Key biochemical markers for diagnosing diabetes mellitus include glycated haemoglobin (HbA1c), fasting plasma glucose, and oral glucose tolerance test (OGTT) results. HbA1c is the primary diagnostic marker, with a threshold of 48 mmol/mol (6.5%) or above indicating diabetes, 39–47 mmol/mol (5.7–6.4%) indicating high risk (prediabetes), and below 39 mmol/mol (5.7%) indicating low probability of diabetes NICE NG3. Fasting plasma glucose and OGTT remain important, especially in pregnancy and when HbA1c is unreliable or unavailable NICE NG3,NICE NG17. For type 1 diabetes diagnosis, clinical features such as ketosis, rapid weight loss, and diabetes-specific autoantibodies are also important, with autoantibody testing recommended to confirm diagnosis NICE NG17. Serum C-peptide measurement may be used selectively to clarify diagnosis when autoantibody results are negative or uncertain NICE NG17.
Recommended monitoring methods in primary care focus on HbA1c measurement for ongoing glycaemic control, with targets individualized but generally aiming for HbA1c levels below 48 mmol/mol (6.5%) in type 1 diabetes and around 53 mmol/mol (7.0%) in type 2 diabetes to reduce complications NICE NG17,NICE NG28. Self-monitoring of capillary blood glucose is routinely recommended for people on insulin or at risk of hypoglycaemia, but not routinely for all type 2 diabetes patients unless specific indications exist (e.g., pregnancy, hypoglycaemia risk) NICE NG28. Continuous glucose monitoring (CGM), including real-time and intermittently scanned systems, is increasingly offered to adults with type 1 diabetes based on individual preference and clinical need, improving glucose control and hypoglycaemia detection NICE NG17[Ajjan et al. 2019].
In primary care, well-controlled patients with stable diabetes may be monitored with HbA1c testing every 6 months, while those with unstable control or treatment changes require more frequent monitoring NICE NG28[Wermeling et al. 2013]. When HbA1c is unreliable due to haemoglobinopathies or altered erythrocyte turnover, alternative markers such as fructosamine or glycated albumin can be used to estimate glycaemic control trends NICE NG17.
Key References
- NG3 - Diabetes in pregnancy: management from preconception to the postnatal period
- NG17 - Type 1 diabetes in adults: diagnosis and management
- NG28 - Type 2 diabetes in adults: management
- (Wermeling et al., 2013): Six-monthly diabetes monitoring of well-controlled patients: experiences of primary care providers.
- (Ajjan et al., 2019): Continuous Glucose Monitoring: A Brief Review for Primary Care Practitioners.