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Determining the type of diabetes is important because it can greatly alter the management
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It is likely that at least 5 - 10% of adult-onset diabetes is not due to type 2 diabetes
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Most of these cases are misdiagnosed as type 2 diabetes resulting in suboptimal care
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It is important to consider other types of diabetes in all patients including:
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Type 1 diabetes may occur at any age with an insidious onset and should be considered more likely if any of the following:
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Symptoms of insulin deficiency at diagnosis e.g. polyuria, polydipsia, weight loss
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Positive capillary ketones/ketonuria at diagnosis
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Normal or low BMI at diagnosis
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Personal or family history of autoimmune disease
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Family history of type 1 diabetes
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Positive Anti-GAD, anti-IA2 or anti-ZnT8 antibody titres are present in > 85% of patients with type 1 diabetes at diagnosis
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Low C-peptide (e.g. fasting < 250 pmol/L or random/post-meal < 600 pmol/L) with a paired glucose > 8 mmol/L within 5 years of diagnosis
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Usually associated with clear precipitant and loss of exocrine pancreatic function but should consider if:
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History of pancreatitis, cystic fibrosis or pancreatic surgery, cancer or trauma
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Diarrhoea and/or features of malabsorption
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Low faecal elastase
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Often never diagnosed, but usually presents in young adults with a strong family history of diabetes and should consider if:
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Family history of monogenic diabetes
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Onset of diabetes < 35 years of age (particularly if < 6 months of age please refer all cases for genetic testing)
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First degree relative(s) with onset of diabetes < 35 years of age (particularly multiple generations)
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Exquisite sensitivity to sulfonylureas
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Associated features such as sensorineural deafness with mitochondrial disease (subtypes only)
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Diagnosis confirmed by gene testing – see NZSSD guidelines on testing for monogenic diabetes
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A careful history and examination should be performed to exclude secondary causes of diabetes including:
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Medications e.g. corticosteroids, anti-psychotics, thiazides, calcineurin inhibitors (post transplant) etc.
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Cushing’s syndrome
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Thyrotoxicosis
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Acromegaly
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Hyperandrogenism in women
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Haemochromatosis
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In addition to a history and examination, investigations that may be helpful if suspicious of other types of diabetes include:
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Point of care capillary ketone levels (may be high in type 1 diabetes or loss of pancreatic function)
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Anti-GAD and anti-IA2 +/- anti-ZnT8 antibodies (positive in type 1 diabetes)
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C-peptide and glucose (C-peptide may be low in type 1 diabetes or loss of pancreatic function)
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Faecal elastase (if diarrhoea to exclude loss of pancreatic function)
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β-hcg (if pregnancy is a possibility to exclude gestational diabetes)
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NB: Please discuss with secondary care if suspicion of type 1 diabetes, diabetes due to loss of pancreatic function, or monogenic diabetes.