Diabetes – diagnosis

Diagnosis, classification and screening of diabetes mellitus. For ongoing management see Type 2 Diabetes Management; for the type-1-specific picture see Type 1 diabetes; for the prediabetes/impaired-glucose group see Prediabetes.

Classification

Type 1 (previously juvenile-onset / insulin-dependent) — T-cell-mediated autoimmune destruction of pancreatic beta cells causing rapid progression to absolute insulin deficiency; needs insulin replacement. About 80% are positive for glutamic acid decarboxylase (GAD) or IA-2 antibodies; a positive antibody gives first-degree relatives a roughly threefold higher risk.

• LADA (latent autoimmune diabetes in adults) — a slowly-progressive form of type 1 presenting around 30–40 years, often with a personal/family history of other autoimmune disease. GAD/IA-2 antibodies confirm it; managed as type 1.

Type 2 (maturity-onset / non-insulin-dependent) — insidious onset with insulin resistance and relative insulin deficiency. At any time more than half of people with diabetes are undiagnosed, and about 35% of the newly diagnosed already have complications. Increasingly prevalent with an ageing, overweight population.

Secondary diabetes

Category	Causes
Pancreatic (type 3c)	Chronic pancreatitis, haemochromatosis, cystic fibrosis, pancreatic surgery/cancer
Endocrine	Cushing syndrome, acromegaly, phaeochromocytoma, PCOS
Drug-induced (often transient)	Corticosteroids, thiazides, high-dose oestrogen (not low-dose HRT), atypical antipsychotics
Other transient	Gestational diabetes; medical or surgical stress

Clinical features

Classic symptoms of uncontrolled diabetes: polyuria, polydipsia, weight loss (especially type 1), tiredness/fatigue, and a propensity for infections (skin, genital thrush).

Murtagh DxT — thirst + polyuria + weight loss ➜ type 1 diabetes. A young person with type 1 typically presents with a brief 2–10 week history of this triad.

Late features: blurred vision, polyneuropathy, erectile dysfunction, and arterial disease (IHD, PVD).

Investigations and diagnostic thresholds

Initial test is a fasting or random blood glucose, with an OGTT or HbA1c as follow-up if indicated. Add tests for cardiovascular risk and end-organ damage (lipids, kidney function, urine ACR, ECG) once diabetes is confirmed.

Diabetes is diagnosed by:

• Symptomatic (≥2 of polydipsia, polyuria, recurrent skin infection or genital thrush) — a single diagnostic value:
  • fasting venous glucose ≥ 7.0 mmol/L, or
  • random venous glucose (≥2 h after eating) ≥ 11.1 mmol/L, or
  • HbA1c ≥ 6.5% (48 mmol/mol).
• Asymptomatic — two separate elevated values (fasting, 2-hour post-prandial, HbA1c, or the two OGTT values).
• Suspected type 1 / juvenile onset — send GAD and IA-2 antibodies.

The 2-hour value on a 75 g OGTT (≥11.1 mmol/L) remains the gold standard for uncertain cases; reserve it for true borderline results and gestational diabetes. If a random/fasting glucose sits in the uncertain 5.5–11.0 mmol/L range in a symptomatic or at-risk patient, proceed to an OGTT (testing threshold 5.5 mmol/L). Urinalysis is unreliable for diagnosis — glycosuria appears at different plasma glucose levels depending on renal threshold.

Prediabetes — fasting glucose 6.1–6.9 mmol/L (impaired fasting glucose) or impaired glucose tolerance on OGTT, not meeting diabetes criteria. It is not a trigger for medication but raises the urgency of weight loss and physical activity — see Prediabetes.

Screening (type 2)

Optimal frequency is every 3 years from age 40 using AUSDRISK, and annually in very high-risk groups. Screen earlier/more often with:

• Impaired fasting glucose or impaired glucose tolerance (prediabetes)
• Age >30 with a first-degree relative with type 2, obesity (BMI >30), or a high-prevalence ethnic background (Aboriginal and Torres Strait Islander, Pacific Islander, Indian subcontinent, Chinese, African/Caribbean)
• Age >18 in Aboriginal and Torres Strait Islander people
• Previous gestational diabetes; PCOS (especially if overweight)
• Long-term corticosteroids or antipsychotics
• Previous cardiovascular event; hypertension/IHD