Blood sugar is a term used to refer to the amount of glucose in the blood. Glucose, transported via the bloodstream, is the primary source of energy for the body's cells.

Blood sugar concentration, or glucose level, is tightly regulated in the human body. Normally, the blood glucose level is maintained between about 4 and 8 mmol/L (70 to 150 mg/dL). The total amount of glucose in the circulating blood is therefore about 3.3 to 7g (assuming an ordinary adult blood volume of 5 liters). Glucose levels rise after meals and are usually lowest in the morning, before the first meal of the day.

Failure to maintain blood glucose in the normal range leads to conditions of persistently high (hyperglycemia) or low (hypoglycemia) blood sugar. Diabetes mellitus, characterized by persistent hyperglycemia of several causes, is the most prominent disease related to failure of blood sugar regulation.

Though it is called "blood sugar" and sugars besides glucose are found in the blood, like fructose and galactose, only glucose levels are regulated via insulin and glucagon.

Normal level of sugar in blood before your meal is between 70 and 110 mg/dl. After food, level will rise in between 100 to 140. The increased level of blood sugar above 140 mg/dl is considered to be the symptoms of diabetics. Low level of sugar, below 70 is also dangerous. It may cause fainting.

The two main tests used to measure diabetes are (a) Fasting Plasma Glucose Test, the direct measurement of glucose levels in the blood after an 8 hour (overnight) fast, and (b) Oral Glucose Tolerance Test, measurement of the body's ability to appropriately handle the excess sugar presented after drinking a high glucose drink

Fasting Plasma Glucose test is positive when the value of elevated blood sugar level is above 140 mg/dl after an overnight fast. If the value is above 140 mg/dl on at least two occasions, it means that a person has diabetes. Normal people have fasting sugar levels that generally run between 70-110 mg/dl.

An oral glucose tolerance test (OGTT) measures plasma glucose at timed intervals over a 3-hour period. The person being tested starts the test in a fasting state (having no food or drink except water for at least 10 hours but not greater than 16 hours). An initial blood sugar is drawn and then the person is given a drink with high amount of sugar in it (75 grams of glucose or 100 grams for pregnant women). The person then has their blood tested again 30 minutes, 1 hour, 2 hours and 3 hour intervals after drinking the high glucose drink. If the plasma glucose level is 200 mg/dl or more in the blood sample taken 2 hours after drinking high glucose shows that the test is positive.

Gestational diabetes is also diagnosed through OGTT. Glucose levels are normally lower during pregnancy, so the basic values for diagnosis of diabetes in pregnancy are lower. If a woman has two plasma glucose values meeting or exceeding any of the following numbers, she has gestational diabetes: a fasting plasma glucose level of 95 mg/dL, a 1-hour level of 180 mg/dL, a 2-hour level of 155 mg/dL, or a 3-hour level of 140 mg/dL.