Glucose is the major carbohydrate present in the peripheral blood. Oxidation of glucose is the major source of cellular energy in the body. Glucose derived from dietary sources is converted to glycogen for storage in the liver or to fatty acids for storage in adipose tissue. The concentration of glucose in blood is controlled within narrow limits by many hormones, the most important of which are produced by the pancreas.
The rapid and precise manner in which fasting blood sugar levels are regulated is in marked contrast to the rapid increase in blood sugar, which occurs during ingestion of carbohydrates. A fall in blood glucose to a critical level (approximately 2.5 mM) leads to dysfunction of the central nervous system. This manifests as hypoglycaemia, and is characterised by muscle weakness, lack of coordination and mental confusion. Further decrease in blood glucose levels leads to hypoglycaemic coma. Blood glucose concentrations show intra-individual fluctuations, which are dependent on muscular activity and the time interval since food intake. These fluctuations are increased further where there is dysregulation, such as occurs in a number of pathological conditions in which blood glucose may be elevated (hyperglycaemia) or depressed (hypoglycaemia).
The most frequent cause of hyperglycemia is diabetes mellitus resulting from a deficiency in insulin secretion or action. This disease is characterised by the elevation of blood glucose to such an extent that the renal threshold is exceeded and sugar appears in the urine (glycosuria). A number of secondary factors also contribute to elevated blood glucose levels. These include pancreatitis, thyroid dysfunction, renal failure, and liver disease.
Hypoglycemia is less frequently observed. A variety of conditions may cause low blood glucose levels such as insulinoma, hypopituitarism, or insulin induced hypoglycemia. Hypoglycaemia is associated with a range of pathological conditions including neonatal respiratory distress syndrome, toxaemia of pregnancy, congenital enzyme defects, Reye’s syndrome, alcohol ingestion, hepatic dysfunction, insulin-producing pancreatic tumours (insulinomas), insulin antibodies, nonpancreatic neoplasms, septicaemia and chronic renal failure.
Blood glucose measurement is used as a screening test for diabetes mellitus, where there is suspected hyperglycaemia, monitoring of therapy in diabetes mellitus, evaluation of carbohydrate metabolism, for example in gestational diabetes acute hepatitis, acute pancreatitis and Addison’s disease.
Glucose measurement in urine is used as a diabetes screening procedure and to aid in the evaluation of glucosuria, to detect renal tubular defects, and in the management of diabetes mellitus.
CSF glucose may be low or undetectable in patients with acute bacterial, cryptococcal, tubular or carcinomatous meningitis, or in cerebral abscess, probably due to consumption of glucose by leucocytes or other rapidly metabolising cells. In meningitis or encephalitis due to viral infections, it is usually normal.
Serum/Plasma (fasting)
| Adults | 4.11‐5.89 mmol/L | 74‐106 mg/dL |
| 60‐90 years | 4.56‐6.38 mmol/L | 82‐115 mg/dL |
| > 90 years | 4.16‐6.72 mmol/L | 75‐121 mg/dL |
| Children | 3.33‐5.55 mmol/L | 60‐100 mg/dL |
| Neonates (1 day) | 2.22‐3.33 mmol/L | 40‐60 mg/dL |
| Neonates (> 1 day) | 2.78‐4.44 mmol/L | 50‐80 mg/dL |
The generally accepted cut-off levels for the diagnosis of diabetes are:
If any one of these criteria is met, results must be confirmed by repeat testing on a subsequent day, unless there is unequivocal hyperglycaemia with acute metabolic decompensation.
Whole blood
| Adults | 3.6‐5.3 mmol/L | 65‐95 mg/dL |
Whole blood glucose levels are 90% of plasma glucose. Hematocrit level may influence the difference between plasma and whole blood glucose levels due to lower glucose values in erythrocytes compared with plasma concentration. Higher hematocrit levels lead to an increased plasma glucose level compared to whole blood.
Urine
| 1st morning urine | 0.3‐1.1 mmol/L | 6‐20 mg/dL |
| Random urine | 0.06‐0.83 mmol/L | 1‐15 mg/dL |
| 24‐h urine | < 2.78 mmol/24 h | < 0.5 g/24 h |
CSF
| Children | 3.33‐4.44 mmol/L | 60‐80 mg/dL |
| Adults | 2.22‐3.89 mmol/L | 40‐70 mg/dL |
CSF glucose values should be approximately 60 % of the plasma values and must always be compared with concurrently measured plasma values for adequate clinical interpretation.