GPAtlas🧭 When to suspect
Type 1 diabetes is an autoimmune condition in which the pancreatic beta cells are destroyed, leaving the body unable to produce insulin. It accounts for roughly 5–10% of all diabetes. Incidence peaks in puberty and early adulthood, but new-onset type 1 diabetes can occur at any age – so it must never be excluded on age alone.
In a child or young person, suspect type 1 diabetes when hyperglycaemia presents with the classic "4 Ts": Thirst (polydipsia), Toilet (polyuria), Tired (lethargy) and Thinner (unexplained weight loss). In an adult, suspect it when hyperglycaemia is accompanied by ketosis, rapid weight loss, onset under 50 years, BMI < 25 kg/m², or a personal or family history of autoimmune disease – while remembering that age and BMI must not be used in isolation to diagnose or exclude it. Blurred vision and recurrent infection (thrush, periodontitis) are common early clues.
Be alert that, in children, a large minority – around a quarter to over a third in UK audit data – first present in diabetic ketoacidosis rather than with the gradual 4 Ts, with the highest rates in the under-5s, who depend on carers to notice the symptoms. A new childhood diagnosis is therefore frequently a medical emergency, and delayed recognition is the single biggest avoidable harm.
Conversely, not every young person with diabetes has type 1. Consider monogenic diabetes (MODY) – autosomal dominant, non-ketotic, antibody-negative, with preserved C-peptide and a strong multi-generational family history – which is commonly mislabelled as type 1. The HNF1A and HNF4A subtypes are sulfonylurea-sensitive (some can come off insulin) and GCK MODY usually needs no treatment, so refer atypical cases for genetic testing rather than committing them to lifelong insulin.
The two priorities in primary care are to act on the emergencies – suspect diabetic ketoacidosis (DKA) whenever there is nausea, vomiting, abdominal pain or deep sighing (Kussmaul) breathing – and, in those already on insulin, to recognise and pre-empt hypoglycaemia and impaired hypoglycaemia awareness.
| Diagnostic test | Supports a diagnosis of diabetes |
|---|---|
| Random plasma glucose (with symptoms) | > 11.1 mmol/L |
| Fasting plasma glucose | ≥ 7.0 mmol/L |
| HbA1c | ≥ 48 mmol/mol (6.5%) confirms diabetes – but do not rely on HbA1c to diagnose or exclude type 1: a normal value does not exclude rapidly developing disease, and treatment must never be delayed awaiting it. |
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⚠️ Common pitfall Anchoring on type 2 diabetes in a slim or older adult and reaching for metformin. Type 1 diabetes (and slowly-evolving LADA) presents across the age range, and age or BMI alone must never decide the type. If there is ketosis or rapid weight loss, treat as type 1, start the same-day referral pathway, and do not withhold insulin. |
Source: NICE NG17 · NICE NG18
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