Diabetes mellitus is one of the most common conditions encountered in clinical practice and represents a significant burden on the health systems of the developed world. It is now estimated that 8% of the total NHS budget is now spent on managing patients with diabetes mellitus.


Type 1 diabetes mellitus (T1DM)Autoimmune disorder where the insulin-producing beta cells of the islets of Langerhans in the pancreas are destroyed by the immune system
This results in an absolute deficiency of insulin resulting in raised glucose levels
Patients tend to develop T1DM in childhood/early adult life and typically present unwell, possibly in diabetic ketoacidosis
Type 2 diabetes mellitus (T2DM)This is the most common cause of diabetes in the developed world. It is caused by a relative deficiency of insulin due to an excess of adipose tissue. In simple terms there isn't enough insulin to 'go around' all the excess fatty tissue, leading to blood glucose creeping up.
PrediabetesThis term is used for patients who don't yet meet the criteria for a formal diagnosis of T2DM to be made but are likely to develop the condition over the next few years. They, therefore, require closer monitoring and lifestyle interventions such as weight loss
Gestational diabetesSome pregnant develop raised glucose levels during pregnancy. This is important to detect as untreated it may lead to adverse outcomes for the mother and baby
Maturity onset diabetes of the young (MODY)A group of inherited genetic disorders affecting the production of insulin. Results in younger patients developing symptoms similar to those with T2DM, i.e. asymptomatic hyperglycaemia with progression to more severe complications such as diabetic ketoacidosis
Latent autoimmune diabetes of adults (LADA)The majority of patients with autoimmune-related diabetes present younger in life. There are however a small group of patients who develop such problems later in life. These patients are often misdiagnosed as having T2DM
Other typesAny pathological process which damages the insulin-producing cells of the pancreas may cause diabetes to develop. Examples include chronic pancreatitis and haemochromatosis.

Drugs may also cause raised glucose levels. A common example is glucocorticoids which commonly result in raised blood glucose levels

Clinical features

The presentation of diabetes mellitus depends very much on the type:

Type 1 DMType 2 DM
Weight loss

May present with diabetic ketoacidosis
  • abdominal pain
  • vomiting
  • reduced consciousness level
Often picked up incidentally on routine blood tests

Remember that the polyuria and polydipsia are due to water being 'dragged' out of the body due to the osmotic effects of excess blood glucose being excreted in the urine (glycosuria).


The diagnostic criteria are determined by WHO.

If the patient is symptomatic:
  • fasting glucose greater than or equal to 7.0 mmol/l
  • random glucose greater than or equal to 11.1 mmol/l (or after 75g oral glucose tolerance test)

If the patient is asymptomatic the above criteria apply but must be demonstrated on two separate occasions.

In 2011 WHO released supplementary guidance on the use of HbA1c for the diagnosis of diabetes:
  • a HbA1c of greater than or equal to 6.5% (48 mmol/mol) is diagnostic of diabetes mellitus
  • a HbAlc value of less than 6.5% does not exclude diabetes (i.e. it is not as sensitive as fasting samples for detecting diabetes)
  • in patients without symptoms, the test must be repeated to confirm the diagnosis
  • it should be remembered that misleading HbA1c results can be caused by increased red cell turnover


Maturity-onset diabetes of the young
Type 1 diabetes mellitus
Type 2 diabetes mellitus