Diabetes Blood Glucose Tests
The terms, "blood glucose" and "blood sugar" should be clarified at the outset. The term, "blood glucose" denotes glucose in blood. The term, "blood sugar", popular in the past, denotes a sum total of glucose and other reducing substances in blood.
Determination of blood glucose is essential for the diagnosis of diabetes, for adjusting the dose of insulin and oral anti-diabetic drugs, for assessing the control of diabetes and for diagnosis of hypoglycaemia (low blood glucose level).
Blood glucose/sugar level can be estimated by several methods. Some methods for example, glucose oxidase (GOD-POD) estimate blood glucose level only. The Somogyi-Nelson method estimates, in addition to glucose, fructose and galactose if present. The Folin-Wu method estimates other sugars and non-sugar substances (total reducing substances) besides glucose. The latter together average about 15-20 mg/dl in fasting state but after a meal or glucose load the changes are unpredictable (from 30 - 80 mg/dl).
The blood glucose level is at times expressed in terms of millimoles per litre (mmol/1). One millimole per litre glucose is equal to 18 mg/dl.
Blood glucose/sugar level varies according to whether blood is collected from a vein (a blood vessel which carries deoxygenated blood) or a capillary (a very minute blood vessel). Ordinarily blood is not collected from an artery (blood vessel carrying oxygenated blood), to estimate blood glucose. Blood collected from a blood vessel around the elbow or wrist is venous blood. Capillary blood is obtained by pricking the finger tip, heel or ear lobe (the latter two sites used usually in infants).
Capillary blood glucose almost equals venous blood glucose in fasting state but exceeds the latter by 8-61 mg/dl (average 24 mg/dl) for more than two hours after a meal containing carbohydrates. However in children, even in fasting state, the difference ranges from 0 to 26 mg/dl (average 10 mg).
The blood glucose/sugar value differs depending on whether whole blood or plasma (clear supernatant obtained by spinning unclotted whole blood) or serum (obtained by letting the whole blood clot and retract) is used for estimation. Plasma glucose is 10-15 mg higher than whole blood glucose. Serum is usually not used for glucose estimation. The following points should be checked in the blood glucose sugar report: the method, source of blood, whether whole blood or plasma was tested, whether after a meal or a glucose load, whether insulin/tablets were taken on the day of the test or whether these were omitted.
The fasting blood glucose is less than 110 mg/dl. It increases after a meal but does not rise above 140 mg/dl.
In mild diabetes, the fasting blood glucose level may be normal or near normal but the post meal level is elevated. When diabetes is more severe, blood glucose level is elevated in fasting state also.
Blood glucose tested without consideration of the time of the day or meals is called random blood glucose. It has limited value in the diagnosis of diabetes. It is useful in emergency situations. It is also useful to confirm or exclude abnormally low blood glucose. It should not be used for follow-up of diabetes and adjustment of drug dosages. The time of random blood glucose test should be noted.
The term "glycaemia" is used to denote "glucose in blood". The terms, "normoglycaemia", "hyperglycaemia" and "hypoglycaemia" denote normal, above normal and below normal blood glucose levels respectively.