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🩺 DIABETES MELLITUS - A Complete Medical Teaching Guide

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QUICK BEGINNER EXPLANATION

• The key is missing (Type 1 - no insulin made), OR
• The lock is broken (Type 2 - cells don't respond to insulin)

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SECTION 1: DEFINITION

• Insulin secretion (the pancreas doesn't make enough)
• Insulin action (the body doesn't respond to it), OR
• Both

Simple version: DM = too much glucose in the blood, for too long, destroying organs silently.

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SECTION 2: CAUSES & RISK FACTORS

Type 1 DM - Causes

• Autoimmune destruction of pancreatic beta cells (islets of Langerhans)
• Triggered by: genetic susceptibility (HLA-DR3, HLA-DR4) + environmental trigger (viral infection, e.g., Coxsackievirus B)
• Results in: absolute insulin deficiency

Type 2 DM - Causes

• Insulin resistance (cells ignore insulin's signal)
• Progressive beta-cell failure (pancreas eventually "burns out")
• Strong genetic component + lifestyle factors

Risk Factors Table

Clinical Pearl: 80-90% of Type 2 DM patients are overweight or obese. Obesity causes fat accumulation, particularly visceral fat, which releases inflammatory cytokines that impair insulin receptor signaling - this is the mechanistic link. (Guyton & Hall)

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SECTION 3: CLASSIFICATION / TYPES

• Pancreatic disease (pancreatitis, cystic fibrosis, hemochromatosis)
• Hormonal excess: Cushing's syndrome, acromegaly, pheochromocytoma, hyperthyroidism
• Drugs: corticosteroids, tacrolimus, atypical antipsychotics (olanzapine), thiazides (high dose)

Mnemonic for secondary DM: "HCAP + Drugs" = Hemochromatosis, Cushing's, Acromegaly, Pancreatitis + Drugs

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SECTION 4: RELEVANT ANATOMY & PHYSIOLOGY

The Pancreas and Beta Cells

Normal Glucose Homeostasis

1. Blood glucose rises
2. Beta cells sense glucose via GLUT-2 transporter
3. Glucose enters beta cell → metabolized → ATP rises → K+ channel closes → cell depolarizes → Ca2+ enters → Insulin released
4. Insulin acts on liver (suppresses glucose production), muscle and fat (increases glucose uptake via GLUT-4)
5. Blood glucose falls back to normal (3.9-5.5 mmol/L / 70-99 mg/dL)

1. Blood glucose falls
2. Alpha cells release glucagon
3. Glucagon acts on liver → glycogenolysis + gluconeogenesis → blood glucose rises

Key concept: The insulin:glucagon ratio controls blood glucose. In DM, this ratio is disrupted - either too little insulin (T1DM) or insulin that doesn't work well (T2DM).

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SECTION 5: PATHOPHYSIOLOGY (Step-by-Step)

Type 1 DM Pathophysiology

TRIGGER (virus, environment)
        ↓
Genetic predisposition activates
        ↓
Auto-reactive T lymphocytes attack beta cells
        ↓
Autoantibodies appear (GAD65, IA-2, IAA, ZnT8)
        ↓
Progressive beta-cell destruction (silent phase, months-years)
        ↓
>80-90% beta cells destroyed
        ↓
ABSOLUTE INSULIN DEFICIENCY
        ↓
No insulin → Liver freely produces glucose (gluconeogenesis)
No insulin → Muscle/fat cannot take up glucose
No insulin → Fat breakdown uncontrolled → Free fatty acids flood liver
        ↓
Liver converts FFAs to KETONE BODIES (beta-hydroxybutyrate, acetoacetate)
        ↓
Blood glucose ↑↑↑ (hyperglycemia)
Blood ketones ↑↑↑ (ketonemia)
Blood pH falls ↓ (metabolic acidosis)
        ↓
DIABETIC KETOACIDOSIS (DKA) if untreated

Type 2 DM Pathophysiology - The "Ominous Octet" (DeFronzo)

The Dual Defect in Type 2 DM:

How Hyperglycemia Causes Damage (Chronic Complications):

Simple version: Prolonged high blood sugar chemically "caramelizes" blood vessel walls, making them stiff, leaky, and inflamed - this is how it destroys the eyes, kidneys, nerves and heart.

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SECTION 6: CLINICAL FEATURES

Classic Symptoms - "The 3 Polys + 1 Weight"

Additional Symptoms

Important Signs on Examination

Key difference: T1DM patients are typically thin (muscle/fat wasted). T2DM patients are typically overweight/obese. This is a useful clinical clue.

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SECTION 7: HISTORY TAKING

What to Ask - and Why

• "How long have you had increased thirst and urination?" - establishes duration
• "Have you lost weight recently without trying?" - red flag for T1DM or uncontrolled T2DM
• "Have you noticed blurry vision?" - osmotic lens changes
• "Any recurrent infections? Skin, urinary, vaginal?" - glucose-rich environment promotes infections

• Previous glucose levels or pre-diabetes diagnosis
• Hypertension, dyslipidemia, cardiovascular disease (metabolic syndrome, T2DM risk)
• Pancreatitis, autoimmune diseases, PCOS
• Previous gestational diabetes (7x higher risk of T2DM later)
• Previous DKA or hypoglycemic episodes

• Corticosteroids, atypical antipsychotics, tacrolimus, HIV antiretrovirals → can cause DM
• Current diabetes medications + adherence

• First-degree relative with T1DM (6% risk vs 0.3% general population)
• T2DM in parents/siblings (3-6x higher risk)

• Diet (carbohydrate intake, processed food)
• Exercise habits
• Smoking (amplifies vascular risk)
• Alcohol (hypoglycemia risk with insulin/sulfonylureas; pancreatitis risk)
• Occupation (important for hypoglycemia risk - driving, machinery)

• Eyes: Visual changes, floaters → retinopathy
• Kidneys: Foamy urine, swelling → nephropathy
• Feet: Numbness, tingling, pain, ulcers → neuropathy
• Heart: Chest pain, breathlessness → macrovascular disease
• Erectile dysfunction (autonomic neuropathy in men)
• Postural dizziness (autonomic neuropathy)

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SECTION 8: DIFFERENTIAL DIAGNOSIS

Differential Diagnosis of Diabetes

Key Rule: Always check blood glucose (not just urine glucose) to diagnose DM. Urine glucose can be absent even with moderate hyperglycemia.

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SECTION 9: INVESTIGATIONS

Basic Tests (Every Patient)

Specific/Diagnostic Tests

Note: Confirmation requires 2 abnormal tests on separate days (unless symptoms + random glucose ≥200 mg/dL - this alone is diagnostic). (ADA Standards of Care 2024, adapted in Harrison's 22E)

Autoantibody Panel (to distinguish T1DM from T2DM)

• GAD65 (Glutamic Acid Decarboxylase) - most sensitive
• IA-2 (Islet Antigen 2)
• IAA (Insulin Autoantibodies)
• ZnT8 (Zinc Transporter 8)

Additional Tests for Complications Screening

Imaging

• Renal ultrasound - if proteinuria or abnormal eGFR
• Echocardiography - if heart failure symptoms (diabetic cardiomyopathy)
• Carotid Doppler / CT coronary angiography - if macrovascular disease suspected
• MRI foot - to assess osteomyelitis in diabetic foot ulcer

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SECTION 10: DIAGNOSIS

Diagnostic Criteria (ADA 2024 / WHO)

┌─────────────────────────────────────────────────────────────────┐
│  DIAGNOSTIC CRITERIA FOR DIABETES MELLITUS                      │
├─────────────────────────────────────────────────────────────────┤
│  1. FPG ≥ 7.0 mmol/L (≥126 mg/dL) after 8h fast               │
│  2. 2-hr PG ≥ 11.1 mmol/L (≥200 mg/dL) on 75g OGTT           │
│  3. HbA1c ≥ 6.5% (48 mmol/mol)                                 │
│  4. Random glucose ≥ 11.1 mmol/L WITH classic symptoms          │
│     (polyuria, polydipsia, unexplained weight loss)             │
│                                                                  │
│  ⚠️ In the ABSENCE of hyperglycemic symptoms, criteria 1-3     │
│  require CONFIRMATION on a SECOND test                          │
└─────────────────────────────────────────────────────────────────┘

Pre-Diabetes Criteria

Clinical Pearl: HbA1c is unreliable in: haemolytic anaemia (falsely low), iron deficiency anaemia (falsely high), haemoglobin variants (HbS, HbC), pregnancy, and haemoglobinopathies. Use FPG or OGTT in these situations.

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SECTION 11: COMPLICATIONS

Microvascular Complications (Small vessel damage)

Macrovascular Complications (Large vessel damage)

Other Complications

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SECTION 12: RED FLAGS & EMERGENCIES

Acute Emergencies

DKA vs HHS Comparison

Admission Criteria

• Any patient with DKA or HHS
• Severe hypoglycaemia not responding to oral glucose
• Blood glucose >25 mmol/L with symptoms
• New-onset T1DM
• Diabetic foot with cellulitis or osteomyelitis
• Altered consciousness, vomiting (unable to take medications)

Referral Criteria

• T1DM → Endocrinology/Diabetologist always
• T2DM not at target after 3 months of dual therapy → Diabetologist
• Diabetic retinopathy → Ophthalmology
• Diabetic nephropathy (proteinuria/declining eGFR) → Nephrology
• Diabetic foot → Vascular Surgery + Podiatry + Orthopaedics (MDT)
• HbA1c >10% at diagnosis with symptoms → Endocrinology early

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SECTION 13: MANAGEMENT

Treatment Goals (ADA 2024 / Harrison's 22E)

Important update from Harrison's 22E (2025): Near-normal glycemia (very tight HbA1c <6%) is NOT the goal in patients with established ASCVD (cardiovascular disease), as it increases hypoglycemia risk and cardiovascular events. GLP-1 agonists and SGLT-2 inhibitors now take priority in patients with CVD, heart failure, or CKD regardless of HbA1c.

Non-Drug Treatment (Lifestyle - The Foundation of All DM Care)

• Reduce total carbohydrate intake - especially refined sugars
• Mediterranean or DASH diet preferred (reduces cardiovascular risk)
• Increase dietary fiber (slows glucose absorption)
• Reduce saturated fats (reduces cardiovascular risk)
• Caloric restriction if overweight (aim 500 kcal/day deficit)
• Avoid sugar-sweetened beverages

• Aerobic exercise: 150 min/week of moderate intensity (brisk walking, cycling, swimming)
• Resistance training: 2-3x/week (increases insulin sensitivity in muscle)
• Reduces HbA1c by ~0.5-1%
• Reduces cardiovascular risk

• 5-10% weight loss improves insulin sensitivity significantly
• 10%+ weight loss can achieve remission of T2DM in some patients
• Bariatric surgery: most effective intervention for T2DM in obese patients (BMI >35 with DM, or >30 with poor control)

• Smoking worsens insulin resistance and cardiovascular risk dramatically

• Glucose monitoring technique
• Insulin injection technique
• Hypoglycemia recognition and treatment
• Foot care (daily inspection, proper footwear)
• Sick day rules

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SECTION 14: DRUG TREATMENT & PHARMACOLOGY

Drug Classes Overview

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1. METFORMIN (Biguanide) - First-Line Agent

Clinical Pearl: Metformin is the ONLY oral antidiabetic shown to reduce cardiovascular mortality (UKPDS 1998). It should be started at diagnosis of T2DM unless contraindicated.

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2. SULFONYLUREAS (e.g., Glibenclamide, Gliclazide, Glimepiride)

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3. GLP-1 RECEPTOR AGONISTS (e.g., Semaglutide, Liraglutide, Dulaglutide, Exenatide)

• Stimulate insulin secretion only when glucose is elevated (glucose-dependent → low hypoglycemia risk)
• Suppress glucagon
• Slow gastric emptying (reduces post-meal glucose spike; causes satiety)
• Act on brain to reduce appetite

2025 Update (Harrison's 22E): GLP-1 receptor agonists are now first-choice add-on therapy (or may even replace metformin as first-line) in T2DM patients with established ASCVD, heart failure, or CKD, given their cardiovascular and renal protective effects.

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4. SGLT-2 INHIBITORS (e.g., Empagliflozin, Dapagliflozin, Canagliflozin)

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5. DPP-4 INHIBITORS (Gliptins - e.g., Sitagliptin, Vildagliptin, Saxagliptin)

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6. THIAZOLIDINEDIONES / TZDs (e.g., Pioglitazone)

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7. ALPHA-GLUCOSIDASE INHIBITORS (e.g., Acarbose)

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8. INSULIN THERAPY

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SECTION 15: TREATMENT ALGORITHM

Type 2 DM Treatment Algorithm (ADA 2024 / Harrison's 22E 2025)

STEP 1: DIAGNOSIS OF TYPE 2 DM
            ↓
    Lifestyle modification (diet + exercise) + METFORMIN
    (Start metformin at diagnosis unless contraindicated)
            ↓
    At 3 months: HbA1c still not at target?
            ↓
STEP 2: ADD SECOND AGENT based on clinical profile:

  ┌─────────────────────────────────────────────────────┐
  │  Does patient have:                                 │
  │  • Established CVD, HF, or high CV risk?           │
  │    → ADD GLP-1 RA (semaglutide/liraglutide)        │
  │       or SGLT-2 inhibitor (empagliflozin)          │
  │                                                     │
  │  • Chronic Kidney Disease?                          │
  │    → ADD SGLT-2 inhibitor (dapagliflozin)          │
  │       + finerenone (if proteinuric)                 │
  │                                                     │
  │  • Need to MINIMIZE weight gain?                    │
  │    → GLP-1 RA (most weight loss) or SGLT-2i        │
  │                                                     │
  │  • Need to MINIMIZE hypoglycemia?                   │
  │    → DPP-4i, GLP-1 RA, or SGLT-2i                 │
  │                                                     │
  │  • Cost is primary concern?                         │
  │    → Sulfonylurea (gliclazide) or TZD              │
  └─────────────────────────────────────────────────────┘
            ↓
    At 3-6 months: still not at target?
            ↓
STEP 3: TRIPLE THERAPY
    (Metformin + GLP-1 RA + SGLT-2i, or
     Metformin + Sulfonylurea + DPP-4i, etc.)
            ↓
    Still not at target after triple therapy OR
    HbA1c >10% at diagnosis / Type 1 DM?
            ↓
STEP 4: ADD INSULIN
    (Usually basal insulin first: Glargine or Degludec)
    If still not controlled → Basal-Bolus regimen

Type 1 DM Algorithm

DIAGNOSIS OF TYPE 1 DM
        ↓
INSULIN THERAPY - MANDATORY (not optional)
        ↓
Basal-Bolus (MDI) or CSII (pump) regimen
        ↓
CGM (Continuous Glucose Monitoring) recommended for all T1DM
        ↓
AID (Automated Insulin Delivery) = Gold standard if accessible
        ↓
Target: HbA1c <7% without undue hypoglycemia
        ↓
Annual screening: Eyes, Kidneys, Feet, BP, Lipids

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SECTION 16: REAL-WORLD CLINICAL APPROACH

OPD (Outpatient) Approach - The Diabetic Review

1. Ask about symptoms: hypoglycemia episodes, hyperglycemia symptoms, foot problems, visual changes
2. Check weight, BMI, blood pressure
3. Review medications - adherence, side effects
4. Review self-monitored glucose diary or CGM data
5. Check HbA1c result
6. Examine feet (inspection + monofilament + pulses)

1. All of the above
2. Dilated fundoscopy (refer ophthalmology)
3. Urine ACR + serum creatinine/eGFR
4. Full lipid profile
5. 12-lead ECG
6. Detailed foot examination
7. Review vaccination status (influenza, pneumococcal, COVID-19)
8. Dental check reminder

Emergency Approach - DKA Protocol (Harrison's 22E)

Patient presents with: N&V, abdominal pain, fruity breath,
Kussmaul breathing, dehydration, altered consciousness
        ↓
1. STABILIZE: Airway, Breathing, Circulation
2. BLOODS: ABG, glucose, ketones, electrolytes (especially K+), 
   renal function, FBC, blood cultures, lactate
3. URINE: dipstick for ketones and glucose; culture if infection
4. ECG: hyperkalemia changes?
        ↓
DKA CONFIRMED (Glucose >13.9 + Ketones >3 + pH <7.3)
        ↓
IV FLUIDS: 0.9% NaCl - 1L over 1h → 1L over 2h → 1L over 4h → 
           reassess (total ~3-6L first 24h)
        ↓
INSULIN: Fixed-rate IV insulin infusion (FRIII) at 0.1 units/kg/hr
         (only start insulin once K+ >3.5 mmol/L)
        ↓
POTASSIUM REPLACEMENT: K+ falls as acidosis corrected
  - K+ 3.5-5.5: add 40 mmol/L to each bag
  - K+ <3.5: DO NOT start insulin - correct K+ first
        ↓
MONITOR: Glucose every hour, electrolytes every 2-4h, ketones every 2h
TARGET: Reduce ketones >0.5 mmol/L/hour; glucose fall 3-5 mmol/L/hr
        ↓
SWITCH to subcutaneous insulin when: 
  ketones <0.6 mmol/L + pH >7.3 + patient eating
        ↓
FIND AND TREAT PRECIPITANT (infection is #1 cause)

Common Mistakes to Avoid

Clinical Pearls

• "Start low, go slow" with metformin to minimize GI side effects
• ACE inhibitor or ARB is the BP drug of choice in diabetic nephropathy (reduces proteinuria)
• Statin is indicated for virtually all DM patients >40 years regardless of cholesterol level
• Aspirin 75-100 mg daily for secondary prevention if established CVD (not for primary prevention in DM)
• A patient on insulin who is "unconscious" should get IV dextrose immediately - always check glucose first
• Microalbuminuria is the earliest detectable sign of diabetic nephropathy - screen annually
• Pregnancy: Switch oral agents to insulin; tight glycemic control (HbA1c <6.5% pre-conception)

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SECTION 17: PRESCRIPTION EXAMPLES

Example 1: Newly Diagnosed T2DM (Overweight, HbA1c 8.5%, no CVD)

Rx
1. Metformin 500 mg tablet - 1 tablet twice daily with meals 
   (increase to 1000 mg BD after 2 weeks if tolerated)
   
2. Atorvastatin 20 mg tablet - 1 tablet at night
   (all diabetic patients >40 yrs benefit from statin)
   
3. Amlodipine 5 mg tablet - 1 tablet once daily 
   (if BP >130/80 mmHg)

Lifestyle advice: 
  - Reduce carbohydrate intake, increase vegetables and fiber
  - 30 minutes brisk walk 5 days/week
  - Target weight loss: 5-10%

Follow-up: 3 months with HbA1c, eGFR, urine ACR, BP

Example 2: T2DM with Established CVD (Post-MI, HbA1c 8%, on Metformin)

Rx - Add:
1. Empagliflozin (Jardiance) 10 mg tablet - 1 tablet once daily 
   in morning
   [SGLT-2i - CV mortality benefit proven in EMPA-REG OUTCOME]
   
   OR
   
   Semaglutide (Ozempic) 0.5 mg injection - once weekly subcutaneous
   (increase to 1 mg at 4 weeks if tolerated)
   [GLP-1 RA - CV benefit proven in SUSTAIN-6]

Continue: Metformin 1000 mg BD
Ensure: Aspirin 75 mg OD (secondary prevention), Statin, ACEi/ARB

Example 3: Type 1 DM - Basal-Bolus Insulin

Rx
1. Insulin Glargine (Lantus) 20 units subcutaneous injection 
   - at bedtime (BASAL)
   [Dose: ~0.2 units/kg/day as starting dose for basal]

2. Insulin Aspart (NovoRapid) 4-6 units subcutaneous injection 
   - before each main meal (BOLUS)
   [Adjust dose based on carbohydrate content: ~1 unit per 10g carbs]

3. Blood glucose monitoring: Before meals and 2h after meals
   OR Freestyle Libre (CGM) - preferred

4. Glucagon Emergency Kit 1 mg - IM injection for severe hypoglycaemia
   [Educate patient AND family member on administration]

Follow-up: 3 months - HbA1c, annual complications screen

Common Prescribing Errors

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SECTION 18: PREVENTION

Primary Prevention of Type 2 DM

Screening Recommendations (ADA 2024)

• Screen all adults ≥35 years with FPG or HbA1c every 3 years
• Screen younger adults if BMI ≥25 (≥23 in Asians) + any risk factor
• Screen all pregnant women at 24-28 weeks gestation for GDM

Secondary Prevention (Preventing Complications)

• Tight glycemic control (HbA1c <7%)
• Blood pressure control (<130/80 mmHg) - most important for microvascular
• Statin therapy - most important for macrovascular
• ACE inhibitor/ARB - for nephropathy prevention/progression
• Annual complication screening
• Smoking cessation
• Foot care education

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SECTION 19: PROGNOSIS

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SECTION 20: PATIENT COUNSELING

What to Tell Your Patient

"Your blood sugar is higher than normal because your body is not using insulin properly. Sugar is building up in your blood instead of going into your cells for energy. Over time, this high sugar damages blood vessels and nerves throughout your body."

"Yes, but it is very manageable. With the right treatment and lifestyle changes, most people with diabetes live a full, healthy life. The key is consistency - taking your medications, watching your diet, and exercising regularly."

1. Diet: "Reduce rice, bread, sweets, and sugary drinks. Fill half your plate with vegetables. Eat at regular times."
2. Exercise: "Walk briskly for at least 30 minutes most days. Exercise helps sugar enter your cells without needing insulin."
3. Medications: "Take your tablets every day with food. Do not skip doses. Even when you feel well, diabetes is still present."
4. Monitoring: "Check your blood sugar regularly as instructed. Keep a record. Bring it to every appointment."
5. Hypoglycemia (low blood sugar): "If you feel shaky, sweaty, dizzy or confused - this is a 'hypo.' Immediately eat 15g of fast sugar (3 glucose tablets, half a can of regular soda, or a glass of fruit juice). Tell someone nearby."
6. Foot care: "Check your feet EVERY DAY. Look for cuts, blisters, redness, or swelling. Never walk barefoot. Wear comfortable, well-fitting shoes. Come to clinic immediately if you see any foot problems."
7. Eye and kidney checkups: "You must have your eyes checked by an eye doctor at least once a year, even if your vision seems fine. We will also check your kidneys and urine once a year."
8. Sick day rules (T1DM patients especially): "If you are ill - vomiting, fever, or unable to eat - DO NOT stop your insulin. Your body needs more insulin when sick. Check your blood sugar and ketones every 2-4 hours and call us."
9. Pregnancy: "If you plan to become pregnant, tell us in advance. We need to get your HbA1c below 6.5% before conception, and some medications must be changed."
10. Driving: "If you take insulin or sulfonylureas, check your blood sugar before driving. Do not drive if glucose is <5 mmol/L."

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SUMMARY TABLE - The Diabetic Patient in One Page

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• Harrison's Principles of Internal Medicine 22E (2025), McGraw-Hill - Chapters 416, 417
• Guyton and Hall Textbook of Medical Physiology, Chapter 79
• Lippincott Illustrated Reviews: Pharmacology, Chapter 24
• Tietz Textbook of Laboratory Medicine, 7th Edition - Chapter on Diabetes
• ADA Standards of Medical Care in Diabetes 2024 (Diabetes Care 47 Suppl 1)
• Goldman-Cecil Medicine - Chapter on T2DMThis comprehensive guide covers all 20 sections you requested. Here is a brief overview of what was covered and the key learning points to remember:

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How to Use This Guide

1. Diagnosis requires confirmation - two abnormal results on separate days (except symptomatic random glucose ≥11.1 mmol/L)
2. HbA1c reflects 3 months - but unreliable in haemolytic anaemia, haemoglobinopathies, and pregnancy
3. Metformin is first-line for T2DM - but GLP-1 agonists/SGLT-2 inhibitors are now co-first-line if CVD, HF, or CKD is present (ADA 2024/Harrison's 22E)
4. DKA = NEVER stop insulin - even if patient is vomiting; start IV fluids first, then insulin only after confirming K+ >3.5
5. T1DM always needs insulin - no oral agents are adequate for T1DM monotherapy
6. The three pillars of DM management: Glycemia (HbA1c), Blood Pressure, and Lipids (Statin) - all three must be managed, not just glucose
7. The diabetic foot is an emergency - even a small ulcer with cellulitis needs urgent MDT review; osteomyelitis can lead to amputation within days if missed