Whole blood was the sample of choice for analysis However, values for glucose in whole blood are less than in plasma since red blood cells contain only about 80% water, compared with 93% in plasma, this despite the identical concentration of glucose in the water phase of both cells and plasma. Therefore, glucose concentration in plasma is about 12% higher than in whole blood (depending on the haematocrit).

Collection and handling of specimens When venous blood is drawn and permitted to clot, the average rate of decrease in serum glucose is approximately 7% in each hour (0.28 – 0.56 mmol/L) or 5-10 mg/dl. This decrease is the result of glycolysis. Serum in contact with RBCs without preservative must be separated from the cells or clot as soon as possible, if glucose values within 0.5 mmol/L (10 mg/dl) of the original values are to be obtained. However, it is preferred to prevent reduction in blood glucose as a result of glycolysis by collecting blood into sodium fluoride containing tubes. Fluoride ions prevent glycolysis by inhibiting enolase enzyme. Oxalate, on the other hand, inhibits coagulation by binding calcium.

Clinical significance Normal range (using enzymatic method) Plasma glucose 3.0 – 5.6 mmol/L ( mg/dl) CSF glucose 2.2 – 4.2 mmol/L (40-75 mg/dl) or 60% of plasma value Hyperglycaemia (raised blood or plasma glucose) is the hallmark of diabetes mellitus. The common underlying defect is a deficiency of insulin action which may be absolute (as in type 1 diabetes) or relative with resistance to insulin action (as in type 2 diabetes).

Oral glucose tolerance test (OGTT) is performed by giving the patient 75 g oral glucose dissolved in 250 – 300 ml water. Blood samples are taken before (fasting) and 2 hr. following the load for the measurement of glucose. A 2 hr. post glucose plasma glucose (2 hPG) level below 7.8 mmol/L ( < 140 mg/dl) is considered as normal, higher than 11.1 mmol/L ( 200 mg/dl) is diagnostic of diabetes, while 2hPG between 7.8 – 11.1 mmol/L (140 – 199 mg/dl) is indicative of impaired glucose tolerance (IGT).

A. Clinical staging Regardless of the underlying cause, DM is subdivided into: 1- Non-insulin requiring. 2- Insulin requiring for control. 3- Insulin requiring for survival. B. Aetiological classification 1- Type 1 DM, which is subdivided into: a- Autoimmune form; which includes both rapidly progressive and slowly progressive forms. b- Idiopathic form. 2- Type 2 DM. 3- Genetic defects of b-cell function. 4- Diseases of exocrine pancreas.

Glycated Hemoglobin (HbA1c): Within the circulation sugars can bind non enzymically to haemoglobin. Binding of glucose to the N terminal valine of the beta chains of HbA (the normal adult haemoglobin) forms a glycated haemoglobin called HbA1c. Normally about 4-6% of the circulating haemoglobin is in this form. The life span of a red blood cell is about 12 weeks. Assuming normal red cell life span, HbA1c thus reflects the average plasma glucose concentration. Normally, the hemoglobin consists of approximately 97% HbA, 2.5% HbA2 and 0.5% HbF. HbA consists of three different fractions: HbA1a, HbA1b and HbA1c. Collectively, these are referred to as HbA1c which reflect the average plasma glucose concentration over the preceding 12 weeks.

Hypoglycaemia On the other hand, hypoglycaemia is defined as plasma glucose less than 2.5 mmol/L (45 mg/dl). It is less common than hyperglycaemia, and it is eventuated in practice in cases including insulinoma, pancreatic tumors, hepatoma, adrenal carcinoma, hypopitutarism, addison’s disease, inborn error of metabolism of glycogen storage disease, galactossemia, fructose intolerance and also in essential reactive hypoglycaemia.

Diagnosis of Diabetes Mellitus Usually by measuring plasma glucose as follow:- 1-Fasting plasma glucose test:- Measures plasma glucose after at least 8 hours without eating. This test is used to detect diabetes or pre-diabetes as follow:- Fasting Plasma Glucose Result (mg/dL)Diagnosis to 125 Pre-diabetes (impaired fasting glucose) 126 and aboveDiabetes

2-Oral glucose tolerance test (OGTT):- Measures plasma glucose after at least 8 hours without eating & 2 hours after drink a liquid containing 75 gm glucose. This test is more precise in diagnose diabetes or especially pre-diabetes as follow:- 2-Hour Plasma Glucose Result (mg/dL)Diagnosis 139 and below 140 to 199 Pre-diabetes (impaired glucose tolerance) 200 and aboveDiabetes

Enzymatic Method for Measurement of Glucose Principle Aldehyde group of glucose molecule is oxidized in the presence of glucose oxidase (GOD) to gluconate with the liberation of hydrogen peroxide, which in turn reacts with phenol and 4-aminophenazone under catalysis of peroxidase enzyme (POD) to form a pink quinoneimine dye as indicator. Glucose + O2 + H2O ---- (GOD)----- Gluconate + H2O2 2H2O2 + phenol + 4-amino-antipyrine---- (POD) Quinoneimine (pink dye) + 4H2O