What are the key biochemical markers for diagnosing diabetes mellitus, and how should they be monitored in primary care?

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 ¹. Fasting plasma glucose and OGTT remain important, especially in pregnancy and when HbA1c is unreliable or unavailable ¹,². 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 ². Serum C-peptide measurement may be used selectively to clarify diagnosis when autoantibody results are negative or uncertain ².

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 ²,³. 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) ³. 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 ²[(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 ³[(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 ².