Beyond HbA1c: Integrating Fasting Plasma Glucose, Oral Glucose Tolerance Test Time Points and Continuous Glucose Monitoring across the Diabetes Care Continuum

Abstract

Glycated haemoglobin (HbA1c) has become the dominant summary measure of glycaemic exposure because it is convenient, standardised and linked to long-term complication risk. Its clinical authority can nevertheless obscure a basic measurement problem: HbA1c is an indirect, time-averaged signal that cannot identify fasting versus post-load dysglycaemia, reveal hypoglycaemia or describe day-to-day glucose dynamics. Fasting plasma glucose (FPG), individual oral glucose tolerance test (OGTT) time points and continuous glucose monitoring (CGM) interrogate different physiological domains and therefore should not be treated as interchangeable competitors. This critical narrative review evaluates how these measures can be integrated from risk identification and diagnosis through treatment monitoring, pregnancy, chronic kidney disease, ageing and other settings in which HbA1c may be insufficient or misleading. Literature published from 1993 to 2 June 2026 was selected through transparent searches of accessible biomedical indexes, full-text repositories and professional guidance sources, followed by citation searching and reference-level verification. The evidence supports HbA1c as an outcome-validated measure of chronic glycaemia, FPG as a pragmatic index of basal glucose regulation, and the OGTT as a challenge test capable of exposing impaired early secretion, delayed glucose disposal and post-load phenotypes missed by fasting measures. One-hour plasma glucose is increasingly supported for earlier risk stratification and diabetes detection, but threshold harmonisation, repeatability and implementation evidence remain incomplete. CGM adds clinically actionable information on time in range, time below range, time above range, variability and temporal patterns, with the strongest therapeutic evidence in insulin-treated diabetes. It is not yet a validated replacement for laboratory diagnosis. An integrated framework should therefore select the measure according to the clinical question, use discordance as a trigger for explanation rather than automatic averaging, and preserve laboratory confirmation where diagnostic classification carries durable consequences. Progress depends on outcome-anchored validation of OGTT and CGM phenotypes, interoperable data standards, equitable access and trials that test whether multi-metric decisions improve outcomes beyond HbA1c-centred care.