In the 2nd century AD, Aretaeus the Cappadocian noted that some people with diabetes were thin, and others were overweight. In the late 19th century Lancereaux called these diabète maigre and diabète gros.1 Since then, these have been named juvenile and maturity-onset diabetes, insulin-dependent and non-insulin-dependent diabetes, type 1 and type 2 diabetes. But of course there are not just two types of diabetes – there may be thousands. In many cases, the cause of a person's diabetes is multifactorial. Sometimes the type of diabetes is unclear. Even when it is clear, diabetes type may be wrongly recorded.2 Don’t miss less common variants of diabetes, especially those that require different treatment or may have an unexpected course. Some diabetes variants are listed in Box 1. Harry was 16 years old when he developed polyuria and polydipsia. His mother had type 2 diabetes as did her father and six more of her relatives. Harry's mother…said: ‘we’d actually taken in a drawn-up family tree, but they didn’t do anything about it, they just said: “He's type 1”.’ Harry was started on insulin. He had great difficulty controlling his blood glucose: ‘The blood glucose levels were all over the place…I didn’t get any warnings, they were just nasty, one minute I’d be kicking a football around and the next minute I’d be flat out on the floor.’ He was often unconscious at night. Seven months after diagnosis an islet-related antibody test proved negative but his consultant still insisted Harry had type 1 diabetes. His mother suspected Harry had maturity-onset diabetes of the young (MODY). She found the expert website www.diabetesgenes.org and a blood test was arranged which confirmed that Harry had a mutation of HNF1A, a cause of MODY. HNF1A mutations cause about 50% of cases of monogenic diabetes. It is estimated that 1–2% of people with diabetes have MODY.3, 4 Harry's insulin was stopped and he was started on a small dose of gliclazide. His life was transformed – from being unsafe to drive and failing at college he passed his exams and got a place in veterinary college. A detailed account of monogenic diabetes was published in this journal recently.5 There were 31,615 children and young people with diabetes in the National Paediatric Diabetes Audit (England and Wales) in 2020–21 but only 162 (0.5%) had a diagnosis of monogenic diabetes. There should have been more.6 Do you have a patient with possible MODY? Use the MODY probability calculator (https://www.diabetesgenes. org/exeter-diabetes-app/ModyCalculator).4 A 37-year-old man, previously well, was admitted with a two-day history of diarrhoea and vomiting. On examination ‘The patient was lucid but in low spirits’; pulse 102 beats/min, respiratory rate 18 breaths/min, and blood pressure 100/58mmHg, but examination was otherwise unremarkable. His blood tests showed diabetic ketoacidosis with a glucose of 74.6mmol/L (1342mg/dl), and HbA1c 6.2% (44mmol/mol). C-peptide and antibodies (ICA, IAA and GAD) were negative. He recovered with standard diabetic ketoacidosis treatment and long-term insulin therapy.7 This man had fulminant type 1 diabetes, first described in Japan where it accounts for about 20% of newly diagnosed acute-onset type 1 diabetes.8 It is seen in other East Asian populations and occasionally in Westerners.9 It is thought that a viral infection in people with genetic susceptibility triggers a rapid immunological process that destroys the beta-cells and damages exocrine pancreas cells. Fulminant diabetes may occur in patients treated with immune checkpoint inhibitors.10 In 70% of patients the hyperglycaemia is preceded by flu-like symptoms or gastrointestinal symptoms. Fulminant diabetes can occur in pregnancy or soon after delivery. Other frequent concomitants are absent islet-related antibodies and raised serum pancreatic enzyme levels (98% of patients).11 ‘Once the diagnosis of this disease is suspected, treatment of diabetic ketoacidosis must be started immediately, as in all other cases of type 1 diabetes. Otherwise, the death of the patient is likely to occur within 24 h. All medical practitioners must remember that this extremely rapidly progressing type of diabetes does exist, and they must pay special attention not to overlook it.’12 Islet cells can suffer autoimmune damage at any age. LADA, also known as slowly evolving immune-mediated diabetes of adults, affects about 12% of people with adult-onset diabetes. Diagnostic criteria included age >30 years, autoimmune history in patient or family, weight normal or slightly overweight, measurable C-peptide, GAD antibody positive, and not needing insulin at first, although it is likely to be needed eventually. Oral agents and non-insulin treatments can be used initially, such as metformin. The possible benefits of insulin use from the start to preserve beta-cell function have yet to be confirmed. SGLT2 inhibitors may help but such patients have a risk of developing diabetic ketoacidosis. It is suggested that C-peptide is monitored, especially if non-insulin treatment has to be increased to reduce hyperglycaemia. Patient and clinician education is important as illness or surgery, for example, may precipitate ketoacidosis. A management strategy is suggested in a consensus statement from an international expert panel.13 A previously well man in his forties of Central African ethnicity with no family history of diabetes had had polyuria and polydipsia for two months. His weight fell from 106kg to 91kg. On admission he looked well but his plasma glucose was 40mmol/L (720mg/dl) with marked ketosis but no acidosis. He was treated with fluid replacement and insulin. Islet-related antibody tests were negative. He was discharged on a total of 82 units of insulin daily which fell to 14–20 units a week later. At three months he was experiencing frequent hypoglycaemia despite further insulin dose reduction. The insulin was stopped and sitagliptin started.14 This man had ketosis-prone type 2 diabetes, also known as Flatbush diabetes, type-J diabetes, type 1.5 diabetes, phasic diabetes, or prairie diabetes. It is common in people of African ethnicity, for example African-Americans, but can affect other ethnic groups. In sub-Saharan Africa it is estimated to account for up to 15% of new diabetes presenting to hospital. It presents with a relatively short history, in people who are overweight who often have a family history of type 2 diabetes. The presenting glucose level is usually high with ketosis or ketoacidosis; insulin treatment causes a rapid fall in glucose, and patients can stop insulin within days or weeks, sometimes needing other glucose-lowering treatment. They may have further episodes of hyperglycaemia and ketosis needing short-term insulin treatment.15 A UK community study among 206 people diagnosed with diabetes aged <45 years included detailed review of diagnosis, GAD antibodies in type 1 diabetes, and targeted genetic testing. Type 1 diabetes was confirmed in 33%, type 2 in 54%, and other diabetes subtypes in 13%. The latter comprised 9% with LADA, 3% with HNF1A MODY (from five families) and 1% with mitochondrial diabetes. Including the families tested as a result of the project ‘led to an estimated minimum population prevalence of 84 (95% CI 31–136) cases per million for HNF1A-MODY.’16 There are many diabetes variants. Remember them when you see both new and existing patients with diabetes. There are almost certainly patients with unusual diabetes variants in every clinic. Misdiagnosis of diabetes type may lead to the wrong treatment and failure to recognise potential dangers. It can cause patients much misery. Always record a family history of diabetes in people with diabetes. Listen to the patient and their family. Many people with monogenic diabetes do not need insulin. An accurate genetic diagnosis is available. Do the test! Fulminant diabetes comes on fast and can kill fast. Beware ketoacidosis in patients with a short history and a very high glucose. Autoimmune diabetes can occur at any age. Don’t miss LADA. Ketosis-prone type 2 diabetes fluctuates from very high glucose and ketosis mandating insulin to not needing insulin at all. Ketosis can recur.