Key Takeaways
Diagnostic criteria for diabetes require either clinical symptoms of hyperglycaemia plus one abnormal test result, or two abnormal results on separate occasions: Fasting Plasma Glucose ≥ 7.0 mmol/L, Random Plasma Glucose ≥ 11.1 mmol/L, 2-hour post-OGTT ≥ 11.1 mmol/L, or HbA1c ≥ 48 mmol/mol (6.5%).
Prediabetes (impaired glucose regulation) is defined by an HbA1c of 42–47 mmol/mol (6.0–6.4%) or fasting plasma glucose of 6.1–6.9 mmol/L, warranting structured lifestyle modification and annual biochemical review.
Under NICE NG28 (February 2026), most adults with type 2 diabetes start modified-release metformin plus an SGLT-2 inhibitor; atherosclerotic CVD adds subcutaneous semaglutide, and eGFR 20–29 needs dapagliflozin or empagliflozin plus a DPP-4 inhibitor (metformin stops below eGFR 30).
Individualised HbA1c targets are 48 mmol/mol for patients managed with lifestyle or a single agent not associated with hypoglycaemia, relaxing to 53 mmol/mol when regimens include drugs that carry hypoglycaemic risk (sulfonylureas, insulin); acute hypoglycaemia (< 4.0 mmol/L) requires 15–20 g of rapid-acting oral carbohydrate for alert patients, or IV 100–200 mL 10% glucose / IM glucagon 1 mg if conscious level is impaired.
Diabetes Mellitus: Diagnostic Criteria & Chronic Management
Diabetes mellitus comprises a group of metabolic disorders characterised by chronic hyperglycaemia resulting from defects in insulin secretion, insulin action, or both. In UK clinical practice, accurate diagnosis and early risk stratification are essential to prevent microvascular and macrovascular complications.
Diagnostic Criteria for Diabetes Mellitus
The World Health Organization (WHO) and National Institute for Health and Care Excellence (NICE) establish diagnostic criteria based on plasma glucose concentrations and glycated haemoglobin (HbA1c). Confirmation requires either:
- Symptomatic patient (polyuria, polydipsia, unexplained weight loss, lethargy, recurrent infections such as candidiasis) plus ONE abnormal laboratory result.
- Asymptomatic individual with TWO abnormal laboratory results on separate occasions.
| Diagnostic Modality | Normal Range | Impaired Glucose Regulation (Prediabetes) | Diabetes Mellitus Threshold |
|---|---|---|---|
| Fasting Plasma Glucose (FPG) | < 6.1 mmol/L | 6.1 – 6.9 mmol/L (Impaired Fasting Glucose, IFG) | ≥ 7.0 mmol/L |
| Random Plasma Glucose (RPG) | < 7.8 mmol/L | N/A | ≥ 11.1 mmol/L (with symptoms) |
| 2-Hour 75g Oral Glucose Tolerance Test (OGTT) | < 7.8 mmol/L | 7.8 – 11.0 mmol/L (Impaired Glucose Tolerance, IGT) | ≥ 11.1 mmol/L |
| Glycated Haemoglobin (HbA1c) | < 42 mmol/mol (< 6.0%) | 42 – 47 mmol/mol (6.0 – 6.4%) | ≥ 48 mmol/mol (≥ 6.5%) |
Clinical Caveats & Invalidation of HbA1c
HbA1c reflects average glycaemia over the approximate 120-day lifespan of circulating erythrocytes. In clinical scenarios where erythrocyte turnover or haemoglobin kinetics are altered, HbA1c is invalid and must not be used for diagnosis:
- Haemoglobinopathies: Sickle cell trait/disease, thalassaemia major or minor.
- Altered erythrocyte lifespan: Haemolytic anaemia, recent blood transfusion (past 3 months), chronic blood loss, splenomegaly, or severe untreated iron-deficiency anaemia.
- End-stage renal disease / Haemodialysis: Uraemia alters red cell survival and carbamylated haemoglobin interferes with assays.
- Pregnancy: Gestational diabetes or pre-existing diabetes in pregnancy (must use fasting glucose and 75g OGTT).
- Rapid-onset hyperglycaemia: Suspected acute Type 1 diabetes in children or young adults, steroid-induced diabetes, and pancreatic resection/acute pancreatitis.
Note
Prediabetes Management: Individuals with an HbA1c between 42 and 47 mmol/mol (6.0–6.4%) or IFG (6.1–6.9 mmol/L) have high progression rates to established type 2 diabetes. Clinical management centres on intensive lifestyle intervention (≥5% body weight reduction, Mediterranean dietary pattern, ≥150 minutes of moderate-intensity physical activity weekly) and annual HbA1c surveillance.
Type 1 Diabetes: Pathophysiology & Management
Type 1 diabetes results from T-cell-mediated autoimmune destruction of insulin-producing beta cells in the pancreatic islets of Langerhans, culminating in absolute insulin deficiency. It characteristically presents in children, adolescents, or young adults with a rapid onset of weight loss, ketonuria, polyuria, polydipsia, and fatigue, frequently progressing to diabetic ketoacidosis (DKA).
Diagnostic Biomarkers
- Autoantibodies: Anti-glutamic acid decarboxylase (anti-GAD65), anti-islet antigen 2 (anti-IA-2), and anti-zinc transporter 8 (anti-ZnT8). At least one antibody is detectable in >90% of new-onset autoimmune cases.
- C-peptide: Low or undetectable fasting C-peptide (< 200 pmol/L) confirms absolute endogenous insulin deficiency.
Insulin Regimens
All patients with type 1 diabetes require lifelong exogenous insulin replacement. NICE guidelines recommend a multiple daily injection (MDI) basal-bolus regimen as the standard of care:
- Basal (background) insulin: Administered once or twice daily using a long-acting insulin analogue (e.g., insulin glargine, insulin degludec, or insulin detemir) to suppress hepatic gluconeogenesis overnight and between meals.
- Bolus (prandial) insulin: Rapid-acting insulin analogue (e.g., insulin aspart, insulin lispro, or insulin glulisine) injected immediately before or with meals, calculated according to carbohydrate counting and ambient capillary glucose.
- Glucose Monitoring: All adults with type 1 diabetes should be offered continuous glucose monitoring (CGM) or flash glucose monitoring (e.g., FreeStyle Libre) to monitor glycaemic trends, identify asymptomatic hypoglycaemia, and optimise time in target range (3.9–10.0 mmol/L).
Type 2 Diabetes: Stepped Pharmacological Management
Type 2 diabetes arises from peripheral insulin resistance combined with progressive pancreatic beta-cell secretory failure. Clinical management follows an individualised stepped pharmacological approach that prioritises organ protection (cardiovascular and renal) alongside glycaemic optimisation.
Initial Medicines: NICE NG28 (Updated February 2026)
The February 2026 update of NICE NG28 made modified-release (MR) metformin plus an SGLT-2 inhibitor the starting combination for almost all adults with type 2 diabetes, because SGLT-2 inhibitors are recommended as much for cardiovascular and renal protection as for glucose lowering. Medicines are introduced one at a time (usually metformin first, then the SGLT-2 inhibitor once metformin is tolerated), with education about sick-day rules and the risk of diabetic ketoacidosis.
| Group (NICE NG28 2026) | Initial Medicines |
|---|---|
| No relevant comorbidity, heart failure, obesity, or CKD with eGFR 30 or more | MR metformin and an SGLT-2 inhibitor; SGLT-2 inhibitor alone if metformin is contraindicated or not tolerated |
| Atherosclerotic cardiovascular disease (e.g. previous MI, stroke, peripheral arterial disease) | MR metformin and an SGLT-2 inhibitor and subcutaneous semaglutide (Ozempic, up to 1 mg once weekly) |
| Early-onset type 2 diabetes (diagnosed before 40) | MR metformin and an SGLT-2 inhibitor; consider adding a GLP-1 receptor agonist or tirzepatide |
| eGFR 20 to under 30 mL/min/1.73m² | Dapagliflozin or empagliflozin and a DPP-4 inhibitor (metformin is not used below eGFR 30) |
| eGFR under 20 mL/min/1.73m² | Consider a DPP-4 inhibitor; if unsuitable or ineffective, pioglitazone or insulin-based treatment |
| Frailty | MR metformin; add an SGLT-2 inhibitor only if frailty does not put the person at risk of volume depletion or hypotension (otherwise consider a DPP-4 inhibitor) |
- Metformin (biguanide): activates AMP-activated protein kinase (AMPK), inhibiting hepatic gluconeogenesis and increasing peripheral glucose uptake. It does not cause hypoglycaemia or weight gain. Start at a low dose with food and titrate (MR preparations reduce gastrointestinal side effects such as diarrhoea, nausea and cramps).
- Renal Monitoring & Lactic Acidosis: Metformin is excreted unchanged by the kidneys; accumulation in renal impairment raises the risk of rare but life-threatening lactic acidosis. Review and reduce the dose if eGFR falls below 45 mL/min/1.73m²; stop metformin if eGFR falls below 30 mL/min/1.73m².
- SGLT-2 inhibitors (dapagliflozin, empagliflozin, canagliflozin): reduce heart failure hospitalisation, cardiovascular events and progression of kidney disease. Warn about genital fungal infection, volume depletion and euglycaemic diabetic ketoacidosis (stop during acute illness, before surgery, and on very low-carbohydrate diets).
- Further medicines when HbA1c remains above the agreed target: options include a DPP-4 inhibitor, pioglitazone, a sulfonylurea, a GLP-1 receptor agonist or tirzepatide (particularly relevant if obesity is a priority), and insulin-based treatment, chosen by comorbidity, hypoglycaemia risk, weight and the person's preferences.
Antidiabetic Drug Classes Comparison
| Drug Class | Examples | Mechanism of Action | HbA1c Reduction | Effect on Weight | Hypoglycaemia Risk | Key Adverse Effects & Clinical Caveats |
|---|---|---|---|---|---|---|
| Biguanide | Metformin | Activates AMPK; inhibits hepatic gluconeogenesis; enhances insulin sensitivity | 10–15 mmol/mol (1.0–1.5%) | Weight neutral / mild loss | Negligible | Gastrointestinal upset, vitamin B12 deficiency. Lactic acidosis risk: review if eGFR < 45, stop if eGFR < 30 mL/min/1.73m² |
| SGLT2 Inhibitors | Dapagliflozin, Empagliflozin, Canagliflozin | Inhibits sodium-glucose co-transporter 2 in renal proximal tubule; promotes glucosuria | 8–10 mmol/mol (0.7–0.9%) | Weight loss (2–4 kg) | Negligible | Mycotic genital infections, UTIs, volume depletion, euglycaemic DKA, Fournier's gangrene. Reduced glycaemic efficacy if eGFR < 45 mL/min/1.73m² |
| DPP-4 Inhibitors (Gliptins) | Sitagliptin, Linagliptin, Alogliptin | Inhibits dipeptidyl peptidase-4; prevents degradation of endogenous incretins (GLP-1/GIP) | 6–9 mmol/mol (0.6–0.8%) | Weight neutral | Negligible | Well tolerated, nasopharyngitis. Rare risk of acute pancreatitis. Linagliptin requires no renal dose adjustment |
| Sulfonylureas | Gliclazide, Glimepiride | Closes ATP-sensitive K+ channels on beta cells, triggering depolarisation and insulin exocytosis | 10–15 mmol/mol (1.0–1.5%) | Weight gain (2–3 kg) | High | Hypoglycaemia, secondary failure over time. Caution in renal impairment and elderly; gliclazide is preferred due to hepatic metabolism |
| Thiazolidinediones (TZDs) | Pioglitazone | PPAR-γ nuclear receptor agonist; promotes adipocyte differentiation and insulin sensitivity | 8–12 mmol/mol (0.8–1.0%) | Weight gain (fluid/fat) | Negligible | Fluid retention, peripheral oedema, heart failure exacerbation, increased fracture risk in postmenopausal women. Contraindicated in heart failure (NYHA I–IV) and bladder cancer |
| GLP-1 Receptor Agonists | Dulaglutide, Semaglutide, Liraglutide | Incretin mimetic; stimulates glucose-dependent insulin release, slows gastric emptying, induces central satiety | 12–18 mmol/mol (1.2–1.8%) | Marked weight loss (3–6 kg) | Negligible | Nausea, vomiting, diarrhoea, gallbladder disease, pancreatitis. NICE NG28 (2026) offers subcutaneous semaglutide with metformin and an SGLT-2 inhibitor in atherosclerotic CVD, and considers a GLP-1 receptor agonist or tirzepatide (dual GIP/GLP-1 agonist) in early-onset type 2 diabetes |
Individualised HbA1c Therapeutic Targets
NICE guidelines mandate individualised glycaemic targets to balance microvascular risk reduction against the danger of hypoglycaemia:
| Patient Clinical Profile | Target HbA1c (mmol/mol) | Target HbA1c (%) | Clinical Rationale |
|---|---|---|---|
| Lifestyle intervention alone or monotherapy not associated with hypoglycaemia (e.g., Metformin, SGLT2i, DPP-4i, Pioglitazone) | 48 mmol/mol | 6.5% | Maximises microvascular protection without risking drug-induced hypoglycaemic episodes |
| Regimen incorporating a drug associated with hypoglycaemia (e.g., Sulfonylurea, Meglitinide, or Insulin) | 53 mmol/mol | 7.0% | Provides a physiological safety margin to prevent neuroglycopenic events and falls |
| Escalation threshold on existing therapy: Inadequate control requiring drug intensification | ≥ 58 mmol/mol | ≥ 7.5% | Action threshold to trigger dual or triple therapy review |
| Frail elderly, severe chronic kidney disease, significant cardiovascular disease, or hypoglycaemia unawareness | 58 – 64 mmol/mol | 7.5 – 8.0% | Avoids severe hypoglycaemia, cardiovascular strain, and fall-related morbidity |
Hypoglycaemia: Recognition & Emergency Management
Hypoglycaemia in adults with diabetes is defined biochemically as a blood glucose concentration < 4.0 mmol/L ("Four is the floor"). Symptoms divide into:
- Autonomic (early warning): Tremor, diaphoresis, palpitations, tachycardia, anxiety, pallor, and hunger (mediated by sympathetic and cholinergic activation).
- Neuroglycopenic (cerebral glucose deprivation): Inattention, confusion, behavioural changes, dysphasia, visual disturbances, seizures, coma, and focal neurological deficits.
[ Capillary Glucose < 4.0 mmol/L ]
│
┌──────────────────────┴──────────────────────┐
▼ ▼
[ Patient Alert, Conscious & Swallowing ] [ Patient Unconscious, Aggressive or NPO ]
│ │
[ Step 1: Rapid Carbohydrate ] ┌──────────────┴──────────────┐
15-20 g fast-acting oral glucose: ▼ ▼
• 150-200 mL pure fruit juice or cola [ IV Access Present ] [ No IV Access / Community ]
• 4-5 Jelly Babies or 4-5 GlucoTabs │ │
│ 100-200 mL 10% Glucose 1 mg Glucagon IM/SC
[ Step 2: Recheck at 15 min ] IV over 10-15 minutes (Recovery in 10-15 min)
│ │ │
┌───────────┴───────────┐ └──────────────┬──────────────┘
▼ ▼ │
[ Still < 4.0 mmol/L ] [ Resolved ≥ 4.0 mmol/L ] ▼
Repeat Step 1 (up to 3x) [ Step 3: Complex Carbohydrate ] [ Once Conscious & Safe ]
Give long-acting carbohydrate: Give oral complex carbohydrate
• Slice of toast / 2 biscuits
• Next planned meal containing carbs
Warning
Glucagon Limitations: Intramuscular glucagon acts by mobilising hepatic glycogen reserves. It is ineffective in patients with depleted hepatic glycogen, such as those with prolonged starvation, severe liver cirrhosis, or acute alcohol-induced hypoglycaemia. In these clinical scenarios, intravenous glucose must be obtained without delay. Furthermore, hypertonic 50% intravenous glucose is no longer recommended due to the high risk of severe extravasation necrosis and chemical thrombophlebitis; 10% intravenous glucose is the standard fluid.
A 64-year-old man with a 12-year history of type 2 diabetes presents for routine diabetic clinic review. His current medications include metformin 1000 mg twice daily and ramipril 10 mg once daily. Recent blood tests reveal an HbA1c of 56 mmol/mol, serum creatinine of 210 µmol/L, and an estimated glomerular filtration rate (eGFR) of 26 mL/min/1.73m² (reduced from 48 mL/min/1.73m² twelve months ago). What is the most appropriate management regarding his diabetic pharmacotherapy?
Stop metformin and start gliclazide 80 mg once daily
Continue metformin at the same dose and add dapagliflozin 10 mg once daily
Reduce metformin to 500 mg once daily with meals
Stop metformin and start dapagliflozin 10 mg plus linagliptin 5 mg once daily
A 56-year-old man with type 2 diabetes attends for a treatment review. Two years ago, he suffered an acute ST-elevation myocardial infarction requiring percutaneous coronary intervention. His current medication is metformin 1000 mg twice daily. His BMI is 29 kg/m², blood pressure is 128/78 mmHg, and his latest HbA1c is 62 mmol/mol. Renal function tests show an eGFR of 78 mL/min/1.73m² and a normal urinary albumin:creatinine ratio. What is the most appropriate next step in his pharmacological management according to clinical guidelines?
Switch metformin to sitagliptin 100 mg once daily
Add empagliflozin 10 mg once daily to metformin
Initiate subcutaneous basal insulin once daily at bedtime
Add gliclazide 40 mg once daily to metformin
A 72-year-old woman with type 2 diabetes treated with gliclazide 80 mg twice daily is brought to the emergency department by ambulance. She was found by her daughter drowsy, diaphoretic, and confused. On arrival, she is unresponsive to verbal commands but responds to painful stimuli. Her airway is patent and breathing spontaneously. A wide-bore intravenous cannula is already in situ. A point-of-care capillary blood glucose measurement is 2.1 mmol/L. What is the most appropriate immediate therapy?
Administer 150 mL of 10% intravenous glucose over 10 minutes
Administer 200 mL of orange juice via a nasogastric tube
Administer 1 mg of glucagon intramuscularly
Administer 50 mL of 50% intravenous glucose bolus via peripheral vein
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