HbA1c forms when red cells are exposed to glucose in the plasma and the test reflects average plasma glucose over the previous eight to 12 weeks. The HbA1c test can be performed at any time of the day without any special preparation and thus avoids the need to fast and follow prior dietary requirements, unlike the oral glucose tolerance test (OGTT). 

HbA1c can be affected by genetic, haematological (e.g. anaemia) and illness-related (e.g. malaria) factors so people with such conditions have red blood cells with an altered life span. HbA1c is therefore not a reliable diagnostic test and fasting glucose testing is recommended in such individuals.

People are classified into moderate risk, high risk or possible type 2 diabetes depending on the fasting plasma glucose or HbA1c result. Those with a high-risk score meet the criteria of non-diabetic hyperglycaemia. 

Individual risk factors should be discussed with those identified as moderate or high risk, with identification of the modifiable factors and how the risks can be reduced through lifestyle changes and evidence-based behaviour change services such as walking programmes and structured weight loss programmes.

People at high risk are invited to at least 13 education and exercise sessions conducted over a minimum of nine months and should have their blood glucose levels checked annually. Current providers of the NHS England DPP include Reed Momenta, WW (formerly Weight Watchers), Changing Health and Liva Healthcare.