People identified with known risk factors associated with the development of type 2 diabetes should be followed up with further diagnostic tests. The aim of the blood test is to check if the person has type 2 diabetes or to confirm their level of risk of progression to type 2 diabetes and discuss how to reduce it.

An HbA1c test measures the amount of glycated haemoglobin in venous blood. As individuals do not need to fast, and the test gives an average blood glucose over the previous 2–3 months, it is the preferred test. An HbA1c level of 42–47 mmol/mol (6.0–6.4%) indicates prediabetes.³⁶

Plasma or capillary blood taken after a fast of 8–10 hours is tested in a fasting plasma glucose (FPG) test. An FPG of 6.1–6.9 mmol/L is diagnostic of prediabetes.³⁶

The 2-hour oral glucose tolerance test (OGTT) assesses the body’s ability to process a large amount of glucose. Following a fast of 8–10 hours a baseline FPG test is carried out. Then the patient is given 75 g of glucose in a solution. A second blood sample is taken 2 hours later and glucose is measured again to assess how well the patient handled the glucose load.

Blood tests should be carried out by accredited methods either within laboratories or by point-of-care testing methodologies. All methods should be monitored appropriately, and clinical governance procedures should be in place to assure the validity of the results produced. These processes must include adequate training of operators and performance of regular quality control processes.

When interpreting results, it is important to consider other clinical conditions and medicines that may cause transient hyperglycaemia, such as long-term high-dose steroid therapy. Consideration should also be given to people with haemoglobinopathies and anaemia, in whom the measurement of HbA1c may not be accurate or may need adjusted.

The following recommendations are from sections 1.4, 1.5 and 1.6 of NICE PH38: Type 2 diabetes: prevention in people at high risk.³⁵

In pregnant women an oral glucose tolerance test is acceptable as initial identification.

Primary care consultations are important opportunities to identify individuals at elevated risk and to make a shared decision on whether or not a diagnostic test is indicated.

People should be fully informed about the blood test and possible implications before consenting. It is vital that robust decision and intervention pathways are available and explained to patients when test results are discussed.

Clinical coding

Record-keeping supports following up and reassessing risk. As part of the system of record-keeping and recall, the clinical coding is essential. Following a more uniform approach nationally to primary care coding of those known to be at high risk of developing type 2 diabetes is suggested.

 

Primary care providers  should consider maintaining a register of patients with prediabetes and annually review and record their weight and risk factors. If the patient has comorbid cardiometabolic conditions these checks could be captured in the same annual review. 

 

On diagnosis, use a single Read code for prediabetes (C11y500 – 'pre-diabetes'), which is inclusive of prediabetes, impaired glucose tolerance, impaired fasting glycaemia and non-diabetic hyperglycaemia. In Vision use #C11y5 to locate the correct code.

The additional recall code should be used to ensure patients with prediabetes are followed up appropriately (66Az - high risk of diabetes annual review).

Testing after gestational diabetes mellitus

Clinical cut-offs for defining individuals at high risk of developing type 2 diabetes differ slightly for those who have had gestational diabetes mellitus (GDM). The following recommendations are from SIGN 171: Management of diabetes in pregnancy and are based on evidence in the post-GDM population. They should not be applied to the general population.

 

R

Advise women who were diagnosed with gestational diabetes and who have tested postnatally with an HbA1c level below 39 mmol/mol (5.7%) or a fasting plasma glucose below 6.0 mmol/L that they have a low probability of having diabetes at present, and they:

• should continue to follow the lifestyle advice (including weight management, diet and exercise) given after the birth
• will need an annual test to check that their blood glucose levels are normal.

 

R

Advise women who were diagnosed with gestational diabetes and who have tested postnatally with an HbA1c level between 39 and 47 mmol/mol (5.7% and 6.4%) or a fasting plasma glucose between 6.0 and 6.9 mmol/L that they are at high risk of developing type 2 diabetes, and offer them advice, guidance and interventions.

 

R

Advise women who were diagnosed with gestational diabetes and who have tested postnatally with an HbA1c level of 48 mmol/mol (6.5%) or a fasting plasma glucose of 7.0 mmol/L or above that they have type 2 diabetes and refer them for further care.

References

1. 35       National Institute for Health and Care Excellence (NICE). Type 2 diabetes: prevention in people at high risk. [cited 11 Sep 2024]. Available from url: https://www.nice.org.uk/guidance/ph38

   36       Davidson MB. Historical review of the diagnosis of prediabetes/intermediate hyperglycemia: Case for the international criteria. Diabetes Res Clin Pract 2022;185:109219.