<p>Diabetes mellitus is a chronic metabolic disorder characterised by impaired glucose homeostasis, resulting in persistent hyperglycaemia. Accurate and prompt diagnosis is essential for early intervention and prevention of long-term complications. Fasting blood glucose levels have traditionally been central to the diagnosis and monitoring of diabetes mellitus. However, growing evidence highlights the clinical relevance of non-fasting glucose measures, including postprandial glucose, random plasma glucose, and glycated haemoglobin (HbA1c) levels. Practical challenges, safety concerns, and evolving insights into glucose physiology have prompted renewed interest in flexible and context-driven approaches to glucose testing. This narrative review examines the physiological basis of fasting and non-fasting glucose regulation and critically evaluates their roles in diabetes screening, diagnosis, and monitoring. It also discusses the strengths and limitations of measuring fasting blood glucose, oral glucose tolerance, HbA1c, random plasma glucose, postprandial glucose, and continuous glucose monitoring. Special attention is given to pre-analytical and practical considerations, patient safety, and the ability of non-fasting measures to capture early metabolic dysfunction and real-world glycaemic exposure. This article reviews evidence supporting the prognostic value of postprandial hyperglycaemia and the expanding role of non-fasting monitoring tools. Non-fasting glucose testing offers substantial advantages in terms of accessibility, safety, and clinical relevance, and is well-suited for population screening and routine diabetes monitoring. Fasting glucose testing remains essential for specific diagnostic and research applications, particularly when strict metabolic standardisation is required. A context-driven framework that prioritises non-fasting approaches while reserving fasting tests for targeted indications provides a balanced and patient-centred strategy for contemporary diabetes care.</p>