Blood glucose is the concentration of glucose dissolved in the circulating blood. Glucose is a six-carbon carbohydrate and one of the body's principal metabolic fuels. It comes primarily from digestion and absorption of carbohydrate, although the liver and kidneys can also produce glucose through glycogenolysis and gluconeogenesis. The bloodstream distributes glucose to tissues throughout the body, including the brain, skeletal muscle and red blood cells.
Blood glucose is continuously regulated because both excessively low and excessively high concentrations can be harmful. Insulin, glucagon, adrenaline, cortisol, growth hormone and other hormonal systems participate in this regulation. In everyday clinical language, “blood glucose”, “blood sugar” and “blood glucose level” are often used interchangeably, although glucose is chemically distinct from the many other sugars found in food.
After a carbohydrate-containing meal, digestion breaks starches and disaccharides into absorbable monosaccharides, with glucose entering the portal circulation and reaching the liver. Rising blood glucose stimulates pancreatic beta cells to release insulin. Insulin promotes glucose uptake in skeletal muscle and adipose tissue, stimulates glycogen synthesis and suppresses hepatic glucose output. Some tissues, including the brain, use glucose without requiring insulin for transport into their cells, although insulin indirectly affects whole-body glucose availability.
Between meals, insulin concentrations fall and glucagon rises, stimulating hepatic glycogen breakdown and gluconeogenesis. During prolonged fasting, additional metabolic pathways maintain glucose supply. This dynamic system means blood glucose is not supposed to remain at one fixed number. It normally rises after eating and declines as glucose is taken up, stored or metabolised. The clinically important issue is whether these fluctuations remain within physiologically appropriate ranges and recover appropriately.
Blood glucose is commonly reported in the United Kingdom in millimoles per litre, abbreviated mmol/L. The United States commonly uses milligrams per decilitre, abbreviated mg/dL. The approximate conversion is mg/dL divided by 18 to obtain mmol/L, while mmol/L multiplied by 18 gives mg/dL. Thus 7.0 mmol/L is approximately 126 mg/dL, 5.6 mmol/L approximately 100 mg/dL and 11.1 mmol/L approximately 200 mg/dL.
Clinical interpretation must always take the unit into account because the same numerical value has very different meanings in the two systems. Blood glucose can be measured from venous plasma in a laboratory, capillary blood using a finger-prick meter, or indirectly through interstitial fluid using a continuous glucose monitor. These measurements are related but are not technically identical and may differ slightly, particularly when glucose is changing rapidly.
The timing of a blood glucose measurement is essential to interpretation. A fasting glucose is measured after an overnight period without caloric intake and provides information about baseline glucose regulation. Pre-meal or preprandial glucose indicates the concentration before eating. Postprandial glucose refers to the concentration after a meal, with clinical guidance often specifying one- or two-hour measurements. Random glucose can be taken at any time and is particularly useful when symptoms suggest hyperglycaemia.
A person can have a normal fasting glucose while experiencing excessive post-meal elevations, or a high fasting glucose with relatively modest post-meal changes. This is one reason why HbA1c, fasting glucose and oral glucose tolerance testing provide complementary information. A single glucose value without knowing when the person last ate, what treatment they use and whether they are ill is therefore difficult to interpret accurately.
Blood glucose changes in response to food, physical activity, insulin, medicines, hormones, illness, stress and sleep. Carbohydrate quantity and type influence the speed and magnitude of post-meal glucose elevation, while physical activity can increase glucose uptake by skeletal muscle independently of some insulin signalling. During infection or physiological stress, hormones such as cortisol and adrenaline can increase glucose production and reduce insulin sensitivity.
Alcohol can produce complex effects because it can inhibit hepatic glucose production and increase hypoglycaemia risk in people using insulin or certain medicines, particularly when food intake is inadequate. Menstrual cycles, pregnancy and other hormonal changes can also affect insulin sensitivity. These variations explain why diabetes treatment is often adjusted over time and why a glucose reading should be interpreted as a measurement within a physiological context rather than as a permanent property of a person.
Blood glucose differs from HbA1c, although the two measurements are closely related. A glucose test measures the concentration of glucose at a particular moment, whereas HbA1c reflects the proportion of haemoglobin molecules that have become glycated during the lifespan of red blood cells and therefore approximates average glycaemic exposure over roughly two to three months. HbA1c is useful because it smooths out daily fluctuations, but that is also its limitation: it can conceal substantial peaks and lows.
Continuous glucose monitoring provides much more frequent measurements and can reveal time in range, glucose variability and trends that HbA1c cannot show. A person with a given HbA1c can therefore have very different glucose patterns from another person with the same HbA1c. For treatment decisions, clinicians increasingly consider both average glycaemia and glucose variability, especially in people receiving insulin.
Blood glucose becomes clinically important when it is persistently too high or too low. Hyperglycaemia refers to glucose above the individual's appropriate range and is central to diabetes. Chronic hyperglycaemia contributes to glycation of proteins, oxidative stress, endothelial dysfunction and vascular injury, which together contribute to diabetic complications. Hypoglycaemia is generally defined in diabetes care as glucose below 3.9 mmol/L, although thresholds can vary slightly according to guideline and context.
It can impair brain function because the central nervous system has a high dependence on glucose availability. Early symptoms include autonomic responses such as sweating, shaking, hunger and palpitations, followed by neuroglycopenic symptoms such as confusion, abnormal behaviour, seizures and unconsciousness if glucose falls severely. Treatment therefore seeks an appropriate range rather than simply maximising the amount of glucose lowering.
Blood glucose measurement has become central to diabetes diagnosis and self-management. Laboratory plasma glucose testing is used to establish diagnostic thresholds because laboratory methods are standardised and quality controlled. Home meters allow people to make treatment decisions using small capillary blood samples, while continuous glucose monitors provide trend information through sensors placed under the skin. None of these measurements should be interpreted without considering device limitations, calibration, timing and clinical circumstances.
Rapid changes can create temporary differences between blood and interstitial glucose because glucose must equilibrate between compartments. Poor peripheral circulation, contaminated fingers, inappropriate technique and device-specific factors can also affect some readings. The purpose of monitoring is therefore not simply to collect numbers but to use reliable measurements to understand glucose physiology and guide treatment safely.
