14th September 2024

Diagnosis

The diagnostic criteria for type 2 diabetes includes the following:

• A fasting plasma glucose of 7 mmol/l or greater
• A random blood sugar 11.1mmol/l or greater
• A HbA1c of 48 mmol/mol (6.5%) or greater
• A plasma glucose of 11.1mmol/l or greater after two hours of oral 75g glucose in the form of the tolerance test

If the patient is symptomatic, NICE advise that a single abnormal HbA1c or fasting plasma glucose result can be used for diagnosis, but it is sensible to repeat the test. In asymptomatic patients, do not diagnose based on a single abnormal result. Repeat testing is required. If the repeat test is normal, arrange for interval testing.

HbA1c is a minor haemoglobin component formed from the chemical condensation of glucose and haemoglobin. It forms slowly and is directly proportional to serum glucose concentration. The normal life of a red blood cell is 120 days. If a larger number of red cells survive for longer periods of time, then they will have more time to form HbA1c. Therefore, factors that reduce the destruction of red cells (such as splenectomy) will falsely increase the concentration of HbA1c. Factors that reduce red cell production (such as B12, folate, and iron deficiency anaemias, as well as aplastic anaemia) will reduce the number of new red cells without HbA1c, thus falsely increasing the concentration of HbA1c. Conversely, reticulocytosis (as seen in hemolytic anaemia or blood loss) and reduced red cell lifespan (as in splenomegaly or pregnancy) will falsely decrease HbA1c concentration. For unknown reasons, chronic alcohol consumption can also falsely increase HbA1c concentrations. Haemoglobinopathies cause unpredictable changes in HbA1c levels. In these circumstances, it might be better to use fructosamine levels or average glucose levels.